Increasing Plant Population Density Research Articles

Eco-physiology of maize crops under combined stresses.

The yield of maize (Zea mays L.) crops depends on their ability to intercept sunlight throughout the growing cycle, transform this energy into biomass and allocate it to the kernels. Abiotic stresses affect these eco-physiological determinants, reducing crop grain yield below the potential of each environment. Here we analyse the impact of combined abiotic stresses, such as water restriction and nitrogen deficiency or water restriction and elevated temperatures. Crop yield depends on the product of kernel yield per plant and the number of plants per unit soil area, but increasing plant population density imposes a crowding stress that reduces yield per plant, even within the range that maximises crop yield per unit soil area. Therefore, we also analyse the impact of abiotic stresses under different plant densities. We show that the magnitude of the detrimental effects of two combined stresses on field-grown plants can be lower, similar or higher than the sum of the individual stresses. These patterns depend on the timing and intensity of each one of the combined stresses and on the effects of one of the stresses on the status of the resource whose limitation causes the other. The analysis of the eco-physiological determinants of crop yield is useful to guide and prioritise the rapidly progressing studies aimed at understanding the molecular mechanisms underlying plant responses to combined stresses.

Suitable duration of grazing exclusion for restoration of a degraded alpine meadow on the eastern Qinghai-Tibetan Plateau

Grazing exclusion (GE) is a key national ecological restoration project widely applied to rehabilitate degraded grasslands. To date, there have been many debates on the effectiveness of GE for grassland recovery, and it is still poorly understood how degraded alpine meadows in the semi-humid area with relative rich rainfall in the Qinghai-Tibetan Plateau respond to GE with different durations. We selected a chronosequence of grazing-excluded alpine meadows to examine the dynamics of grassland functions. The results showed that the dominant functional group shifted from forbs to graminoids after 8 to 10 years GE and then returned to forbs. The plant population density increased significantly with GE time (P < 0.01). The total biomass, species richness, Shannon diversity index, and relative importance value of graminoids first increased and then decreased with increasing plant population density, whereas the relative importance value of forbs presented the opposite trend. All turning points were between 1350 and 1650 plants per m2 which occurred from 8 to 10 years of GE. As the duration of GE increased, the ecosystem multifunctionality first dramatically increased and then became steady after 8 to 10 years of GE. These findings imply that GE is of great practical significance to prevent degradation in semi-humid regions of the Qinghai-Tibetan Plateau, where we recommend that GE should cease after approximately 8 to 10 years to restore degraded alpine meadows from the perspective of plant productivity, diversity, community structure, and ecosystem multifunctionality.

Intense winter grazing impairs subsequent Italian ryegrass re‐establishment in soybean‐pasture rotations

AbstractWe evaluated Italian ryegrass (Lolium multiflorum Lam.) establishment in a factorial of grazing intensity (intense, moderate, moderate‐light, light, and no grazing, with target average sward heights of 10, 20, 30, and 40 cm in the grazed plots) and additional seeding (self‐seeding with or without additional seeding) in an integrated soybean‐beef cattle system in southern Brazil. Grazing treatments were imposed in winter (June–July) and followed by a direct‐seeded soybean crop (November). The establishment was quantified prior to the stocking periods in 2017 and 2018, corresponding to self‐seeding from 2016 and 2017. Intense grazing impaired ryegrass re‐establishment, resulting in larger individual plants (0.54 vs. 0.15 g dry matter plant−1) but lower population density (57 vs. 1,355 plants m−2) than in the other treatments. Additional broadcast ryegrass seeding did not fully compensate for the reduced natural reseeding in this treatment. The addition of seeds under intense grazing increased plant population density to values comparable to moderate grazing intensities but reduced individual plant mass, limiting herbage mass attained by the end of the pasture establishment phase. Contrastingly, the various combinations of individual plant mass and population density were sufficient to maintain herbage masses in moderate to light grazing intensities that were comparable to that in the ungrazed treatment, regardless of seed addition (63 vs. 180 g DM m−2 for intense grazing vs. the average of other treatments over the whole establishment phase). Moderate grazing, e.g., target sward heights of 20 cm or greater, is necessary to achieve a system that can sustain itself without seed addition.

