Assimilate Availability Research Articles

Spring Maize Kernel Number and Assimilate Supply Responses to High‐Temperature Stress under Field Conditions

Core Ideas Spring maize mono‐cropping system is shown to be a water‐saving and high‐yield maize farming system. Spring maize was traditionally planted in late April or early May but seldom obtained desirable yield. Altering sowing date reduced frequency of high‐temperature stress of spring maize around silking and grain‐filling stage. Altering sowing date improved assimilate availability of spring maize during the grain‐filling stage. Late May turned out to be the better sowing date, rather than traditional sowing dates in late April or early May under current conditions. The productivity of spring maize (Zea mays L.) in North China Plain (NCP) is primarily limited by high‐temperature stress (HTS) during the grain‐filling stage. To avoid HTS and promote productivity, variable sowing date (SD), treatments were established from 2013 to 2015 (early April, SD1; mid‐April, SD2; late April, SD3; early May, SD4; mid‐May, SD5; and late May, SD6). During the grain‐filling stage, the leaf area index (LAI) and dry matter accumulation (DMA) were greatest at the final SD (SD6) compared with all other treatments. Additionally, the daily mean air temperature (Tmean), daily maximum air temperature (Tmax), and cumulative stressful days (CSD) during this stage across 3 yr decreased at SD6 by 1.4 to 3.2°C, 1.0 to 2.7°C, and 5.7 to 14.7 d, respectively, compared with the values at SD1 to SD5. Planting at SD6 increased the final grain yield by an average of 5.4 to 19.3%. A linear regression analysis showed that the kernel number (KN) per plant increased by 4 with every 100 kg ha−1 DMA. Kernel number was negatively correlated with Tmax and CSD from 15 d pre‐silking to 15 d post‐silking. Compared to SD1 and SD4, ear leaf at 15 d post‐silking under the SD6 treatment had the highest net photosynthetic rate (Pn) and the lowest ABA concentration. In conclusion, to avoid HTS and achieve the greatest corn grain yields in NCP, spring sowing maize should be postponed until late May.

Plasticity of seed weight compensates reductions in seed number of oilseed rape in response to shading at flowering

Abstract Understanding the response of the number of seeds and seed weight to the availability of assimilates is crucial for designing breeding strategies aimed to increase seed and oil yield in oilseed rape. This study aims to answer the questions: i) do seed number and seed weight in oilseed rape differ in their plasticity in response to the availability of assimilates at flowering? and ii) how sensitive are oil and protein concentrations to the availability of assimilates during flowering? A spring oilseed rape hybrid was sown in two field experiments and the treatments were combinations of i) two plant densities and ii) shading or no shading between the beginning and end of flowering. Seed yield was not affected by plant density or by shading. Lower plant density was compensated by an increase in seed number per plant, without effects on single-seed weight. However, the negative effect of shading during flowering on seed number per area was fully compensated by an increase in single-seed weight by 47–61%. The plasticity of single-seed weight observed in the present study of oilseed rape has never been reported for annual seed crops. Shading at flowering increased both the seed filling rate and the duration of the seed filling period at all positions in the canopy. We also observed that the reduction of the source-sink ratio at flowering increasing seed weight does not necessarily modify oil or protein concentrations. Thus, single-seed weight could be targeted to increase seed yield in oilseed rape without compromising oil content.

Variation in Seed Protein Concentration and Seed Size Affects Soybean Crop Growth and Development

Developing high protein (HP) cultivars is often precluded by the inverse relationship between protein and yield. We hypothesized that attaining HP concentration based on contrasting seed size impacts crop growth and development differently. We screened 97 soybean genotypes and found lines with HP concentration (∼450 g kg−1) associated with (i) increased protein content (mg seed−1) in large seed genotypes, and (ii) reduced oil and carbohydrate contents in small seed ones. Then, we evaluated different growth traits in a subset of three HP large and three HP small seed genotypes, as well as in three high‐yielding genotypes with average seed size and protein concentration. High‐yielding genotypes showed higher leaf area duration and harvest index when compared with HP genotypes, regardless of seed size. High protein large seed was associated with more assimilate availability per seed during seed filling, while HP small seed showed higher leaf area at the beginning of seed fill, more canopy biomass production, and very low levels of assimilate per seed. Results show that selecting for seed protein concentration can impact crop growth and development differently, depending on the strategy used for selection in terms of seed size. These findings, if utilized for parental selection, might contribute to eliminating negative correlations between seed protein and yield, since these strategies may be under different genetic control and/or determine different biophysical constraints.

