A.J. Hall

The effects of water stress and genotype on the dynamics of pollen-shedding and silking in maize

Abstract The effects of exposure to water stress at three developments stages (start of tasseling, start of pollen-shedding, and silking) on the viability of pollen and the dynamics of flowering in maize (Zea mays L.) were examined using a single genotype. Viability was not affected by stress at any stage, but alterations in the dynamics of flowering reduced the number of pollen grains available for pollination of the last ears to silk within the stressed populations. In a second experiment the effects of stress at the start of pollen-shedding on the availability of pollen during the presentation of silks was studied in a set of six cultivars. There was considerable variability between cultivars in the number of pollen grains produced per tassel, and water stress decreased the quantity of pollen grains harvested. There was a strong association between pollen availability and kernel number in the apical ear (as a proportion of control values). Two effects of stress on pollen availability were distinguished: a reduction in the duration of exposure of silks of the apical ear to pollen, and a fall in the rate of pollen production during the period in which receptive silks were present. The relative importance of these effects in determining pollen availability varied with cultivar.

Ontogenetic changes in radiation use efficiency of sunflower (Helianthus annuus L.) crops

Abstract Radiation use efficiency (ϵ) values for sunflower were derived using data from periodic determinations of crop aerial or total (aerial + root) biomass and radiation interception in two experiments conducted at Junin and Buenos Aires, Argentina. Analysis showed that three distinct phases, each characterized by a different value of ϵ, could be distinquished. The phases were establishment (0–47 days after emergence ( dae )), rapid growth (47 dae -anthesis) and postanthesis (anthesis-physiologcial maturity). The crops had reached the bud-visible stage and intercepted at least 85% of incident radiation by 47 dae . Efficiency values based on aerial biomass were highest during the rapid growth (ϵrg = 2.4 ± 0.22 g/MJ PAR) phase and lower during the establishment (ϵe = 1.01 ± 0.25 g/MJ PAR) and postanthesis (ϵpa = 1.3 ± 0.15 g/MJ PAR) phases. The differences between ϵpa and ϵrg are attributed to the increased respiration load, high synthesis costs of the oil-rich grains and leaf senescence during grain filling. The value of ϵe was less than of ϵrg even when efficiencies were calculated on the basis of total crop biomass (i.e. including root biomass). Hence, ontogenetic changes in partitioning of biomass between the root and aerial organs do not seem to be the cause of the changes in the value of ϵ. No variations in photosynthetic capacity with leaf position could be found, and it is concluded that the most likely reason for the lower values of ϵe lies in saturation of canopy photosynthesis at less than full sunlight while crop cover is incomplete.

Land allocation conditioned on El Nino-Southern Oscillation phases in the Pampas of Argentina

Abstract The El Nino-Southern Oscillation (ENSO) contributes to the vulnerability of crop production to climate variability in the Pampas region of Argentina. Predictability of regional climate anomalies associated with ENSO may provide opportunities to tailor decisions to expected climate, either to mitigate expected adverse conditions or to take advantage of favorable conditions. Model analysis was used to explore the potential for tailoring land allocation among crops to ENSO phases at the farm scale in two sub-regions of the Pampas. The model identifies as a function of risk preferences and initial wealth the crop mix that maximizes expected utility of wealth at the end of a 1-year decision period based on current costs and prices, and crop yields simulated for each year of historical weather. The model reproduced recent land allocation patterns at the district scale under moderate risk aversion, and predicted increasing diversification with increasing risk aversion. Differences in land allocation among ENSO phases were consistent with known climate response to ENSO, and crop response to water availability. Tailoring land allocation to ENSO phase increased mean net farm income between US

Radiation-use efficiency of sunflower crops: effects of specific leaf nitrogen and ontogeny

5 and

Spatial and interannual variations in the photothermal quotient: implications for the potential kernel number of wheat crops in Argentina

15 ha −1 year −1 relative to optimizing the crop mixture for all years, depending on location, risk aversion and initial wealth. The relationship between potential value of ENSO information and risk aversion was not monotonic, and differed between locations. Crop mix and information value also varied with crop prices and initial soil moisture. There are potential financial benefits of applying this approach to tailoring decisions to ENSO phases.

