Increased Tiller Number Research Articles

Responses of tiller growth and related genes expression in rice to red and blue radiation

In the present study, we investigated tiller formation in rice cultivated under red radiation (R), red supplemented with 10 % of blue radiation (RB), and white radiation (W). In addition, expression of three genes related to tiller formation, OSH1, MOC1, and OsTB1, was analyzed at both vegetative and reproductive stages. RB promoted the outgrowth of tiller buds and increased tiller numbers significantly. Transcription of MOC1 and OsTB1 in RB was higher than in R, whereas OSH1 expression was independent on radiation quality.

Introduction of genes encoding C4 photosynthesis enzymes into rice plants: physiological consequences.

Transgenic rice plants expressing the maize phosphoeno/pyruvate carboxylase (PEPC) and pyruvate, orthophosphate dikinase (PPDK) exhibit a higher photosynthetic capacity (up to 35%) than untransformed plants. The increased photosynthetic capacity in these plants is mainly associated with an enhanced stomatal conductance and a higher internal CO2 concentration. Plants simultaneously expressing high levels of both enzymes also have a higher photosynthetic capacity. The results suggest that both PEPC and PPDK play a key role in organic acid metabolism in the guard cells to regulate stomatal opening. Under photoinhibitory and photooxidative conditions, PEPC transgenic rice plants are capable of maintaining a higher photosynthetic rate, a higher photosynthetic quantum yield by PSII and a higher capacity to dissipate excess energy photochemically and non-photochemically than untransformed plants. Preliminary data from field trials show that relative to untransformed plants, the grain yield is about 10-20% higher in selected PEPC and 30-35% higher in PPDK transgenic rice plants, due to increased tiller number. Taken together, these results suggest that introduction of C4 photosynthesis enzymes into rice has a good potential to enhance its tolerance to stress, photosynthetic capacity and yield.

Effects of time of first harvest, total amount of nitrogen, and nitrogen application method on total dry matter yield in twice harvesting of rice

We examined the effects of time of first harvest, total amount of nitrogen, and nitrogen application method on the total dry matter yield of a twice-harvested rice ( Oryza sativa L.) cultivar Taporuri grown in southwestern Japan. The highest total dry matter yield was 27.1 t ha −1. We examined the effects of two times of first harvest (booting stage versus full heading stage). Plants harvested at the full heading stage produced 50% higher dry matter yield of first crop than those harvested at the booting stage, because the plants had greater weights per tiller than those harvested at the booting stage. Plants harvested at the booting stage produced 37% higher dry matter yield of second crop than those harvested at the full heading stage, because the plants had greater dry weights per tiller than those harvested at the full heading stage. Overall, plants harvested at the full heading stage produced 8% higher total dry matter yield than those harvested at the booting stage. We also examined the effects of two total amounts of nitrogen (300 kg N ha −1 versus 150 kg N ha −1) and three nitrogen application methods (method 1: 33.3% applied at transplanting, 66.7% at the maximum tiller number stage, 0.0% at 7 days after the first harvest, and 0.0% at 21 days after the first harvest; method 2: 33.3, 33.3, 16.7, and 16.7%, respectively; method 3: 33.3, 0.0, 33.3%, and 33.3%, respectively). For the first crop, plants that received 300 kg N ha −1 produced 10% higher dry matter yield than those that received 150 kg N ha −1 in 2004. Plants that underwent nitrogen application methods 1 and 2 produced 21% and 23% higher dry matter yield of first crop than plants that underwent method 3, respectively. The increase in number of tillers per meter in the 300 kg N ha −1 treatment and in nitrogen application methods 1 and 2 resulted in the higher dry matter yields of first crop in these treatments. For the second crop, plants that received 300 kg N ha −1 produced 10% higher dry matter yield of second crop than those that received 150 kg N ha −1. Plants that underwent nitrogen application methods 2 and 3 produced 57% and 56% higher dry matter yield of second crop than method 1, respectively. The increase in number of tillers per meter in the 300 kg N ha −1 treatment and in nitrogen application methods 2 and 3 resulted in the higher dry matter yields of second crop in these treatments. Overall, plants that received 300 kg N ha −1 produced 7% higher dry matter yield than those that received 150 kg N ha −1 and plants that underwent nitrogen application method 2 produced 21% and 11% higher dry matter yield than those that underwent methods 1 and 3, respectively. Therefore, our findings suggest that to produce the highest total dry matter yield the best time for first harvest is the full heading stage and the best nitrogen management is to provide 300 kg N ha −1 in accordance with nitrogen application method 2.