Vegetative and reproductive growth of salt-stressed chickpea are carbon-limited: sucrose infusion at the reproductive stage improves salt tolerance.

Reproductive processes of chickpea (Cicer arietinum L.) are particularly sensitive to salinity. We tested whether limited photoassimilate availability contributes to reproductive failure in salt-stressed chickpea. Rupali, a salt-sensitive genotype, was grown in aerated nutrient solution, either with non-saline (control) or 30mM NaCl treatment. At flowering, stems were either infused with sucrose solution (0.44M), water only or maintained without any infusion, for 75 d. The sucrose and water infusion treatments of non-saline plants had no effect on growth or yield, but photosynthesis declined in response to sucrose infusion. Salt stress reduced photosynthesis, decreased tissue sugars by 22-47%, and vegetative and reproductive growth were severely impaired. Sucrose infusion of salt-treated plants increased total sugars in stems, leaves and developing pods, to levels similar to those of non-saline plants. In salt-stressed plants, sucrose infusion increased dry mass (2.6-fold), pod numbers (3.8-fold), seed numbers (6.5-fold) and seed yield (10.4-fold), yet vegetative growth and reproductive failure were not rescued completely by sucrose infusion. Sucrose infusion partly rescued reproductive failure in chickpea by increasing vegetative growth enabling more flower production and by providing sucrose for pod and seed growth. We conclude that insufficient assimilate availability limits yield in salt-stressed chickpea.

Contribution of incident solar radiation on leaves and pods to soybean seed weight and composition

Abstract The weight and composition of soybean seeds ( Glycine Max L. Merrill) depend on changes in carbon and nitrogen assimilate supply during grain filling. Soybean pods and seeds are green, evidencing their capacity to capture light. However, the current physiological knowledge does not consider any effect of incident solar radiation reaching the pods on seed weight and composition. The objective of this work was to investigate the response of seed weight and composition to changes in assimilate supply from leaves, to the incident solar radiation reaching the pods and to the combination of both, changes in assimilate supply from the leaves and incident solar radiation on pods of soybean plants. Field experiments were performed during two growing seasons at Balcarce, Argentina. Treatments modified the amount of assimilates supplied by the leaves (plant shading, defoliation), the solar radiation reaching the pods (pod shading) or both (defoliation and pod shading) during seed filling. Plant shading and defoliation reduced seed weight, oil concentration and oil and protein content and increased the concentration of saturated and poli-unsaturated fatty acids while reduced oleic acid percentage. Pod shading increased the concentration of stearic acid and reduced the concentration of linolenic acid. When pods were shaded on defoliated plants, seed weight and oil and protein content decreased while fatty acid composition was similar to values obtained under defoliation treatment. Based on these results, a conceptual model that considers photoheterotrophic nature of reproductive structures of soybean is proposed. Seed weight, oil and protein content and oil fatty acid composition depended on assimilate availability for the seeds. The response of oil and protein content to assimilate supply depended on whether leaves were present or not. The effect of solar radiation incident on pods depended on the amount of assimilates available for the seeds: (i) when carbon allocated was low (defoliation treatments), pods contributed to seed carbon economy but solar radiation incident on them did not affect fatty acid composition; (ii) when carbon allocated to the seeds was high (intact plants), contribution of pods to seed carbon economy was not significant, but the amount of solar radiation incident on pods produced significant changes in fatty acid composition.

Identification of yield limiting phenological phases of oats to improve crop management