Varietal Response to Double Plant Population Density in Maize: Implications for Breeding

Purpose : Suboptimal plant population has been identified as one of the causes of low yield of maize in Nigeria, hence the need for increased plant population per hectare. Maize varieties respond differently to increased plant population density. There is need to identify maize varieties that respond well to increased plant population density towards developing maize for tolerance to population stress.Research Method : Four new maize varieties were tested under population density of 53,333 (ND) and 106,666 plants/ha (DD). The trials were conducted at two locations in two seasons. The experimental design was split plot in three replications. The DD plots received additional 50% of fertilizer. Data were collected on agronomic and yield traits. Findings : Mean squares due to density were significant for grain yield and the number of ears per plant. Mean squares due to variety was significant for all traits except the number of ears per plant. Yield and plant appeal under DD were generally lower than under ND except in few instances. For instance, grain yield under DD was higher (4.96tons/ha) than under ND (4.61tons/ha) for BR9943DMRSR at Ile-Ife, and for BR9928DMRSR (2.47tons/ha under DD, 2.09tons/ha under ND) at Ilora. Only BR9928DMRSR had an increased grain yield from ND (2.41tons/ha) to DD (2.48tons/ha) across locations. Originality / Value : The variety, BR9928DMRSR, with increased yield and reduced plant height from ND to DD seemed promising for improvement for tolerance to plant population stress.

Maize Yield and the Associated Weeds as Affected by Plant Population Density and Weed Control Treatments

Two field experiments were carried out in the Farm of Agricultural Research and the Experimental Center of Faculty of Agriculture at Moshtohor, (Toukh Directorate, Qalyubia Governorate) Benha Univ. Egypt, during 2017 and 2018 seasons to investigate the effect of five plant population densities, i.e. 18, 21, 24, 27 and 30 thousand maize plants/feddan (fed) {one fed = 4200 m2} and five weed control treatments, i.e.pendimethalin 682.5 g a.i./fed (stomp extra 45.5 % CS),acetochlor840 g a.i./fed (harness 84 % EC), nicosulfuron24 g a.i./fed (active 6 % SC), hand hoeingtwice and the unweeded check (the control) on yield, some of its components and associated weeds in maize (single cross hybrid 2036 for MisrHytech Seed Int.). Increasing plant population density from 18 to 30 thousand maize plants/fed reduced weed biomass at 50 days after sowing as well as significantly increased mean values of silking date, leaf area index at 80 days after sowing, plant height, ear height, No. of barren plants/fed, No. of ears/fed and stover yield/fed in both seasons. On the other hand, mean values of area of topmost ear leaf at 80 days after sowing, leaf area/plant at 80 days after sowing, stem diameter, No. of plants carried two ears/fed, ear length, No. of kernels/ear, weight of kernels/ear, shelling %, 100-kernel weight and harvest index were significantly decreased in the two seasons.Maize planted at 24000 plants/fed produced the highest mean values of grain and biological yields/fed in both seasons. Weed control by hand hoeing twice or using nicosulfuron herbicide surpassed the other weed control treatments in depressing mean values of fresh and dry weights of broad-leaved, grassy and total annual weeds at 50 days after sowing maize, as well as gave the best mean values of all maize traits without significant difference between them. Maize planted at 30000 plants/fed under mechanical weed control (hand hoeing twice) or using nicosulfuron herbicide gave the lowest mean values of all weed measurements at 50 days after sowing as well as gave the greatest mean values of leaf area index, plant height, ear height, No. of ears/fed and stover yield/fed in both seasons. The greatest mean values of area of topmost ear leaf, leaf area/plant, No. of kernels/ear, weight of kernels/ear and 100-kernel weight were obtained from planting 18000 maize plants/fed with hand hoeing twice. Maize planted at 24000 plants/fed and hand hoeing twice produced the greatest mean values of biological yield/fed in both seasons. The maximummean values of grain yield/fed were obtained by maize planted at 24000 and 21000 plants/fed and hand hoeing twice the first and second seasons, respectively.Planting maize by 21000 plants/fed with hand hoeing twice gave the highest mean values of nitrogen uptake/fed and protein yield/fed in both seasons.

