Increasing Sowing Density Research Articles

Effect of Different Crop Densities of Winter Wheat on Recovery of Nitrogen in Crop and Soil within the Growth Period

Previous experiments have shown that, at harvest of winter wheat, recovery of fertilizer N applied in early spring [tillering, Zadok’s growth stage (GS) 25] is lower than that of N applied later in the growth period. This can be explained by losses and immobilization of N, which might be higher between GS 25 and stem elongation (GS 31). It was hypothesized that a higher crop density (i.e. more plants per unit area) results in an increased uptake of fertilizer N applied at GS 25, so that less fertilizer N is subject to losses and immobilization. Different crop densities of winter wheat at GS 25 were established by sowing densities of 100 seeds m–2 (Slow), 375 seeds m–2 (Scfp= common farming practice) and 650 seeds m–2 (Shigh) in autumn. The effect of sowing density on crop N uptake and apparent fertilizer N recovery (aFNrec = N in fertilized treatments − N in unfertilized treatments) in crops and soil mineral N (Nmin), as well as on lost and immobilized N (i.e. non‐recovered N = N rate − aFNrec), was investigated for two periods after N application at GS 25 [i.e. from GS 25 to 15 days later (GS 25 + 15d), and from GS 25 + 15d to GS 31] and in a third period between GS 31 and harvest (i.e. after second and third N applications). Fertilizer N rates varied at GS 25 (0, 43 and 103 kg N ha–1), GS 31 (0 and 30 kg N ha–1) and ear emergence (0, 30 and 60 kg ha–1). At GS 25 + 15d, non‐recovered N was highest (up to 33 kg N ha–1 and up to 74 kg N ha–1 at N rates of 43 and 103 kg N ha–1, respectively) due to low crop N uptake after the first N dressing. Non‐recovered N was not affected by sowing density. Re‐mineralization during later growth stages indicated that non‐recovered N had been immobilized. N uptake rates from the second and third N applications were lowest for Slow, so non‐recovered N at harvest was highest for Slow. Although non‐recovered N was similar for Scfp and Shigh, the highest grain yields were found at Scfp and N dressings of 43 + 30 + 60 kg N ha–1. This combination of sowing density and N rates was the closest to common farming practice. Grain yields were lower for Shigh than for Scfp, presumably due to high competition between plants for nutrients and water. In conclusion, reducing or increasing sowing density compared to Scfp did not reduce immobilization (and losses) of fertilizer N and did not result in increased fertilizer N use efficiency or grain yields.

The Response of Root/Shoot Partitioning and Root Morphology to Light Reduction in Maize Genotypes

In maize (Zea mays L.), increasing sowing density in early maturing genotypes modifies the root/shoot ratio, which generates more frequent lodging. The effects of available light on root emergence and biomass were thus studied to investigate the genetic variation and its interaction with light in traits related to root lodging. Isolated plants of eight hybrids of contrasting maturities and lodging resistance were grown in pots in outdoor conditions, in full sunlight and under neutral shading which captured 61% of the incident light. Plants were harvested at four sampling dates for root and shoot dry matter determination and measurement of some major traits involved in the architecture of the adventitious root system. The partitioning of biomass between root and shoot varied among genotypes. Biomass allocation was significantly affected by light treatment, and the effects varied among genotypes. Some genotypes seemed better able to maintain biomass allocation to the shoot, without major effects on root morphology, than were others. At silking, the final number of emerged roots from the first six internodes was similar across treatments. On the other internodes, the number of roots was lower in the shaded treatment, but the reduction observed (between 8 and 26%) depended on genotype and earliness. In addition, some genotypes with similar root biomass clearly differed in root morphology (58% variation in the average weight of root primary axes under shading), indicating a variation in the structure of the root system. The relationships between root biomass and root number indicated significant interactions between genotypes and shading, with possible consequences on root lodging resistance in the field.