Simulating the effects of localized red:far‐red ratio on tillering in spring wheat (Triticum aestivum) using a three‐dimensional virtual plant model

The outgrowth of tiller buds in Poaceae is influenced by the ratio of red to far-red light irradiance (R:FR). At each point in the plant canopy, R:FR is affected by light scattered by surrounding plant tissues. This paper presents a three-dimensional virtual plant modelling approach to simulate local effects of R:FR on tillering in spring wheat (Triticum aestivum). R:FR dependence of bud outgrowth was implemented in a wheat model, using three hypothetical responses of bud extension to R:FR (unit step, curvilinear and linear response). Bud break occurred when a threshold bud length was reached. Simulations were performed for three plant population densities. In accordance with experimental observations, fewer tillers per plant were simulated for higher plant population densities. The linear and curvilinear responses caused a delay in the increase in tiller number compared with experimental data. The unit step response approached experimental results best. It is suggested that a model based on relatively simple relations can be used to simulate degree of tillering. This study has shown that the virtual plant approach is a promising tool with which to address crop morphological and ecological research questions where the determining factors act at the level of the individual plant organ.

EFFECT OF PROPAGATION TIME AND METHOD ON SWEET CORN MORPHOLOGY

In our experiments, we tried to find out how the method and time of propagation, out of the technological elements applied by us for enhancing earliness, influenced some of the morphological characteristics of sweet corn. The test variety was Spirit, a normal sweet variety. The following treatments were applied during the experiment: P1 = uncovered direct seeded plants (Apr 6th) P2 = covered direct seeded plants (Apr 6th) P3 = covered transplanted plants (Apr 6th) P4 = uncovered direct seeded plants (Apr 20th) P5 = covered direct seeded plants (Apr 20th) P6 = uncovered transplanted plants (Apr 20th) P7 = covered transplanted plants (Apr 20th) At the first top dressing 20 ears were selected from each treatment in a random fashion and the following measurements were carried out after harvest: plant height (to flag leaf, after harvest), stem diameter (under the ear in ripening phase), number of tillers and ear node height. Direct seeding resulted in a significantly larger stem diameter, in a greater ear node height, in an increased number of tillers and, in combination with early propagation, in a greater plant height. Early propagation resulted in a lower ear node height and, in the case of direct seeding, in a greater stem height, in a lower number of tillers and, in the stand with row cover, in a smaller stem diameter as compared to the common (Apr 20th) propagation date. The transplanted treatments with cover showed practically no evidence of tillering. From the point of the effect of the propagation method we compared the results of the treatments P2-P3 at the earlier date, while those of P5-P7 and P4-P6 at the later date. In the investigation of the effect of propagation date we compared the treatments P1-P4, P2-P5 and P3-P7.

Trinexapac-ethyl Affects Kentucky Bluegrass Root Architecture

Trinexapac-ethyl (TE) [4-(cyclopropyl-a-hydroxy-methylene)-3,5-dioxocyclohexanecarboxylic acid ethyl ester] effects on turfgrass root architecture are not known. It has been postulated that PGR application could cause photoassimilate that is normally used for shoot growth to be funneled to root growth. This study evaluated the effects of a single TE application on kentucky bluegrass (KBG) root and shoot growth for seven weeks. Individual KBG plants were grown in a hydroponic system and harvested weekly. At each harvest, tiller height, tiller number, and color ratings were recorded. Estimates of total root length (TRL), root surface area (SA), and average root diameter were measured using the WinRhizo system. Trinexapac-ethyl reduced plant height for 4 weeks followed by a period of postinhibition growth enhancement. Trinexapac-ethyl increased tiller number over the course of the study and slightly enhanced plant color. Trinexapac-ethyl reduced TRL and SA 48% and 46% at 1 week after treatment (WAT) followed by an accelerated growth rate 1 to 4 WAT. Trinexapac-ethyl had no effect on root diameter. On a tiller basis, TE initially reduced TRL and SA 30% and 31%, respectively. Total root length per tiller and root surface area per tiller were reduced by TE treatment, but by 7 WAT, those differences were no longer significant. Initial reductions in TRL and SA per tiller may reduce tiller competitiveness for water and nutrients. Based on data for TRL and SA per tiller, shoot and root growth must be considered in total to fully understand TE effects on plant growth. Field research is needed to corroborate results from hydroponic-studies and examine the effect of various TE rates and multiple applications on turfgrass root and shoot growth.