SUMMARYThe optimization of management practices for oats is hindered by a lack of knowledge of the critical phenological phases at which management should be focussed. The objective of the present review was to identify the yield-limiting phases in the growth of the oat crop in order to optimize management and to maximize yield. The methodology employed was to identify characteristics associated with either the pre-anthesis grain number determination phase or with the post-anthesis grain filling phase. Characteristics associated with the pre-anthesis phase were identified as a positive linear relationship between yield and grain number, in addition to insensitivity of grain weight to changes in assimilate supply. Characteristics associated with the post-anthesis grain filling phase were identified as an absence of a relationship between yield and sink size (grain number) and changes in grain weight in response to changes in assimilate supply. Data was taken from published literature. Yields of both winter- and spring-sown hulled oats increased linearly with grain number showing a strong influence of grain number on yield. Grain weight of both winter- and spring-hulled oats, however, decreased with increasing grain number suggesting that competition for assimilates may exist at high grain number. Further evidence of the influence of assimilate supply on grain yield was obtained from several studies which showed that yield increased with leaf area duration as well as from studies where grain weight was found to decrease after reductions in assimilate supply per grain, whereas grain weight increased when assimilate supply to grain was increased. Oat crops also feature a grain abortion mechanism when assimilate supply is constrained. Yield of naked oats increased with grain number before reaching a plateau, a trend which suggests source limitation at high grain numbers. The available evidence suggests that yield is primarily determined by grain number determination but that grain yield is also potentially limited by assimilate availability in the post-anthesis period. It is recommended that crop management strategies for oats should aim both to increase grain number in the pre-anthesis period but also prolong the grain filling period after anthesis. Such a post-anthesis strategy should both reduce the possibility of yield being limited by assimilate availability and compensate for the production of smaller grains at higher grain numbers.

Soybean fruit development and set at the node level under combined photoperiod and radiation conditions.

In soybean, long days during post-flowering increase seed number. This positive photoperiodic effect on seed number has been previously associated with increments in the amount of radiation accumulated during the crop cycle because long days extend the duration of the crop cycle. However, evidence of intra-nodal processes independent of the availability of assimilates suggests that photoperiodic effects at the node level might also contribute to pod set. This work aims to identify the main mechanisms responsible for the increase in pod number per node in response to long days; including the dynamics of flowering, pod development, growth and set at the node level. Long days increased pods per node on the main stems, by increasing pods on lateral racemes (usually dominated positions) at some main stem nodes. Long days lengthened the flowering period and thereby increased the number of opened flowers on lateral racemes. The flowering period was prolonged under long days because effective seed filling was delayed on primary racemes (dominant positions). Long days also delayed the development of flowers into pods with filling seeds, delaying the initiation of pod elongation without modifying pod elongation rate. The embryo development matched the external pod length irrespective of the pod's chronological age. These results suggest that long days during post-flowering enhance pod number per node through a relief of the competition between pods of different hierarchy within the node. The photoperiodic effect on the development of dominant pods, delaying their elongation and therefore postponing their active growth, extends flowering and allows pod set at positions that are usually dominated.

Carpel size, grain filling, and morphology determine individual grain weight in wheat.

Individual grain weight is a major yield component in wheat. To provide a comprehensive understanding of grain weight determination, the carpel size at anthesis, grain dry matter accumulation, grain water uptake and loss, grain morphological expansion, and final grain weight at different positions within spikelets were investigated in a recombinant inbred line mapping population of bread wheat (Triticum aestivum L.)×spelt (Triticum spelta L.). Carpel size, grain dry matter and water accumulation, and grain dimensions interacted strongly with each other. Furthermore, larger carpels, a faster grain filling rate, earlier and longer grain filling, more grain water, faster grain water absorption and loss rates, and larger grain dimensions were associated with higher grain weight. Frequent quantitative trait locus (QTL) coincidences between these traits were observed, particularly those on chromosomes 2A, 3B, 4A, 5A, 5DL, and 7B, each of which harboured 16-49 QTLs associated with >12 traits. Analysis of the allelic effects of coincident QTLs confirmed their physiological relationships, indicating that the complex but orderly grain filling processes result mainly from pleiotropy or the tight linkages of functionally related genes. After grain filling, distal grains within spikelets were smaller than basal grains, primarily due to later grain filling and a slower initial grain filling rate, followed by synchronous maturation among different grains. Distal grain weight was improved by increased assimilate availability from anthesis. These findings provide deeper insight into grain weight determination in wheat, and the high level of QTL coincidences allows simultaneous improvement of multiple grain filling traits in breeding.