Interactions between root‐infecting fungi and plant density in an annual legume, Kummerowia stipulacea

Summary Population dynamics of an annual legume, Kummerowia stipulacea, were followed in permanent plots in a mown field for 4 years. There was a large reduction in the number of plants within seasons. Laboratory studies showed that biotic factors were responsible for seed death in the soil. Using soil from the field plot location, we isolated fungi pathogenic to K. stipulacea seeds and seedlings. Two pathogenic fungi (Rhizoctonia solani and Pythium irregulare) were used in a glasshouse study of the effects of pathogens and intraspecific competition on plant mortality and growth. Our null hypothesis was that death of individual plants due to disease would be balanced by increased growth of survivors. A factorial experiment was established with five soil treatments (three controls, P. irregulare and R. solani) and seven seed densities. Inoculation by both R. Solani and P. irregulare led to a reduction in final numbers of plants, with the reduction greatest for populations inoculated with R. solani. Average biomass per plant was reduced with increased sowing density in all treatments except the R. solani inoculation, where surviving plants in high‐density pots may be larger because of the reduced density resulting from pathogen‐induced death. No treatment differences in size hierarchies were evident. Surviving plants in both inoculation treatments could not take full advantage of the reduced density, and thus total biomass and number of trifoliate leaves per population (a surrogate for seed production) were reduced compared to controls, leading us to reject the null hypothesis. The reduction was more severe at high sowing densities. These results suggest that disease has the potential to reduce seed production and thus plant population numbers, if populations are seed limited. Shapes of yield‐sowing density relationships were asymptotic for all treatments; thus, disease‐induced ‘overcompensation’ leading to fluctuating population dynamics was not suggested.

The root morphology of some legume spp. in the south-western Cape and the relationship of vesicular-arbuscular mycorrhizas with dry mass and phosphorus content of Acacia saligna seedlings

The root systems of Acacia saligna (Labill.) Wendl. have more extensive laterals than those examples of the Fabaceae indigenous to the Cape. Aspalathus flexuosa Thunb. and Rafnia angulata Thunb. had lateral roots infected with vesicular-arbuscular (VA) mycorrhizal fungi whereas those of Aspalathus albens L. were non-mycorrhizal. VA mycorrhizal fungi were found in the roots of seedlings of A. saligna. Chlamydospores of the genus Glomus and three types of Gigaspora auxiliary cells were identified. VA mycorrhizal colonization was correlated with the dry mass and phosphorus content of A. saligna seedlings. Seedling dry mass, phosphorus content, nodule production and VA mycorrhizal colonization decreased with increased sowing density. The more extensive root system and abundance of root nodules and VA mycorrhizas apparently contribute to the success of A. saligna as an invasive weed of the fynbos vegetation.

GAMETOPHYTE ONTOGENY, SEX EXPRESSION, AND GENETIC LOAD AS MEASURES OF POPULATION DIVERGENCE IN BLECHNUM SPICANT

Early gametophyte ontogeny was quantitatively distinct for Olympic Peninsula, Alaskan, and disjunct Idaho populations of the homosporous fern Blechnum spicant (L.) J. Sm. Although variable, gametophyte sex expression was shown to have a genetic component. Statistically different patterns of sex expression characterize each population. The Olympic Peninsula populations were distinct from each other but consistent in having a predominantly unisexual pattern. The disjunct Idaho population was predominantly bisexual at the time when comparable field collected gametophytes bear sporophytes. Preliminary experiments suggest that an antheridogen operates in this species. Increased sowing density favors maleness, and an extract from soil cultures of gametophytes shifts cultures to an exclusively male pattern after a dramatic suppression of growth. Mating experiments revealed that all populations are interfertile, although fertility was highest when the test Idaho population underwent intergametophytic‐selfing. The Idaho population evidenced a low level of genetic load consistent with predictions based on its sex expression. Although Olympic Peninsula populations evidenced apparent high genetic load in some experiments, failure to produce abundant sporophytes in other experiments suggested that additional cultural factors operated to reduce sporophyte formation. Moderate density mating experiments produced single sporophytes that were comparable to field collections. Isolated gametophytes underwent polyembryony after a time delay and gametophyte proliferation. Cultural conditions which allow sporophyte formation on isolated gametophytes without this delay or proliferation must be sought before further genetic analysis is undertaken.