Response of white maize hybrids to plant densities and nitrogen fertilizer rates

Two field experiments were conducted on the Farm of Agric. Res. and Exp. Center of Fac. of Agric. Moshtohor, Benha University, Toukh Directorate, Qalyubia Governorate, Egypt, during the two successive summer growing seasons of 2016 and 2017 to study the effect of three plant population densities i.e. 20000, 24000 and 28000 plants/feddan (fed) and three nitrogen fertilizer rates, i.e. 90, 120 and 150 kg N/fed on the growth traits, yield components, yield and some kernels chemical properties of three white single cross hybrids of maize (S.C. 7071, S.C. 30K8 and S.C. 2031). The obvious results of this investigation can be summarized as follows: Increasing plant population density from 20000 to 28000 plants/fed significantly increased mean values of No. of days from planting to 50 % tasseling and silking, leaf area index, plant height (cm), ear height (cm) No. of barren plants/fed, No. of ears/fed, stover yield/fed (kg) and biological yield/fed (kg) in both seasons. On the other hand, mean values of No. of green leaves/plant, leaf area/plant (cm2), No. of plants carried two ears/fed, No. of kernels/ear, ear weight (g), shelling (%), 100-kernel weight (g), ear yield/fed (kg), grain yield/fed (kg), harvest index, nitrogen uptake/fed (kg) and protein yield/fed (kg) were significantly decreased in the two seasons. Allmost traits of maize under study were significantly increased by increasing nitrogen fertilizer rates from 90 and 120 to 150 kg N/fed except, No. of days to 50 % tasseling and silking as well as No. of barren plants/fed were significantly decreased with increasing nitrogen rates in the both seasons, nitrogen fertilizer rate of 150 gave the best mean values for all maize traits under study. White single cross hybrids of maize were significantly differed in all maize traits under study in the both seasons. Maize hybrid of S.C. 30K8 was significantly surpassed S.C. 7071 and S.C. 2031 in mean values of No. of plants carried two ears/fed, No. of ears/fed, No. of kernels/ear, grain yield/fed and harvest index as well as gave the lowest mean values of No. of barren plants/fed and the shortest period from planting to 50 % tasseling or silking in the both seasons. Moreover, S.C. 2031 surpassed the other two maize hybrids in mean values of leaf area/plant, leaf area index, ear weight, 100-kernel weight, ear yield/fed, biological yield/fed, nitrogen uptake/fed and protein yield/fed in the two seasons. Meanwhile, S.C. 7071 recorded the highest mean values of No. of green leaves/plant, plant height, ear height, shelling % and stover yield/fed in the two seasons. The first order interactions between (20000 plants/fed X 150 kg N/fed), (24000 plants/fed X S.C. 30K8), (20000 plants/fed X S.C. 2031) and (150 kg N/fed X S.C. 30K8) as well as the second order interactions between (24000 plants/fed X 150 kg N/fed X S.C. 30K8) and (20000 plants/fed X 150 kg N/fed X S.C. 2031) were significantly recorded the greatest grain yield/fed as compared with the others interactions in the both seasons.

Does increasing plant population density alter sugar yield in high stalk-sugar maize hybrids?

Escalating demands for food and green energy have renewed interest in the dual-purpose use of maize (Zea mays L.) for a biofuel and high-energy forage crop. Recently, maize hybrids with high stalk-sugar (sugarcorn) have been developed. It is important to determine how agronomic practices, for example altering plant population density (PPD), affect stalk-sugar yields of these newly developed hybrids and to advance knowledge required for producing sugarcorn as a dual-purpose bioenergy–high energy silage crop in short-season regions unable to grow sugarcane. A field experiment was conducted for 3 years to assess the effect of PPD on stalk-sugar accumulation, dry matter production, silage and sucrose yields of sugarcorn compared with two commercial silage hybrids. Targeted PPD ranged from 75 000 to 150 000 plants ha–1 in increments of 25 000 plants ha–1. We found that increasing PPD from 75 000 to 125 000 plants ha–1 increased stalk sugar concentrations by up to 25% in some of the sugarcorn hybrids, with minimum change in the conventional check hybrids. The sugarcorn hybrid CO348 × C103 had the highest stalk sugar concentration (128 g kg–1) and sucrose yield of up to 3.8 Mg ha–1 at the targeted PPD of 125 000 (or actual 118 000 ± 7000) plants ha–1. By contrast, the check silage hybrids produced at most 2.0 Mg ha–1 of sucrose yield with much lower stalk sugar concentrations (53–65 g kg–1). Sugarcorn hybrids had generally lower grain yield with greater plant barrenness (the failure of a plant to produce a normal ear) and severer head smut infestation than the conventional silage hybrids. Our results suggest that as a silage crop, the current recommended PPD of 75 000–85 000 plants ha–1 for commercial silage maize production in the region is likely suitable for sugarcorn, and a higher PPD is required if sugarcorn hybrids are designated as a biofuel crop or for dual-purpose use.