Physiological differences in yield related traits between flint and dent Argentinean commercial maize genotypes

Argentina is the worldwide single maize (Zea mays L.) exporter of non-GMO flint maize, also called plata maize. This grain is known for high dry-milling yields, the production of large endosperm grits and specific cooking functional properties. But, this special maize has lower yields at farmer fields when compared to regular dent germplasm, and studies describing the physiological characteristics behind this are scarce. Our objective was to understand differences in yield determination mechanisms between flint and dent commercial germplasm for the temperate area. We characterized 31 genotypes (24 dent and 7 flint) growing at five different environments for describing their yield differences, and also described specific physiological traits to unravel the mechanisms behind these yield differences.Grain yield, KNP, KW, plant growth rate and biomass partitioning around flowering, kernel set efficiency per unit of accumulated ear biomass at flowering and assimilate availability per kernel during flowering all showed significant kernel type (flints vs. dents) effects (p<0.05). And significant genotype differences within each kernel type were evident for all traits (p<0.01). Flint kernel type showed lower yields (ca. 80% of dents) due to reduced KNP and KW. This lower KNP in flints was mostly related to a lower plant growth rate around flowering, although they also showed a reduced biomass partitioning to the ear during this period. Flint genotypes, however, showed higher kernel set efficiency per unit of accumulated ear biomass when compared to dents (p<0.01). Lower KW in flints was related to a reduced assimilate availability per kernel around flowering (p<0.01), both kernel types showed similar assimilate availability per kernel during grain filling (p>0.05). This indicated flint and dent kernel types had the same amount of assimilates to fulfill their early established potential KW. Our results emphasize the importance of the flowering period for understanding yield differences between flints and dents, and biomass accumulation rate during this period was identified as a key trait for increasing flint yields.

Effects of weather conditions during different growth phases on yield formation of winter oilseed rape

Winter oilseed rape (WOSR) is an important oil and protein crop in Europe, used in biofuel production and as protein source in livestock farming. In contrast to cereals, WOSR seed yields are still increasing in most countries but yield stability was not improved during the last decades.In our study, we analyzed the effects of weather conditions during different growth stages on maximum seed yield, maximum oil yield, number of seeds per m2 and 1000-seed weight to get further information on yield formation processes.Field trials performed at 34 environments (site×year combinations) representing different soil characteristics and climate regions in Germany were used for the analysis. About 40% of seed yield variability could be explained by weather conditions during specific growth phases. The most important phenological phases thereby were: onset of pods and seeds (BBCH 50–65) and seed development (BBCH 71–79). During onset of pods and seeds, yield was significantly influenced by temperature, radiation and drought stress. Assimilate availability during this phase determines the number of seeds per m2 (sink size). After flowering, only temperature significantly affected WOSR yield. Temperature is the major parameter determining the duration of growth stages. Lower temperature elongates the time of assimilate production and translocation to the seeds. During this growth stage, seed weight is determined. In our data sets, low sink size was not yield limiting due to compensatory effects between the yield components number of seeds per m2 and 1000-seed weight. Yield response pattern suggests that WOSR yield is predominantly source-limited, especially during the late reproductive phase.

Pod Set in Soybean: Investigations with SOYPODP, a Whole Plant Model

The ability of soybean [Glycine max (L.) Merr.] to adjust its pod load to environmental conditions is an important, but not well understood, part of the yield production process. To better understand this process at the whole plant level, a single‐node model (SOYPOD) was extended to predict pod set of whole plants. Pod survival in SOYPODP was determined by comparing assimilate from photosynthesis (calculated from solar radiation) to pod requirements during initial pod growth. Measured profiles of flowers per node and pod and seed growth rates were used as input. Pods that do not receive adequate assimilate for a specified number of consecutive days do not survive. When inter‐nodal movement of assimilate was prohibited and the nodal assimilate production profile matched the input flower profile, pod set on the stem matched measured profiles of two field‐grown cultivars. In this form, SOYPODP accurately mimicked known responses of pod number to variation in assimilate supply, nodes per plant and individual seed growth rate. Competition for assimilate at a node‐controlled pod set, so increasing uniformity of pod development (more flowers or a shorter flowering period) increased pod set. Increasing nodal variation in the beginning of pod development reduced the sensitivity of the plant to short‐term fluctuations in assimilate supply. Variation in uniformity of pod development at a node was responsible for increases in pod set with no change in assimilate availability. The value of these increases is not clear, but it seems unlikely that they would affect yield.