OsPTF1, a Novel Transcription Factor Involved in Tolerance to Phosphate Starvation in Rice

We report here on a novel transcription factor with a basic helix-loop-helix domain for tolerance to inorganic phosphate (Pi) starvation in rice (Oryza sativa). The gene is designated OsPTF1. The expression of OsPTF1 is Pi starvation induced in roots while constitutively expressed in shoots, as shown by northern-blot analysis. Overexpression of OsPTF1 enhanced tolerance to Pi starvation in transgenic rice. Tillering ability, root and shoot biomass, and phosphorus content of transgenic rice plants were about 30% higher than those of the wild-type plants in Pi-deficient conditions in hydroponic experiments. In soil pot and field experiments, more than 20% increase in tiller number, panicle weight, and phosphorus content was observed in transgenic plants compared to wild-type plants at low-Pi levels. In Pi-deficient conditions, transgenic rice plants showed significantly higher total root length and root surface area, which results in a higher instantaneous Pi uptake rate over their wild-type counterparts. Microarray analysis for transgenic plants overexpressing OsPTF1 has been performed to investigate the downstream regulation of OsPTF1.

Suppression of Tiller Bud Activity in Tillering Dwarf Mutants of Rice

In this study, we analyzed five tillering dwarf mutants that exhibit reduction of plant stature and an increase in tiller numbers. We show that, in the mutants, axillary meristems are normally established but the suppression of tiller bud activity is weakened. The phenotypes of tillering dwarf mutants suggest that they play roles in the control of tiller bud dormancy to suppress bud activity. However, tillering dwarf mutants show the dependence of both node position and planting density on their growth, which implies that the functions of tillering dwarf genes are independent of the developmental and environmental control of bud activity. Map-based cloning of the D3 gene revealed that it encodes an F-box leucine-trich repeat (LRR) protein orthologous to Arabidopsis MAX2/ORE9. This indicates the conservation of mechanisms controlling axillary bud activity between monocots and eudicots. We suggest that tillering dwarf mutants are suitable for the study of bud activity control in rice and believe that future molecular and genetic studies using them may enable significant progress in understanding the control of tillering and shoot branching.

Evaluating salt tolerance of wheat genotypes using multiple parameters

Salt tolerance of wheat is known to change with growth stage. Identifying the multiple parameters associated with salt tolerance during different growth stages is important for evaluating wheat genotypes and improving their salt tolerance. Thirteen wheat genotypes from Egypt, Germany, Australia and India were grown in soil and exposed to four salinity levels (control, 50, 100 and 150 mM NaCl). Tiller number, leaf number and leaf area per plant at vegetative stage; dry weight per plant at vegetative, reproductive and maturity stages; and yield components of main spike and total grain yield at maturity were determined. The results showed that tiller number was affected more by salinity than leaf number and leaf area at the vegetative stage. Salinity decreased dry weight per plant significantly at all growth stages. Spikelet number on the main stem decreased much more with salinity than spike length, grain number and 1000-grain weight at maturity. According to cluster analysis with multiple agronomic parameters at all growth stages, the Egyptian genotypes Sakha 8 and Sakha 93 and the Indian genotype Kharchia were ranked as the most tolerant to salinity. A change in salt tolerance with growth stages was observed for Sids 1, Gemmeza 7 and Westonia. Drysdale and Sakha 69 were ranked as moderate tolerant. The remaining genotypes showed the lowest tolerance to salinity at all growth stages. We conclude that an increase in tiller number per plant and spikelet number per spike will improve the salt tolerance of wheat genotypes in breeding programs. Cluster analysis with multiple agronomic parameters simultaneously to evaluate the salt tolerance facilitates the rankings of salt tolerance of wheat genotypes.

