Roxana Savin

Source—sink relationships and grain mass at different positions within the spike in wheat

Abstract Studies manipulating the source-sink relationships during postanthesis in wheat have shown inconsistent results, concluding that wheat yield could be either sink- or source-limited. Most of the studies reported data of grain mass as an average of all grains preventing the recognition of different sensitivities to the availability of assimilates of grains placed at different positions within the spike. The response in growth and final mass of grains from the base, middle and apical spikelets to increases in assimilate availability was studied by modifing source-sink relationships during the grain growth period of wheat. In addition, a quantitative analysis of the responses in grain mass to manipulations in the source—sink relationships was attempted. Two wheat cultivars, ‘Condor’ (semi-dwarf) and ‘Thatcher’ (standard-height) were grown in 1992 at the experimental station of The University of Melbourne under two different photoperiods between emergence and heading at field conditions to have two different environments during grain-filling period (early and late). Twenty main shoots in each plot were chosen and detillered at anthesis. Ten days after anthesis, the source—sink relationship was altered in half of them by removing all the spikelets along one side of the spike. Grain growth (averaged over all grains per spike) in both cultivars and under the two grain-filling periods appeared to be largely insensitive to the increase in the availability of assimilates. Neither grain growth rate nor the affective period of grain-filling was significantly affected by potentially doubling the source capacity per grain by trimming. Similarly, there was no effect of trimming on grain mass and its components when grains from apical, central or basal spikelets were analysed separately for the two grain-filling periods. This lack of effect of changes in source—sink relationships on grain growth would imply that the assimilate availability in control plants in this study was sufficient to fully satisfy grain growth requirements. These results were similar when grains of different size (those at different position within the spike) were individually considered, suggesting that there is no association between the size of the grains in the control treatment and their response to changes in the assimilate availability. Comparing these results and many others from the literature, in which there was not agreement about the fact that grain yield was source- or sink-limited, and using a quantitative approach, we suggest that during post-anthesis period, grain yield of wheat is either sink-limited or co-limited by both source and sink but never source-limited.

Shading effects on the yield of an Argentinian wheat cultivar

Shading treatments of 50% of the incident radiation were applied to the semidwarf wheat cultivar Leones INTA before and after anthesis in two field experiments in Argentina in 1987 and 1988. The treatments reduced biological (above-ground dry matter) yield, grain yield and number of grains/m2. Number of grains/m2 was closely and linearly correlated with ear dry weight at anthesis and with the photothermal quotient, calculated from 20 days before to 10 days after anthesis. Grain yield was sink limited, and the shading treatments reduced sink strength. The contribution of preanthesis assimilates to grain yield was smaller in the shaded crops than in the unshaded controls; in unshaded crops, almost 40% of grain yield was contributed by preanthesis assimilates whilst in preanthesis shaded crops this contribution was negligible. The proportion of preanthesis assimilates contributed to the grain was closely related to the decrease in stem dry weight during grain filling. The effects of shading on main stems and tillers were the same.

Effect of temperature and carpel size during pre-anthesis on potential grain weight in wheat

The effect of environmental conditions immediately before anthesis on potential grain weight was investigated in wheat at the experimental field of the Faculty of Agronomy (University of Buenos Aires, Argentina) during 1995 and 1996. Plants of two cultivars of wheat were grown in two environments (two contrasting sowing dates) to provide different background temperature conditions. In these environments, transparent boxes were installed covering the spikes in order to increase spike temperature for a short period (c. 6 days) immediately before anthesis, i.e. between ear emergence and anthesis. In both environments, transparent boxes increased mean temperatures by at least 3n8 mC. These increases were almost entirely due to the changes in maximum temperatures because minimum temperatures were little affected. Final grain weight was significantly reduced by higher temperature during the ear emergence–anthesis period. It is possible that this reduction could be mediated by the effect of the heat treatment on carpel weight at anthesis because a curvilinear association between final grain weight and carpel weight at anthesis was found. This curvilinear association may also indicate a threshold carpel weight for maximizing grain weight.

Floret development of durum wheat in response to nitrogen availability

In Mediterranean durum wheat production, nitrogen (N) fertilization may be important to stabilize and increase yields. Wheat yield responses to N fertilization are usually related to grains per m2, which in turn is the consequence of processes related to floret development (floret initiation followed by floret death/survival) during stem elongation. The literature is rather scarce in terms of the relevance of floret developmental dynamics, determining the final number of grains in general terms and in particular regarding responsiveness to N. The aim of this study was to determine whether durum wheat responses to N under different water regimes are related to the dynamics of development of floret primordia to produce fertile florets. During the 2006–2007 and 2007–2008 growing seasons, experiments with a factorial combination of two N levels (0 and 100 or 250 kg N ha−1) and two levels of water availability (rainfed and irrigated) were carried out (although the water regime was only effective in the second season). The response of yield was largely a consequence of that in grain number per spike. Floret initiation was similar for both N levels in each experiment and water regime, for which the survival of a higher proportion of initiated florets was critical in the response of the crop. The diminished rate of floret abortion during the late part of stem elongation in response to N was associated with a slightly accelerated rate of floret development which allowed a higher proportion of the primordia initiated to reach the stage of fertile floret by flowering.