Comparison of Salt and Drought‐Stress Effects on Maize Growth and Yield Formation with Regard to Acid Invertase Activity in the Kernels

AbstractThe long‐term effects of salt stress (11 dS m−1) and drought stress (35 % WHC) were investigated for two maize genotypes, focusing on the relation between metabolic changes around the time of pollination and the impact on yield determinants at maturity. The relatively salt‐resistant hybrid Pioneer 3906 and the relatively drought‐resistant hybrid Fabregas were compared. The experiments were conducted in large plastic containers in a vegetation hall in two consecutive years (2011 and 2012). Plant height and leaf area were significantly reduced under both stress conditions. The transpiration rate was only slightly reduced under drought stress; but under salt stress, a significant reduction occurred 40–53 days after sowing. As a significant increase in sucrose concentrations was observed in the salt‐treated maize kernels 2 days after pollination, the availability of assimilates was not limiting and the plants could afford to save water by reduced stomatal opening. Although under both stress conditions the soluble acid invertase activity was reduced 2 days after pollination, concomitantly, an increase in hexose concentrations was observed. Thus, in these experiments, the delivery of hexoses by acid invertase activity did not limit kernel development. Differences in grain yield at maturity between salt and drought stress were most likely caused by salt‐specific effects (Na+ toxicity), Fabregas being more affected than Pioneer 3906.

Fruit illumination stimulates cell division but has no detectable effect on fruit size in tomato (Solanum lycopersicum).

Light affects plant growth through assimilate availability and signals regulating development. The effects of light on growth of tomato fruit were studied using cuvettes with light-emitting diodes providing white, red or blue light to individual tomato trusses for different periods during daytime. Hypotheses tested were as follows: (1) light-grown fruits have stronger assimilate sinks than dark-grown fruits, and (2) responses depend on light treatment provided, and fruit development stage. Seven light treatments [dark, 12-h white, 24-h white, 24-h red and 24-h blue light, dark in the first 24 days after anthesis (DAA) followed by 24-h white light until breaker stage, and its reverse] were applied. Observations were made between anthesis and breaker stage at fruit, cell and gene levels. Fruit size and carbohydrate content did not respond to light treatments while cell division was strongly stimulated at the expense of cell expansion by light. The effects of light on cell number and volume were independent of the combination of light color and intensity. Increased cell division and decreased cell volume when fruits were grown in the presence of light were not clearly corroborated by the expression pattern of promoters and inhibitors of cell division and expansion analyzed in this study, implying a strong effect of posttranscriptional regulation. Results suggest the existence of a complex homeostatic regulatory system for fruit growth in which reduced cell division is compensated by enhanced cell expansion.

Heat stress in temperate and tropical maize hybrids: Kernel growth, water relations and assimilate availability for grain filling

Several studies have indicated that maize (Zea mays L.) kernel weight is severely affected by heat stress, but this response was never evaluated under field conditions. Our objective was to assess the effect of brief episodes of above-optimum temperatures on the dynamics of biomass and water accumulation in kernels of maize hybrids with contrasting tolerance to heat stress. Heat effect on assimilate supply from the plant (i.e., current biomass production and water-soluble carbohydrates in stem) to developing grains was also analyzed. Field experiments included a factorial combination of (i) three hybrids (Te: temperate; Tr: tropical; TeTr: Te×Tr), (ii) two temperature regimes, control and heated during daytime hours (ca. 33–40°C at ear level), and (ii) three 15-d periods (GS1: immediately before anthesis; GS2: from silking onwards; GS3: early phase of active grain filling). Heat effects on final kernel weight were larger (i) when they occurred during the first half of effective grain filling (−23.1% for GS3) than around flowering (−4.8% for GS1, −6.3% for GS2), and (ii) for the Te hybrid (−20.4%) than for the TeTr (−8.6%) and the Tr (−6.8%) hybrids. Heating around flowering (i) enhanced the assimilate availability per kernel during the effective grain-filling period, (ii) increased carbohydrates reserves in stem at physiological maturity, (iii) and had no significant effect on the dynamics of biomass and water accumulation in kernels. The opposite trend was detected among plots heated during GS3, which mostly exhibited the interruption of grain filling. Robust associations were established between (i) carbohydrate reserves in stem at physiological maturity and assimilate availability per kernel during effective grain filling (r2=0.49; P<0.001), and (ii) the rate of water loss from kernels and the duration of effective grain filling (r2=0.71; P<0.001). These responses underlay the enhanced sensitivity to heat stress of the hybrid with full temperate genetic background.