Corn Response to Row Spacing and Seeding Rate Varies by Hybrid and Environment in Kentucky

There has been recent interest in growing corn in narrow rows (&lt;30 inches wide) and at dense plant populations to increase grain yields. The purpose of this study was to evaluate three corn hybrids adapted to the midsouthern United States at multiple row spacings and seeding rates. Studies were established at Lexington and Hodgenville, KY during the 2011 and 2012 growing season. The corn hybrids were planted in 15‐inch rows, twin 8‐inch rows on 30‐inch centers, and 30‐inch rows at seeding rates of 30,000; 35,000; 40,000; and 45,000 seeds/acre. Plant stands, stalk diameter, intercepted photosynthetically active radiation, and grain yield were evaluated for all hybrid × row spacing × seeding rate combinations. Stalk diameter decreased with increasing seeding rates, but no lodging was observed. At Hodgenville in 2011, corn planted in twin rows yielded 6.7% more than corn in 30‐inch rows, but row spacing did not affect yield in the other site‐years. Under favorable growing conditions at Lexington in 2011, grain yield increased by 1.5 bu/acre per 1000 seed increase in seeding rate; however, under hot and dry conditions in 2012, grain yield decreased 0.87 bu/acre for every 1000 seed increase in seeding rate. At Hodgenville in 2012, a hybrid × seeding rate interaction was observed in which one hybrid responded positively, one responded negatively, and one was unresponsive to seeding rate. Corn producers considering decreasing row widths or increasing plant population density should ensure that they choose hybrids appropriate for their management systems and environmental conditions.

DETERMINATION OF THE CRITICAL PERIOD OF WEED CONTROL IN MAIZE GROWN UNDER DIFFERENT PLANT DENSITIES

Tow field experiments were carried out during the two successive summer seasons of 2010 and 2011 at the Agric. Res. and Exp. Center of Fac. Of Agric. Moshtohor to study the effect of five plant population densities (20, 22, 24, 26 and 28 thousand maize plants per fed.) and seven periods for weed control (un-weeded control (1), weed control at 20 (2), 20+35 (3), 20+35+50 (4), 20+35+50+65 (5), 20+35+ 65+80 (6) and 20+35+80+95 days after sowing (7)) on growth, yield and its components of maize, associated weeds as well as the net economic return of maize. The obvious results of this investigation can be summarized as follows: Increasing plant population density from 20 to 28 thousand plants /fed. significantly decreased total number and fresh weight of removed weeds in the first season, total fresh and dry weights of weeds at harvest, number of green leaves / plant, leaf area / plant, stem diameter, ear length, number of grains / ear, ear weight, shelling %, 100-grain weight and grain yield / plant in both seasons. On the other hand, plant height, leaf area index, number of ears /fed., Stover yield, nitrogen up-take and protein yield / fed. were significantly increased. Generally, the greatest grain and biological yields / fed. were results from maize planting by 26000 plants/ fed. This was true in the two growing seasons. Un-weeded check significantly decreased all these characters except total fresh and dry weights of weeds at harvest compared with weed removal even once after maize emergence. The maximum grain and biological yields/ fed. were produced from planting maize under weed control at 20, 35, 50 and 65 days after sowing. The critical period of weed control (CPWC) in maize crop based on 5% acceptable yield loss was 20 - 35 days after sowing. The highest values of leaf area index and stover yield / fed. were recorded from planting maize at the highest density under weed control by treatment 5. While, the lowest plant density under the same weed control gave the maximum values for No. of green leaves / plant, leaf area / plant, ear length, No. of grains /ear, 100-grain weight, ear weight and grain yield / plant in the two growing seasons. Meanwhile, the greatest maize grain and biological yields / fed. were resulted from maize planting by 26000 plants / fed. under the same weed control (weed control at 20 +35 + 50 + 65 days after sowing). The best treatment was that planting 24000 maize plants / fed. under weed control by either treatment 5 or 6 in the first season and planting 26000 maize plants /fed. with weed control by treatments 5 in the second season, where the net farm return valued 1945.2 and 2981.0 L.E. / fed. in the first and second seasons respectively.

Inter-plant competition for resources in maize crops grown under contrasting nitrogen supply and density: Variability in plant and ear growth

Increased plant population density in irrigated and fertilized maize crops enhances plant-to-plant variability since early vegetative stages, because the most suppressed individuals of the stand intercept less radiation per unit leaf area than the dominant ones (i.e. a size-asymmetric competition for light). Contrarily, a size-symmetric competition has been proposed for the acquisition of soil resources in a plant community (e.g. N capture per unit root length is similar among plants of different size). Hence, N fertilization effect on the variability of maize plants would depend on the initial plant-to-plant variability or on that promoted by a high plant population density. Two maize hybrids with contrasting tolerance to crowding (tolerant AX820 and intolerant AX877) were cultivated under different combinations of stand densities (6, 9 and 12 plants m −2) and N supplies (0 and 200 kg N ha −1) without water restrictions. Variability in plant growth rate among plants was computed along the cycle, especially after fertilizer was applied (i.e. the early reproductive period; PGR ER) and during the critical period around silking (PGR CP). Plant-to-plant variability in biomass partitioning to the ear (partition index; PI), ear growth rate during the critical period (EGR CP) and kernel number per plant (KNP) was also established. Reduced N supply increased the coefficient of variation (CV) of PGR ER, PGR CP, EGR CP and KNP (0.05 < P < 0.10). The CVs of PGR CP, PI, EGR CP and KNP augmented (0.001 < P < 0.10) at the highest stand density. The CVs of PGR ER, PGR CP, PI and KNP were larger for hybrid AX877 than for hybrid AX820 (0.001 < P < 0.10). N fertilization smoothed the initial plant-to-plant variability, but the extent of this benefit in a maize crop is genotype dependent; it was much larger in the hybrid tolerant to crowding stress than in the intolerant one. For the latter, the variability held during the critical period around silking and produced a high CV of KNP.