Identification of quantitative trait loci for plant growth of rice in paddy field flooded with salt water

To gather more information regarding salt-tolerance in rice ( Oryza sativa L.), we identified gene loci controlling growth in paddy fields flooded with salt water. We analyzed the quantitative trait loci (QTL) using 98 backcross-inbred lines (BIL) derived from the backcross between Nipponbare (moderately salt-tolerant rice, as recurrent parent) and Kasalath (salt susceptible rice) in the four cropping seasons from 1999 to 2002. Shoot length, tiller number and shoot fresh weight were determined in the plants grown in a paddy field flooded with fresh water (NaCl; less than 10 mM, control group) and in the plants grown in a paddy field flooded with salt water (NaCl; 50–120 mM, saline group). A total of 27 QTLs for these traits were detected in the saline and control groups. Among them in the saline group, QTLs detected all four seasons were located on four regions on chromosomes 1, 2, 3 and 7. QTLs located on the long-arm of chromosome 2 regulated tiller number, and QTLs located on the long-arms of chromosomes 1 and 3, and the short-arms of chromosome 7 regulated shoot length. QTLs for shoot length located on the long-arms of chromosome 1 were detected in the control and saline paddy fields. QTLs in the other three regions were detected only in the saline group. The Nipponbare alleles of the two QTLs on chromosomes 3 and 7 contributed to the increase in shoot length, while the Kasalath allele of QTL on chromosome 2 contributed to the increase in tiller number. Therefore, these results suggest that three QTLs on chromosomes 2, 3 and 7 play essential roles in the growth of rice in the paddy field flooded with salt water.

Expression of recombinant protoporphyrinogen oxidase influences growth and morphological characteristics in transgenic rice

Transgenic rice plants expressing a Bacillus subtilis protoporphyrinogen oxidase (Protox), the last shared enzyme of the porphyrin pathway, in the cytoplasm (C89) or the plastids (P72) were compared with wild-type rice plants in their growth characteristics. Production of tiller buds 18 d after seeding was more profuse in transgenic plants than in wild-type plants, especially in plastid-targeted plants. Transgenic plants had 12-27% increase in tiller number and 17-33% increase in above-ground biomass compared with wild-type plants 4 and 8 weeks after transplanting of 2-week-old rice seedlings, demonstrating that tiller production and above-ground biomass correlate with each other. Cytoplasm-expressed and plastid-targeted transgenic plants also had a distinct phenotypic characteristic of narrower and more horizontal leaves than wild-type plants. Phenotypic and anatomical characteristics of the transgenic plants were clearly different from wild-type plants, indicating that regulation of porphyrin biosynthesis by expression of B. subtilis Protox in rice influences morphological characteristics of plant growth as well as biomass.

Reversal of dormancy in switchgrass with low-light photoperiod extension

Some switchgrass ( Panicum virgatum L.) cultivars originating in the northern USA show limited late-summer and fall growth when grown in more southerly locations despite adequate temperature and moisture. Our objective was to determine the effects of low-light photoperiod extension on the dry matter yield of switchgrass cultivars originating from contrasting latitudes. Seedlings of the four cultivars (Cave-in-Rock, ‘Caddo’, ‘Kanlow’, and ‘Alamo’) were grown for 100 d in a greenhouse in winter under ambient (11.2–12.2 h) and extended (18 h) photoperiods. Photoperiod extension was with 7 μmol m −2 s −1 photosynthetic photon flux density. Cultivars responded differently to photoperiod extension ( P<0.05). Large increases in dry matter yield at extended photoperiods were observed in Cave-in-Rock (+98%) and Caddo (+129%). The dry matter yield of Kanlow increased by 31%, whereas dry matter yield of the southernmost cultivar Alamo was not affected by photoperiod extension. Yield increases for Cave-in-Rock and Caddo were associated with increased tiller number and weight. A reversal of dormancy for some cultivars with a photoperiodic signal verified that dormancy was not simply a result of a low radiant input.

Defoliation Impacts on Festuca campestris (Rydb.) Plants Exposed to Wildfire

Wildfires commonly occur in the Fescue Prairie of Alberta, but little information exists to provide a basis for making grazing recommendations after burning. A wildfire in April 1999 provided an opportunity to study the effect of season and intensity of post-burn defoliation on foothills rough fescue (F. campestris Rydb.) in southwestern Alberta. A 3 (date of defoliation) x 2 (defoliation intensity) factorial experiment with 10 replicates (plants) was established in both a burned and a non-burned grassland and analyzed as a nested design. Plants were defoliated once during active vegetative growth (17 May), inflorescence development (2 July), or dormancy (30 September), at either 5 or 15-cm clipped stubble heights in the first growing season after fire. Burning increased tiller numbers by 54% compared to non-burned plants but reduced plant ANPP by 51% in the second growing season. While a single defoliation of burned plants, particularly early in the year, had little effect on growth, delaying defoliation into July decreased tillers 1 year later. Increasing defoliation intensity had the greatest impact on non-burned plants, reducing plant height (15%) as well as tiller (21%) and plant (32%) ANPP in the second year. May defoliation reduced etiolated growth 1 year later regardless of burn treatment. A single grazing event after wildfire does not necessarily appear to detrimentally affect rough fescue; however, the low herbage available immediately after fire may not justify the increased risk to the plant with subsequent grazing. DOI:10.2458/azu_jrm_v56i4_bogen