A physiological framework to explain genetic and environmental regulation of tillering in sorghum

Tillering determines the plant size of sorghum (Sorghum bicolor) and an understanding of its regulation is important to match genotypes to prevalent growing conditions in target production environments. The aim of this study was to determine the physiological and environmental regulation of variability in tillering among sorghum genotypes, and to develop a framework for this regulation. Diverse sorghum genotypes were grown in three experiments with contrasting temperature, radiation and plant density to create variation in tillering. Data on phenology, tillering, and leaf and plant size were collected. A carbohydrate supply/demand (S/D) index that incorporated environmental and genotypic parameters was developed to represent the effects of assimilate availability on tillering. Genotypic differences in tillering not explained by this index were defined as propensity to tiller (PTT) and probably represented hormonal effects. Genotypic variation in tillering was associated with differences in leaf width, stem diameter and PTT. The S/D index captured most of the environmental effects on tillering and PTT most of the genotypic effects. A framework that captures genetic and environmental regulation of tillering through assimilate availability and PTT was developed, and provides a basis for the development of a model that connects genetic control of tillering to its phenotypic consequences.

Combined effect of irrigation regime and fruit load on the patterns of trunk-diameter variation of ‘Hass’ avocado at different phenological periods

The objective was to investigate the combined effect of irrigation regime and fruit load on trunk-diameter variation patterns of ‘Hass’ avocado trees grown in lysimeters, at different phenological periods. Plant water uptake of both fruited and defruited trees was monitored at high temporal resolution during successive growth stages. The trunk growth rate (TGR) during all the experiments was not affected by the irrigation treatments, yet daily TGR fluctuated significantly during the season, probably in association with periodic changes in the priority of partitioning of carbohydrates between reproductive and vegetative plant organs, i.e., flushes of shoot or root growth. Fruit load clearly played a dominant role in determining TGR, very likely because of the dramatic effect of fruit load on stomatal conductance and leaf carbohydrate concentration and, therefore, on the overall availability of assimilates. Thus, analyses of trunk diameter variation (TDV) data that lead to evaluation of TGR and maximum trunk diameter variation (MTDV) reflect phenological stages and periodicity of shoot, fruit and root growth, and also may provide an integrative, “holistic viewpoint” of overall tree status.The observed high dependency between the MTDV and irrigation treatments may indicate that MTDV actually depends on water-stress history rather than on the actual plant-water status. Thus, MTDV cannot be used on an absolute basis for irrigation management control. On the other hand, it may be a very efficient aid to control irrigation when used on a relative basis, after identifying the impacts of local phenological and environmental factors on the MTDV. Furthermore, the immediate MTDV response to accidental faults may illustrate the advantages of using dendrometer devices for remotely controlling irrigation systems.

Manipulation of the availability of assimilates for kernel growth in wheat in Japan: Effects of crop thinning and planting geometry

Wheat often overproduces kernels when grown in favorable environments, resulting in low kernel weight with high kernel screenings. This indicates that photoassimilation after heading would not be sufficient to support the growth of a large number of kernels. The objective of this study was to examine the effects of crop thinning at heading (experiment 1) and double-row planting (compared with equidistant row spacing; experiment 2) on kernel weight and yield of Japanese wheat cultivars under different levels of soil fertility. In experiment 1, elimination of aboveground competition for radiation by thinning increased yield by up to 32%, mainly due to increased kernel weight. In experiment 2, double-row planting increased kernel weight significantly (by 4–10%) without yield loss by moderately suppressing maximum stem number (by 7–14%) and spike and kernel number (by up to 14%) under high fertility. In both experiments, the effect of crop thinning or row configuration on wheat yield and kernel weight was negligible under low fertility because of little competition for radiation. The results suggested that control of vegetative growth and kernel production would help avoid the shortage of assimilates for kernel growth in fertile environments. For further increasing wheat yield under favorable conditions in Japan, the appropriate crop management options (e.g., planting geometry) should be combined with the suitable varieties having a more communal ideotype (sparse tillering with upright posture) than a competitive phenotype (profuse tillering with lax posture) to enhance photoassimilation after heading.

Linking ascorbic acid production in Ribes nigrum with fruit development and changes in sources and sinks.