Plasticity of winter wheat modulated by sowing date, plant population density and nitrogen fertilisation: Dimensions and size of leaf blades, sheaths and internodes in relation to their position on a stem

Mathematical models to describe crop–environment interaction on organ scale need to take crop or plant architecture into account. Up to now, architectural plant models are largely descriptive and parameters need to be estimated for each species, cultivar and environment. Required measurements are extensive and time-consuming. Hence investigating morphological patterns and their modulation as a response to environmental conditions may help to reduce measurement efforts and to predict plant architecture in crop models. In this paper, we describe the plasticity of winter wheat – expressed as the dimensions and sizes of leaf blades, sheaths and internodes in relation to their position on a stem – under the climatic conditions of the Paris region. Results are discussed with respect to: (i) genotypic variability, (ii) inter-annual variability, (iii) sowing date and plant population density, (iv) Nitrogen fertilisation and (v) tiller rank. Eight wheat cultivars grown in the same season showed similar patterns of leaf and internodes dimensions in relation to their position on the stem. For the cultivar ‘Soissons’, main stem architecture at flowering was remarkably stable when similar growth conditions were reproduced in the different seasons. Increased plant population density yielded longer juvenile, but shorter adult leaf blades and sheaths. Earlier sowing led to an increase in the number of juvenile phytomers – growing before the onset of stem elongation – on the main stem, whereas the number of adult phytomers was almost identical. Further there were little differences in the size of leaf blades, sheaths and internodes between the main stem and axillary tillers. We found remarkable differences in the size of adult leaf blades and sheaths in different growing seasons, with different timing of nitrogen fertilisation and we discuss decreased availability of nitrogen in the soil in spring as a likely cause. Data presented here can be used to enlarge the understanding of wheat plasticity regarding the regulation of organ size by temperature, light, plant-available nitrogen and size-mediated effects towards a mechanistic modelling of these responses.

Low crop plant population densities promote pollen-mediated gene flow in spring wheat (Triticum aestivum L.)

Transgenic wheat is currently being field tested with the intent of eventual commercialization. The development of wheat genotypes with novel traits has raised concerns regarding the presence of volunteer wheat populations and the role they may play in facilitating transgene movement. Here, we report the results of a field experiment that investigated the potential of spring wheat plant population density and crop height to minimize gene flow from a herbicide-resistant (HR) volunteer population to a non-HR crop. Pollen-mediated gene flow (PMGF) between the HR volunteer wheat population and four conventional spring wheat genotypes varying in height was assessed over a range of plant population densities. Natural hybridization events between the two cultivars were detected by phenotypically scoring plants in F(1) populations followed by verification with Mendelian segregation ratios in the F(1:2) families. PMGF was strongly associated with crop yield components, but showed no association with flowering synchrony. Maximum observed PMGF was always less than 0.6%, regardless of crop height and density. The frequency of PMGF in spring wheat decreased exponentially with increasing plant population density, but showed no dependence on either crop genotype or height. However, increasing plant densities beyond the recommended planting rate of 300 cropped wheat plants m(-2) provided no obvious benefit to reducing PMGF. Nevertheless, our results demonstrate a critical plant density of 175-200 cropped wheat plants m(-2) below which PMGF frequencies rise exponentially with decreasing plant density. These results will be useful in the development of mechanistic models and best management practices that collectively facilitate the coexistence of transgenic and nontransgenic wheat crops.