Seasonal changes in the effects of elevated CO2 on rice at three levels of nitrogen supply: a free air CO2 enrichment (FACE) experiment

AbstractOver time, the stimulative effect of elevated CO2 on the photosynthesis of rice crops is likely to be reduced with increasing duration of CO2 exposure, but the resultant effects on crop productivity remain unclear. To investigate seasonal changes in the effect of elevated CO2 on the growth of rice (Oryza sativa L.) crops, a free air CO2 enrichment (FACE) experiment was conducted at Shizukuishi, Iwate, Japan in 1998–2000. The target CO2 concentration of the FACE plots was 200 µmol mol−1 above that of ambient. Three levels of nitrogen (N) were supplied: low (LN, 4 g N m−2), medium [MN, 8 (1998) and 9 (1999, 2000) g N m−2] and high N (HN, 12 and 15 g N m−2). For MN and HN but not for LN, elevated CO2 increased tiller number at panicle initiation (PI) but this positive response decreased with crop development. As a result, the response of green leaf area index (GLAI) to elevated CO2 greatly varied with development, showing positive responses during vegetative stages and negative responses after PI. Elevated CO2 decreased leaf N concentration over the season, except during early stage of development. For MN crops, total biomass increased with elevated CO2, but the response declined linearly with development, with average increases of 32, 28, 21, 15 and 12% at tillering, PI, anthesis, mid‐ripening and grain maturity, respectively. This decline is likely to be due to decreases in the positive effects of elevated CO2 on canopy photosynthesis because of reductions in both GLAI and leaf N. Up to PI, LN‐crops tended to have a lower response to elevated CO2 than MN‐ and HN‐crops, though by final harvest the total biomass response was similar for all N levels. For MN‐ and HN‐crops, the positive response of grain yield (ca. 15%) to elevated CO2 was slightly greater than the response of final total biomass while for LN‐crops it was less. We conclude that most of the seasonal changes in crop response to elevated CO2 are directly or indirectly associated with N uptake.

Defoliation impacts on Festuca campestris (Rydb.) plants exposed to wildfire

Wildfires commonly occur in the Fescue Prairie of Alberta, but little information exists to provide a basis for making grazing recommendations after burning. A wildfire in April 1999 provided an opportunity to study the effect of season and intensity of post-burn defoliation on foothills rough fescue (F. campestris Rydb.) in southwestern Alberta. A 3 (date of defoliation) x 2 (defoliation intensity) factorial experiment with 10 replicates (plants) was established in both a burned and a non-burned grassland and analyzed as a nested design. Plants were defoliated once during active vegetative growth (17 May), inflorescence development (2 July), or dormancy (30 September), at either 5 or 15-cm clipped stubble heights in the first growing season after fire. Burning increased tiller numbers by 54% compared to non-burned plants but reduced plant ANPP by 51% in the second growing season. While a single defoliation of burned plants, particularly early in the year, had little effect on growth, delaying defoliation into July decreased tillers 1 year later. Increasing defoliation intensity had the greatest impact on non-burned plants, reducing plant height (15%) as well as tiller (21%) and plant (32%) ANPP in the second year. May defoliation reduced etiolated growth 1 year later regardless of burn treatment. A single grazing event after wildfire does not necessarily appear to detrimentally affect rough fescue; however, the low herbage available immediately after fire may not justify the increased risk to the plant with subsequent grazing.