The effect of sowing density and application of gibberellic acid on male sterility and ear emergence in ethephon‐treated field‐grown wheat

SUMMARYThe induction of male sterility with ethephon and improvement in ear emergence with GA3 (1‐1 kg a.i./ha) applied 3 days later, was investigated in the winter wheat cv. Maris Nimrod sown in the field at72, 138 and 276 spaced plants/m2. Increased sowing density resulted in fewer surviving tillers per plant, a larger number of tillers per unit area and consequently a narrower range of stage of tiller development. This permitted a more accurate assessment of the correct time for ethephon application and resulted in a larger proportion of tillers responding to GA3 by having fully emerged ears. The most effective treatments for the production of emerged male sterile ears were 6‐4 and 12‐8 kg ethephon/ha applied to crops sown at the highest density.

Selection of B chromosomes in Secale cereale and Lolium perenne

Secale cereale A series of three experiments was carried out to see if varying sowing density had any differential effect upon the survival of plants with and without B chromosomes. In all experiments it was shown that as sowing density increased, and therefore the selection pressure increased, so the mean B frequency decreased. This was accounted for by an increasing survival at high densities of plants without B chromosomes. Lolium perenne In contrast to the results in rye, increasing sowing density in Lolium perenne caused an increase in the mean B frequency. This was accounted for by a decrease in the number of plants without B chromosomes and a corresponding increase in the frequency of plants with 2B chromosomes. From the results of these experiments in Secale and Lolium it is concluded that the change in B frequency within and between populations is an adaptive response to lhe different kinds, and degree of selection to which the populations are exposed. The results argue against the view that B chromosomes are “nuclear parasites” with, invariably, detrimental consequences.

Development and structure of foliage in wheat stands of different density

The effect of sowing density (21, 85, 340 and 3400 plants per 1 m on the leaf area index (LAI), vertical arrangement of developing foliage and dry matter production was investigated in stands of wheat (Triticum aestivum L. cv. Niva). 1. With increasing sowing density. a) The LAI increased and the highest LAI = 9.6 was obtained with 3400 plants. m-2. The largest development of foliage took place at ear emergence with 85, 340 and 3400 plants. rn-2, and in the flowering phase with 21 plants. m-2. b) The overal leaf area duration was reduced, with a difference of 11 days between the lowest and the highest density. c) The number of leaves formed per one stem increased from 5 to 8. 2. An increase in theLAI affected the vertical distribution of leaf area and resulted in increased differenoes in relative light intensity between the top and bottom layers of the leaf canopy. Differences inLAI were linked with differences in leaf area duration and in the relative proportions of leaves of different age in the total number of leaves per 12 3. a) Shoot dry matter production increased with increasing density; the maximum of 3811 g. m-2 was obtained with 3400 plants.M-2. b) The dependence of grain yield on sowing density had a parabolic character; the maximum yields of 47001 g. m-2 and 442.8 g m-2 were obtained with 340 and 85 plants. respectively. c) The optimum LAI for the formation of grain yield was 2.3 to 3.2. d) The crop growth rate (C) increased with increasing density; the highest mean recorded C = 594 g. m. week-1 corresponded to LAI = 9.6. 4. Thinning the stands of 3400, 340 and 85 plants. m-2 in the phases of the 3rd leaf (22 days after sowing) and of tillering (30 days after sowing) to the next lower density resulted in a reduced LA leaf and tiller number, crop growth rate, shoot dry matter production and grain yield as compared with both the initial and standard densities. The role of foliage with respect to both maximum dry mater production and maximum grain yield was defined by an optimum range of relations between the rate of foliage development, leaf area, vertical structure and distribution of light intensity within the stand operating as a self-controlling system.