Root lodging in sunflower. Variations in anchorage strength across genotypes, soil types, crop population densities and crop developmental stages

Root lodging is an important adversity affecting sunflower ( Helianthus annuus L.) production in Argentina under current husbandry practices, and may limit progress towards the achievement of higher yields via increased plant population density. Although there are perceptions that lodging susceptibility varies across developmental stages, crop population densities, genotypes and soil types, these perceptions have not been tested for sunflower using a standardized experimental protocol. This study aimed at: (1) identifying the sources of the variation in root lodging susceptibility in response to variations in crop population density in two genotypes of reputedly different susceptibility; (2) detecting the crop developmental stages most susceptible to root lodging; and (3) examining the relationships between root failure moment, root plate diameter and soil shear strength. We mechanically induced lodging at three developmental stages in plants rooted in pre-wetted plots. The crops were grown at 5.6 plants m −2 over 3 years on either Typic Argiudoll or Typic Hapludoll soils and at 3, 5.6, 10 and 16 plants m −2 on a Typic Argiudoll. The force needed to induce root lodging (root failure moment) and root plate diameter varied across genotypes, plant densities and developmental stages. Root failure moment and root plate diameters were greater ( p < 0.05 for both variables) in the resistant hybrid across the three development stages and almost all crop population densities. For both hybrids, the most susceptible development stage was R2, and root failure moment and root plate diameter diminished ( p < 0.05) as crop population density increased. Although root failure moment did not differ between soil types, root plate diameter was greater ( p < 0.0001) in the coarser soil. The relationship between root failure moment and the product of root plate diameter cubed by soil shear strength (a measure of plant anchorage strength) for both hybrids, both soil types, and all crop population densities could be described by a single linear relationship ( y = 0.2382 x; R 2 = 0.812; p < 0.025).

Evaluation of sweet potato ( Ipomoea batatas) live mulch at different spacings for weed suppression and yield response of maize ( Zea mays L.) in southwestern Nigeria

The use of living plants for ground cover management in food crops is very recent in spite of the fact that food crop production in chemically killed sod has been practiced for years. A randomized complete block field experiment was conducted over 2 years to evaluate the influence of sweet potato live mulch at different intra row spacings (60 cm×25 cm, 60 cm×50 cm and 60 cm×75 cm) on weed suppression and yield response of maize in southwestern Nigeria. Hoe-weeding once at 3 and 7 weeks after planting was included in the experimental set-up as the standard cultural weed control practice. Results indicated that all the spacing regimes tested significantly ( P<0.05) suppressed weed growth and increased grain yield over the unweeded sole maize. Regressing weed growth parameters ( Y) against decreasing intra row spacing expressed as increasing plant population density of the sweet potato intercrop ( X) showed highly significant ( P⩽0.001) negative relationships. Of the treatments tested, maize plus sweet potato at 60 cm×25 cm and weeding once at 3 and 7 weeks after planting (WAP) produced the highest yields, but bearing in mind the economics of labour input, weed control and yield, maize plus sweet potato at 60 cm×50 cm would appear to be the most promising of all the treatments. The results of this study further confirm the potential of sweet potato as a suitable crop for use as live mulch that can be effectively exploited for weed suppression and improved growth and yield responses in maize.

Intra-specific competition in maize: Early established hierarchies differ in plant growth and biomass partitioning to the ear around silking

Early interferences among plants within a maize stand determine the establishment of extreme plant types (i.e. dominant and dominated individuals). The development of these hierarchies takes place well before [from the seventh leaf stage (V 7) onwards] the start of the critical period for kernel set (i.e. a 30-day period centered in silking). Kernel number per plant (KNP) is significantly related to plant growth rate around silking (PGR S) and biomass partitioning to the ear during this period. Previous evidence has demonstrated that at high stand densities, extreme plant types may exhibit similar PGR S values but set different KNP. We tested the hypothesis that early established plant hierarchies differ in biomass allocation to the ears during the period around silking. Two hybrids of contrasting tolerance to crowding (DK752 and DK765 as the tolerant and the intolerant hybrid, respectively) were cropped at different interplant competition intensities (6, 12, 12 pl m −2 thinned to 6 pl m −2 at V 9 and 6 pl m −2 shading from V 9 onwards) during 2003/2004 and 2004/2005 in Argentina. For all treatments, the coefficient of variation (CV) of plant biomass increased from V 3 (ca. 1.2%) to V 9-10 (ca. 22%). From V 7 onwards, plant growth rate of dominant individuals was higher ( P < 0.05) than that of the dominated plants. Hence, dominant plants exhibited higher ( P < 0.05) PGR S (ca. 4.5 g pl day −1) than dominated individuals (ca. 3.7 g pl day −1). As PGR S declined in response to increased plant population density (ca. 5.1 and 2.8 for 6 and 12 pl m −2, respectively), biomass partitioning to the ear was reduced (ca. 0.44 and 0.33 for 6 and 12 pl m −2, respectively). For all treatments, however, dominant plants exhibited a greater biomass partitioning to the ear (ca. 0.41) than the dominated individuals (ca. 0.36). Consequently, the former were the individuals with the highest ear growth rate (ca. 1.9 and 1.4 g per ear per day for the dominant and dominated plant, respectively) and KNP (ca. 623 and 490 kernels per plant for the dominant and dominated plant, respectively) of the stand. We identified three traits on DK765 related to the low tolerance to high-density stress of this genotype: (i) a higher plant-to-plant variability (CV ca. 26% and 19%, for DK765 and DK752, respectively), (ii) a lower biomass partitioning to the ear around silking (ca. 0.26 and 0.39 for DK765 and DK752, respectively), and (iii) a higher response rate of KNP to ear growth rate around silking (ca. 370–738 and 360–414 kernels per g, for DK765 and DK752, respectively). Hence, as stand density was increased, KNP of DK765 was sharply reduced, especially in the dominated individuals of the stand.