Involvement of ethylene in the morphological and developmental response of rice to elevated atmospheric CO2 concentrations

We tested the hypothesis that increased carbohydrate flux under elevatedCO2 regulates accelerated development using rice (Oryzasativa L. cv. Jarrah). Plants were grown either in flooded soil orsolution culture at either 360 or 700 μL CO2L−1. Total dry mass, shoot elongation rates (SER),tiller appearance rates (TAR) and ethylene release from intact rice seedlingswere measured from 5 to 42 days after planting (DAP). At maturity, shoot andsheath length, tiller number and grain mass were also measured. ElevatedCO2 had a profound effect on growth, morphology and development andthe effects were more pronounced during the early growth phase. Total aboveground biomass increased at elevated CO2 and this was accounted for by enhanced tiller number. Grain yield was increased by 56% under elevated CO2mainly due to increased tiller number and hence panicle number. TAR and SERwereenhanced at elevated CO2 but SER increased only untill 25 DAP.Elevated CO2 stimulated a 2-3-fold increase in endogenous andACC-mediated ethylene release but the ACC concentration in the leaves waslittleaffected showing that rates of ACC synthesis matched its oxidation. Inhibitionof ethylene action by 1-aminocyclopropane (1-MCP) had a more pronouncedinhibitory effect on ethylene release in plants that were grown at 700 ascompared to 360 μL CO2 L−1. Feedingsucrose to intact plants enhanced ethylene synthesis and these results areconsistent with the hypothesis that increased accumulation of sucrose atelevated CO2 may enhance expression of genes in the ethylenebiosynthetic pathway. We conclude that increase in ethylene release may becentral in promoting accelerated development under elevated CO2 andthis coincides with the release of auxiliary buds and accelerated rates oftiller appearance hence increased grain yield at elevated CO2.

Intermediate Wheatgrass and Russian Wildrye Responses to Defoliation and Moisture

Perennial forage grasses in the Northern Great Plains are often grazed under water-limiting conditions. The effects of defoliation and soil moisture dynamics on herbage yield, tiller recruitment and number of crown positions for 2 perennial forage grasses were evaluated in a greenhouse experiment at Mandan, N.D. Intermediate wheatgrass (Thinopyrum intermedium (Host) Barkw. &D.R. Dewey] and Russian wildrye (Psathyrostachys juncea (Fisch.) Nevski] were grown at 75, 50, or 40% of field capacity and left as undefoliated controls or defoliated at 10-day intervals to an 8- or 4-cm stubble height. Cumulative herbage yield exhibited defoliation level by soil moisture (P = 0.0001) and species by defoliation (P = 0.007) interactions. Yield decreased with increasing defoliation intensity at 75 and 50% of field capacity, but at 40% of field capacity only the most intense defoliation level was significantly affected. Russian wildrye produced more herbage (1.97 g plant(-1)) than intermediate wheatgrass (1.36 g plant(-1)) under severe defoliation level. Increased defoliation intensity (P = 0.0001) but not water availability (P > 0.05) decreased tiller numbers. Pooled across all treatments, Russian wildrye produced 10 tillers per plant and intermediate wheatgrass produced 7 (P = 0.0001). This may partially explain Russian wildrye's greater grazing tolerance. The number of crown positions (potential axillary bud sites in the bottom 20mm of the plant) was similar between species suggesting that increased tiller numbers in Russian wildrye occurred because its axillary buds were more readily activated than intermediate wheatgrass. Moderate and severe defoliation reduced the number of crown positions on parent seedlings to only 62 and 50% of the number of crown positions of control seedlings, respectively. Water stress decreased (P = 0.004) number of crown positions at 40% of field capacity but only when crown positions of both parent and daughter tillers were pooled. In this experiment, defoliation had a greater effect than water stress. Moisture level mainly affected tiller size not numbers. The interaction between defoliation and water stress should be examined in field studies to foster improved management of these 2 grasses. DOI:10.2458/azu_jrm_v55i1_hendrickson

Intermediate wheatgrass and Russian wildrye responses to defoliation and moisture