Crop Development: Genetic Control, Environmental Modulation and Relevance for Genetic Improvement of Crop Yield

Publisher Summary This chapter discusses the particularities of development of wheat and soybean to highlight the importance of identifying the genetic and environmental controls of the phenological pattern. This knowledge is a prerequisite to understand, predict, and manipulate the association between crop cycles, the resources, and the environmental constraints to favor the coincidence of the critical period with the most favorable conditions. Although the cycle to match crops and environmental factors has been determined in most production systems, further improvement is feasible by manipulation of critical periods. The critical period may occur before (e.g., in wheat) or after (e.g., in soybean) flowering, but it is clear that in both species, in spite of their large morphological and physiological differences, the growth during this period defines crop yield in most environments. Improving the knowledge of genetic and environmental drivers of the expression of the genes that control flowering time should improve the precision in positioning the critical period, when the highest level of resources is expected and stresses are less likely.

Assessing strategies for wheat cropping in the monsoonal climate of the Pampas using the CERES-Wheat simulation model

Abstract We evaluated a series of management strategies for rainfed wheat cropping in a monsoonal climate in the Pampas using the CERES-Wheat model with 24 years of daily weather records. These strategies included the combination of two cultivars of different maturity (intermediate and short) and two sowing dates (early and late), in two different locations (Pergamino and Marcos Juarez). In addition, the impact of not controlling weeds during the fallow preceding each crop was assessed. Simulation experiments showed that without limitations of nitrogen, maximum yields could be achieved by early sowing of the intermediate-cycle cultivar. The highest yield achieved by this strategy was partially associated with greater water consumption during the crop cycle. However, in the lowest-yield years of both sites, but particularly in Marcos Juarez, the intermediate cultivar sown early did not appear to be the best option. An analysis of stability of grain yield indicated that late sowing of the short-cycle cultivar was the most stable system. For weedy fallows, the yield of the short-cycle cultivar was more affected than the intermediate one, particularly in years of low yield.

Co-limitation of nitrogen and water, and yield and resource-use efficiencies of wheat and barley

In semiarid Mediterranean environments, low nitrogen (N) and water availabilities are key constraints to cereal productivity. Theoretically, for a given level of N or water stress, crops perform better when co-limitation occurs. Empirical evidence of this theoretical concept with field crops is rather scarce. Using data from field experiments we evaluated whether N-use efficiency (NUE) and water-use efficiency (WUE) in small grain cereals increases with the degree of co-limitation. Four field experiments were carried out during three growing seasons including factorial combinations of bread wheat, durum wheat and barley, grown under different N fertiliser rates and water regimes. Yield gap was calculated as the difference between maximum attainable yield and actual yield while stress indices for N (NSI) or water (WSI) were calculated as the ratios between actual N uptake or water use and those required to achieve maximum yields, respectively. Water and N co-limitation was calculated as C WN = 1 – |NSI–WSI|. The relationships of yield gap, NUE and WUE with the different co-limitation indices were evaluated. Yield gap (range from –3.8 to –8.1 Mg ha–1) enlarged (was more negative) with the highest levels of stress and, as expected from theory, it was reduced with the degree of co-limitation. WUE ranged from 6.3 to 21.8 kg ha–1 mm–1 with the maximum values observed under conditions in which co-limitation increased. Reduction in yield gap with increased degree of co-limitation was mainly due to a positive effect of this variable on WUE.

Leaf appearance, tillering and their coordination in response to NxP fertilization in barley

The aim of this work was to study the effects of nitrogen (N) and phosphorus (P) deficiencies and their interaction on the appearance of leaves and tillers of barley (Hordeum vulgare spp. distichum L.) by analyzing the rate and duration of the appearance period. Three microcrops experiments were carried out in 200 L containers using malting barley cv. Quilmes Palomar. Treatments were a factorial combination of two levels on N and two or three levels of P fertilization. Both N and P deficiencies delayed leaf and tiller appearance rates but leaf appearance was less susceptible to nutrient deficiencies than tiller appearance. P deficiency effects on leaf emergence differed from N effects. While N deficiency diminished the rate of leaf appearance (RLA), it has no significant effect on the duration of leaf appearance or on the duration of the period from emergence to flowering. On the other hand, P deficiency diminished RLA, and increased both the duration of leaf appearance and the duration of the period from emergence to flowering.

Grain weight and malting quality in barley as affected by brief periods of increased spike temperature under field conditions

High temperature is usually one of the most important stresses during grain filling affecting both yield and quality in barley crops. In the present study, an attempt was made to assess in the field the effects of short periods of high temperature, using transparent boxes covering only the spikes, with thermostatically controlled electric resistance for increasing the temperature. Treatments consisted of 2 malting cultivars and 5 heat treatments of high temperatures (8°C above the environmental temperature for 6 h/day for 5 consecutive days) over different periods during grain filling. Final grain weight was reduced by 2–14%, depending on the timing of heat stress and the genotype. There was a significant increase in grain nitrogen percentage in both cultivars, and grain β-glucans decreased with high temperatures in Logan and were unchanged in Beka. The resulting malt extract was reduced with exposure to high temperatures, depending on the cultivar, implying that even mild heat stress may change malting performance.

Testing the root growth subroutine of the CERES-wheat model for two cultivars of different cycle length

Abstract The objective of this study was to test the root growth subroutine from the CERES-Wheat model during a growing season. The study involved observation and analysis of root growth and distribution throughout the growing season, and thier simulation, using two wheat cultivars of different cycle length and two water regimes. The model accurately predicted crop development and yield, but overpredicted root depth by 90 cm at terminal spikelet and by 50 cm booting, anthesis and mid grain-filling period. In addition, the model underpredicted root mass, presumably because of the way it handles root—shoot partitioning. There was good agreement between observed and simulated values of root-length density and soil water content ( R 2 = 0.66 and 0.86, respectively). The value of the specific root-length parameter in CERES-Wheat lies closer to data reported from a winter wheat crop experiment than to data from spring wheat reported in this and other studies. The models ability to predict these attributes is discussed.