Effects of thermoperiodicity and plant population density on stem and flower elongation, leaf development, and specific fresh weight in single stemmed rose ( Rosa hybrida L.) plants

Thermoperiodicity, i.e. growth in the alternating temperature regime with the same diurnal mean compared with growth at the constant temperature at which optimal growth occurs, was studied at three plant population densities in four cultivars of Rosa hybrida L. Single-node cuttings with five-leaflet leaves were excised and grown as single-stemmed rose plants at an average photosynthetic photon flux density of about 260 μmol m −2 s −1 and supplied with carbon dioxide at about 1000 μmol mol −1. The optimal constant temperature regime was 22 °C day (20 h)/22 °C night (4 h); alternating temperatures were 23 °C day (20 h)/18 °C night (4 h). The plant population densities were 100, 131 and 178 plants m −2 of bench area. Thermoperiodicity was absent, or could not be detected, in the parameters related to the growth period, the formation of fresh biomass, the bloom quality, and most parameters related to shoot elongation. However, classic thermoperiodic effects of alternating regime were significant in the cultivars Red Velvet and Sonia, with shoot elongation promoted (7.1 and 10.5%, respectively) in the growth phase from onset of axillary bud growth until the flower bud became visible. Compared with the other two cultivars, plants of Red Velvet and Sonia tended to develop longer internodes. The results, obtained concurrently at three different plant population densities, suggest that thermoperiodicity can affect (single-stemmed) plant growth and development in R. hybrida. Increased plant population density also increased plant height at visible flower bud, but the bloom quality, expressed as specific fresh weight, and the flower height at anthesis was decreased at the highest density. Increased plant population density increased the number of five-leaflet leaves developed in Red Velvet, but had no effect on leaf number in Texas and Sonia, while, in Lambada the leaf number was decreased at the highest density.

Morphological plasticity of chickpea in a semiarid environment

Chickpea (Cicer arietinum L.) is being rapidly adapted to the semiarid northern Great Plains, but little is known about the morphological responses of this annual grain legume to the dry environment. This study, conducted in southwestern Saskatchewan, examined the morphological plasticity of three market classes of chickpea by growing the crop at four plant population densities. Chickpea grown at high (50 plants m−2) population density produced approximately half as many fertile pods per plant as those grown at low (20 plants m−2) density, but total number of pods per unit area increased with increasing plant population density. Large‐seeded kabuli chickpea produced fewer pods per unit area, or &lt;60% of that produced by small‐seeded kabuli, and &lt;50% of that by desi chickpea. Infertile pods accounted for 17 to 23% of the total pods for large‐seeded kabuli, compared with 9 to 12% for small‐seeded kabuli, and 6% for desi chickpea. The large‐seeded kabuli produced &lt;87 seeds for every 100 pods produced, whereas desi and small‐seeded kabuli produced &gt;110 seeds for every 100 pods. Consequently, the large‐seeded kabuli chickpea produced &lt;90% of seed yield per unit area than small‐seeded kabuli and desi chickpea. As plant population increased from 20 to 50 plants m−2, the seed yield m−2 increased by 20% for desi and 27% for small‐seeded kabuli, but only 17% for the large‐seeded kabuli chickpea. In the semiarid northern Great Plains, seed yield potential of desi and small‐seeded kabuli chickpea can be increased by increasing plant population density, whereas the seed yield of large‐seeded kabuli can be improved by increasing percentage pod fertility.