Perennial forage grasses in the Northern Great Plains are often grazed under water-limiting conditions. The effects of defoliation and soil moisture dynamics on herbage yield, tiller recruitment and number of crown positions for 2 perennial forage grasses were evaluated in a greenhouse experiment at Mandan, N.D. Intermediate wheatgrass (Thinopyrum intermedium (Host) Barkw. D.R. Dewey] and Russian wildrye (Psathyrostachys juncea (Fisch.) Nevski] were grown at 75, 50, or 40% of field capacity and left as undefoliated controls or defoliated at 10-day intervals to an 8- or 4-cm stubble height. Cumulative herbage yield exhibited defoliation level by soil moisture (P = 0.0001) and species by defoliation (P = 0.007) interactions. Yield decreased with increasing defoliation intensity at 75 and 50% of field capacity, but at 40% of field capacity only the most intense defoliation level was significantly affected. Russian wildrye produced more herbage (1.97 g plant(-1)) than intermediate wheatgrass (1.36 g plant(-1)) under severe defoliation level. Increased defoliation intensity (P = 0.0001) but not water availability (P &gt; 0.05) decreased tiller numbers. Pooled across all treatments, Russian wildrye produced 10 tillers per plant and intermediate wheatgrass produced 7 (P = 0.0001). This may partially explain Russian wildrye's greater grazing tolerance. The number of crown positions (potential axillary bud sites in the bottom 20mm of the plant) was similar between species suggesting that increased tiller numbers in Russian wildrye occurred because its axillary buds were more readily activated than intermediate wheatgrass. Moderate and severe defoliation reduced the number of crown positions on parent seedlings to only 62 and 50% of the number of crown positions of control seedlings, respectively. Water stress decreased (P = 0.004) number of crown positions at 40% of field capacity but only when crown positions of both parent and daughter tillers were pooled. In this experiment, defoliation had a greater effect than water stress. Moisture level mainly affected tiller size not numbers. The interaction between defoliation and water stress should be examined in field studies to foster improved management of these 2 grasses.

Survival, Gap Formation, and Recovery Dynamics in Grassland Ecosystems Exposed to Heat Extremes: The Role of Species Richness

A field experiment was performed in which the richness of perennial grasses (S) was varied in model ecosystems exposed to a simulated heat wave (free air temperature increase and drought). The proportion of individuals that survived the heat wave decreased with S, which could be ascribed to higher water consumption in the species-rich systems. Higher transpiration at high diversity was also observed in other studies using functional groups and could have originated from increased leaf area, less intense stomatal closure, or a combination of both. The increased tiller number per plant that we observed, while leaf area per tiller remained constant, suggests that an enhanced leaf area index was most likely responsible. However, competitive interactions also seemed to play a role in the influence of S on survival. Regrowth of the surviving individuals, expressed as leaf area per living plant after a recovery period following the heat wave, increased with S, most likely due to the dominance of productive species, which was facilitated by the additional space yielded by more intense gap formation at higher S (due to higher plant mortality). Species richness affected both the size and density of the gaps. Mean size increased exponentially with S, while density increased at low S but decreased at higher S when connectance of the gaps occurred. Size distribution of the gaps was not affected.

Eficiência da aplicação de nitrogênio no perfilhamento do arroz em três manejos de irrigação

Com o objetivo de avaliar a eficiência de três modalidades de aplicação de nitrogênio (N) na cultura de arroz foi conduzido um experimento, no ano agrícola 1998/99, na Universidade Federal de Santa Maria (RS). O delineamento experimental foi bifatorial 3 x 5 em blocos ao acaso com parcelas subdivididas e quatro repetições. Nas parcelas principais, foram estabelecidas, no início do perfilhamento do arroz, três modalidades de aplicação da uréia: 1) Uréia + banho, na qual a uréia foi aplicada superficialmente e, no dia seguinte, deu-se um banho, inundando-se a área 14 dias depois; 2) Uréia + inundação, na qual a uréia foi aplicada um dia antes da inundação definitiva; e 3) Inundação + uréia, com a aplicação da uréia um dia após a inundação. Nas subparcelas, foram aplicadas quatro doses de N (0, 20, 40 e 60kg ha-1 de N) e uma testemunha absoluta, sem aplicação de N durante todo o ciclo da cultura. Em todos os tratamentos que receberam N, foram aplicados 10kg ha-1 de N na semeadura e 23kg ha-1 de N no início do alongamento de entrenós. Concluiu-se que o N aplicado na etapa vegetativa até o início de alongamento de entrenós incrementa a quantidade de perfilhos e o N acumulado total. Este incremento depende das modalidades de aplicação da uréia, sendo maior nos tratamentos "uréia + banho" e "uréia + inundação", em relação à aplicação da uréia na água. O incremento das doses de nitrogênio proporciona elevação na produção de grãos enquanto as modalidades de aplicação de uréia no perfilhamento não afetam a produção de grãos, utilizando-se nitrogênio na iniciação do primórdio floral.