Light-associated nitrogen distribution profile in flowering canopies of sunflower (Helianthus annuus L.) altered during grain growth

Abstract Loss of nitrogen from the leaves and a reduction in specific leaf nitrogen (SLN, g N m −2 ) is associated with grain filling in sunflower ( Helianthus annuus L.). To explore the relationship between crop radiation-use efficiency (RUE, g MJ −1 ) and SLN, crop biomass accumulation and radiation interception were measured between the bud-visible and physiological-maturity stages in crops growing under combinations of two levels of applied nitrogen (0 and 5 g N m −2 ) and two population densities (2.4 and 4.8 plants m −2 ). Both nitrogen fertilization and density had significant ( P = 0.05) effects on crop biomass yield, nitrogen uptake, leaf area index and SLN, but the nitrogen effects were more pronounced for these and other crop variables. Linear regressions of accumulated biomass (OCdwt, corrected for the energy costs of oil synthesis in the grain) on accumulated intercepted short-wave radiation between bud visible and early grain filling provided appropriate and significantly ( P = 0.05) different estimates of RUE for the pooled 0 g N m −2 (1.01 g OCdwt MJ −1 ) and 5 g N m −2 (1.18 g OCdwt MJ −1 ) treatments. When calculated for each inter-harvest interval, crop RUE varied in a curvilinear fashion during the season, with a broad optimum from 40 to 70 days after emergence of the crops, and with lower values earlier and later in the season. The reduction in RUE toward physiological maturity was particularly marked. A plot of RUE against SLN revealed a reduction in RUE at small SLN values, but the relationship may be confounded by ontogenetic changes in other factors. A published model (Sinclair and Horie (1989), Crop Sci., 29: 90–98) was used to explore the RUE/SLN relationship. The model was unable to reproduce the decline in RUE during the second half of the grain-filling period. It is suggested that an important cause of this failure may be the partition, in the model, of a fixed, rather than a variable, fraction of crop gross photosynthesis to respiration.

Contribution of pre-anthesis assimilates to grain-filling in irrigated and water-stressed sunflower crops I. Estimates using labelled carbon

Abstract Analyses were performed to evaluate the hypothesis that, under conditions of good water availability, variations in yield in the Argentine wheat belt were related to variations in the photothermal quotient (Pq, the ratio of mean daily incident short-wave radiation to mean daily temperature in excess of 4.5°C). Data relating to spring bread wheat yields collected between 1978 and 1985 at several sites of the national wheat nursery, for which good weather data were available, were filtered to eliminate trials which may have been exposed to drought and other adversities, such as disease and hail. There were significant (P = 0.05) differences in yield, kernel number (Nk, grains m−2) and, to some extent, kernel weight among crops grown at different sites under close to optimum levels of water and nutrient availability. Nk was the most important determinant of yield, variations in Nk accounting for 78% of the total variance in the yield. Cultivar effects, presumably cultivar-specific differences in kernel weight, accounted for a further 14% of this variance. For the filtered data set, the relationship between Nk and Pq for the 25 days prior to anthesis, accounted for 52% of the total variability in Nk. The Pq and cultivar together accounted for 82% of total variability. Associations between Nk and either radiation or temperature alone were weaker, and site effects not accounted for by Pq were very small. The Pq estimated from 12- to 15-year weather records for the currently accepted optimum date of anthesis differed significantly (P = 0.05) between each of three representative sites in the northern, central and southern parts of the wheat belt. Reductions in potential Nk associated with interannual variations in Pq at each site may be as much as 34% of maximum Nk for the site. The Pq for anthesis dates of up to 20 days before or after the currently accepted optimum at any of these sites did not differ with respect to the optimum date, thus no advantage in terms of greater Nk is to be expected from changes in anthesis dates.