Path Analyses of Population Density Effects on Short‐Season Soybean Yield

Yield component analysis provides a framework for identifying potentially useful traits for yield improvement. Consideration of how population density affects other yield components has not been addressed specifically for short‐season soybean [Glycine max (L.) Merr.] production. We assessed the direct and indirect contributions of population density for short‐season soybean yield and its components over a wide range of population densities (6–134 plants m−2) using path‐coefficient analysis. Data were from field tests conducted in 1997, 1998, and 1999 at Keiser, AR. Although population density had a large inverse association with pods plant−1, the large direct effect of population density on yield was greater than its negative indirect effect via pods plant−1. The direct effects of pod number plant−1 and seeds pod−1 on yield were positive, whereas mass seed−1 had a negligible effect. Pods fertile‐node−1 differed between cultivars, and it was reduced by increasing population density. For early sowing, the contribution of population density to yield was less because pods m−2 could be achieved at low population densities by a large number of fertile‐nodes plant−1 and pods fertile‐node−1. In contrast, at late sowing, the decreased potential for fertile‐nodes plant−1 was compensated by increasing plant population density. In short seasons, maximizing nodes m−2 and pods m−2 can be achieved by high population densities and early canopy closure, rather than the conventional system of larger plants with greater numbers of pods plant−1 and pods fertile‐node−1

Growth, Flowering, and Postharvest Performance of Single-stemmed Rose (Rosa hybrida L.) Plants in Response to Light Quantum Integral and Plant Population Density

The cut rose, grown as a single-stemmed crop, resembles a potted plant and can be adapted to transportable bench systems. Potentially, this cultivation method could increase control of rose development, flexibility of production and produce, and automation of difficult or laborious cultural operations. Synchronous growth and flowering is considered important. The effects of increased quantum irradiation integral and plant density on shoot growth, fresh biomass production, and bloom quality were studied as single-stemmed rose plants (Rosa hybrida L.) `Kordapa' Lambada, `Tanettahn' Manhattan Blue, `Tanorelav' Red Velvet, and `Sweet Promise' Sonia grown under 20 hours photoperiods at 23 °C average air temperature. Plants were grown in rockwool cubes on ebb and flood benches irrigated with a complete nutrient solution, and were supplied with carbon dioxide at 1000 μmol·mol-1. Increased the daily quantum integral from 17.8 to 21.0 mol·m-2·d-1 increased fresh biomass efficiency, stem diameter, and specific fresh mass while number of nodes, number of five-leaflet-leaves, plastochron value, and stem length at anthesis decreased. Fresh mass at anthesis was not affected by the treatments. Increasing plant population density from 100 to 178 plants/m2 increased stem length at visible flower bud, and reduced both fresh biomass efficiency and specific fresh mass. These effects are suggested to be related to assimilate supply and translocation, and light perception of the roses. High quantum integral slightly reduced flower diameter but in general, quantum integral or plant density did not affect bloom quality or vase life. Use of preservative floral solution generally improved rose flower diameter and vase life. In Lambada increased light quantum integral prolonged vase life, but use of preservative solution did not. The cultivars Sonia and Red Velvet required 19 to 20 days from cutting/planting until onset of bud growth, 29 to 34 days until visible flower bud, and 39 to 49 days until anthesis. Red Velvet roses were ≈60 cm long at anthesis, and had larger stem diameter and growth rate, accumulated more fresh biomass, were most efficient producing fresh biomass, and had higher specific fresh mass among the cultivars. Light quantum integral is suggested to be used as a means to synchronize single-stemmed rose plant development.

Maximizing Sugarcane Yield by Increasing Plant Population Density, Minimizing NO3‐N Leaching and Improving Soil Organic Matter in Different Crop Rotations

AbstractIn a field experiment conducted during 1992–95 at Lucknow, India, sugarcane was planted in rows 60 and 90 cm apart in three crop rotations (rice‐sugarcane‐ratoon, Sesbania aculeata for green manure‐sugarcane‐ratoon, and cowpea‐sugarcane‐ratoon) with 0, 150 and 300 kg N ha−1 as urea either with or without farmyard manure (FYM) at 10 t ha−1. Sugarcane yields were significantly greater in the Sesbania rotation than in the other because of a larger N uptake. N uptake of the crop was significantly affected by soil organic carbon, and available N and K contents. Ratoon yields, however, were largest in the cowpea sequence followed by the rice rotation, probably due to a prolonged residual effect of cowpea and rice root residues. The residual effect of a Sesbania green manure was negligible as demonstrated by the low NO3‐N content of the soil profile after sugarcane harvest compared to the other two crop sequences. The total cane productivity (main sugarcane plus ratoon) was greater (156 t ha−1) in the cowpea rotation than the Sesbania (152 t ha−1) and rice (140 t ha−1) rotations.