Grain number in sunflower as affected by shading during floret growth, anthesis and grain setting

In vegetative canopies of many species, the vertical gradient of lamina nitrogen concentration (NW) parallels the profile of light distribution in such a way that the actual nitrogen partitioning approaches the optimum pattern for canopy photosynthesis. This paper evaluates the hypothesis that a strong sink for nitrogen, viz. growing grain, affects the pattern of lamina nitrogen distribution usually described for vegetative canopies. The light and NW profiles of sunflower (Helianthus annuus L.) crops were characterised from anthesis to physiological maturity. The factorial combination of two plant populations (2.4 and 4.8 plants m−2) and two levels of nitrogen supply (0 and 5 g N m−2) were the sources of variation for NW and light profiles. Before the onset of nitrogen accumulation in grain, the pattern of NW was similar to that described for other species and it was related to the distribution of light in the canopy. Important changes in the profile of NW occurred during grain filling that were unrelated to the light regime. Nitrogen was mobilised from leaves in all positions in the canopy and the rate of NW change was greater in leaves closer to the grain, which were also the leaves where nitrogen was more concentrated. It is concluded that the physiological mechanisms involved in determining the distribution of leaf nitrogen in vegetative canopies do not apply to sunflower during grain filling.

Genotype by environment interaction and indirect selection for yield in sunflower: I. Two-mode pattern analysis of oil and biomass yield across environments in Argentina

Hall, A.J., Connor, D.J. and Whitfield, D.M., 1989. Contribution of pre-anthesis assimilates to grain-filling in irrigated and water-stressed sunflower crops. I. Estimates using labelled carbon. Field Crops Res., 20: 95-112. Labelled carbon was used to estimate pre-anthesis assimilate contributions to seed yield in sunflower crops irrigated throughout the season or stressed during grain-filling. At weekly intervals, commencing 23 days before anthesis and continuing through to maturity, previously unlabelled plots were exposed to labelled carbon dioxide. At maturity, the partitioning of label between the seed and the remainder of the crop (including roots) was determined. Estimates of pre-anthesis assimilate contributions to grain carbon were obtained by weighting these partitioning coefficients by seasonal crop gross photosynthesis data (Variant 1 ) and by crop dry-weight changes over the pre- and post-anthesis periods (Variant 2 ). Estimates of absolute contributions varied slightly between treatments and with estimation technique. Taking the synthesis costs of grain oil content into account, about 60 g C m- e (or about 22 or 27 % of the C content of seeds in irrigated and droughted crops, respectively ) originated in pre-anthesis assimilation. Estimates for stressed crops obtained using Variant 1 were lower than those obtained using Variant 2. The amount of water-soluble carbohydrate stored in stem, tap-root and receptacle at anthesis was equivalent to 86 g C m -2. During grain-filling, the losses from the non-seed portion of the crops in dry weight, in nitrogen-associated C, and in water-soluble carbohydrates were equivalent to 73, 14 and 60 g C m -s, respectively. Thus, labile C stored in the crop at anthesis considerably exceeded that required to sustain the estimated transfer to seeds, and observed losses from nonseed organs were similar in magnitude to these estimates.

On-farm assessment of regional and seasonal variation in sunflower yield in Argentina

Abstract The effects of shading (to 20% of incident radiation) on grain number (GN) in sunflower were examined by shading the crop in each of the six separate 10-day intervals running from the end of floret differentiation through to early grain growth (roughly, from mid-anthesis minus 15 days to mid-anthesis plus 26 days). An ontogenic index, based on floret morphology, male and female floret organ development, and early embryo development was formulated and used to describe the timing of shading and to categorize floret responses at three positions (periphery, mid-section, centre) on the head. Floret number per head was not affected by treatment, but shading reduced grain set (and, consequently, GN) significantly (P