Understanding the synthesis of ascorbic acid (l-AsA) in green tissues in model species has advanced considerably; here we focus on its production and accumulation in fruit. In particular, our aim is to understand the links between organs which may be sources of l-AsA (leaves) and those which accumulate it (fruits). The work presented here tests the idea that changes in leaf and fruit number influence the accumulation of l-AsA. The aim was to understand the importance of leaf tissue in the production of l-AsA and to determine how this might provide routes for the manipulation of fruit tissue l-AsA. The experiments used Ribes nigrum (blackcurrant), predominantly in field experiments, where the source-sink relationship was manipulated to alter potential leaf l-AsA production and fruit growth and accumulation of l-AsA. These manipulations included reductions in reproductive capacity, by raceme removal, and the availability of assimilates by leaf removal and branch phloem girdling. Natural variation in fruit growth and fruit abscission is also described as this influences subsequent experimental design and the interpretation of l-AsA data. Results show that fruit l-AsA concentration is conserved but total yield of l-AsA per plant is dependent on a number of innate factors many of which relate to raceme attributes. Leaf removal and phloem girdling reduced fruit weight, and a combination of both reduced fruit yields further. It appears that around 50 % of assimilates utilized for fruit growth came from apical leaves, while between 20 and 30 % came from raceme leaves, with the remainder from 'storage'. Despite being able to manipulate leaf area and therefore assimilate availability and stored carbohydrates, along with fruit yields, rarely were effects on fruit l-AsA concentration seen, indicating fruit l-AsA production in Ribes was not directly coupled to assimilate supply. There was no supporting evidence that l-AsA production occurred predominantly in green leaf tissue followed by its transfer to developing fruits. It is concluded that l-AsA production occurs predominantly in the fruit of Ribes nigrum.

Increasing the harvest index of wheat in the high rainfall zones of southern Australia

The harvest index (HI) of wheat in the high rainfall zones (HRZ) of Australia is low compared with values in similar rainfall environments in Europe. From 2005 to 2007, we examined dry matter (DM) partitioning, yield formation, post-anthesis dry matter accumulation, and remobilization of water soluble carbohydrates (WSC) of a number of wheat genotypes (5 spring wheat and 1 winter wheat) to determine the factors influencing the HI in the HRZ of south-western Australia. Grain yield was strongly correlated with grain number (GN) in all three years and with the sum of post-anthesis DM accumulation and WSC in stems at anthesis when the amount of assimilates available to fill grains was less than 600gm−2. However, when the amount of assimilates exceeded 600gm−2, grain yield reached a plateau and did not increase with the increased availability of assimilates because of sink limitations. Genotypes with more GN tended to extract more WSC to fill grains than ones with fewer GN in 2005 and 2007. GN increased with increased spike DM at anthesis, the number of grainsg−1 of spike DM, and the duration of spike growth, but was not correlated to the fraction of DM partitioned to the spike at anthesis for individual genotypes and across the 3 years. Compared to winter wheat in the UK, Australian spring and winter wheat had a lower number of grainsg−1 spike DM and g−1 chaff DM. HI was positively related to WSC at anthesis and post-anthesis DM accumulation in the drought year of 2006, but not in average (2007) and above-average rainfall (2005) years. HI was negatively related to the proportion of stem DM to above-ground dry matter (AGDM), but positively related to the proportion of spike DM to AGDM. Dry matter partitioning indicated that the proportion of stem DM to AGDM and chaff DM to AGDM was 20% and 50% higher than that of winter wheat grown in the UK, respectively. This was related to a lack of remobilization of WSC and a net increase between spike DM at anthesis and chaff DM at maturity. Therefore, we suggest that increasing the number of grainsm−2 by breeding for more grainsg−1 spike DM, reducing dry matter partitioning into stem and chaff, and remobilizing WSC can lead to an improved HI. We conclude that the yield of wheat can be increased without increasing AGDM and the potential HI can reach 0.50 in the HRZ of south-western Australia if the number of grainsm−2 can be increased.

Post‐Flowering Assimilate Availability Regulates Oil Fatty Acid Composition in Sunflower Grains

ABSTRACTFatty acid composition of Helianthus annuus L. (sunflower) oil depends on intercepted solar radiation per plant (ISR) during grain filling. This effect could be accounted for by the assimilate availability of the grains (the source‐sink ratio). However, the current physiological‐biochemical knowledge does not consider any effect of carbon availability on oil fatty acid composition. The objective of this work was to address the regulation of fatty acid composition by assimilate supply to sunflower grains. A wide range of source‐sink ratios was obtained by manipulating either the source or the sink during grain filling. Assimilate supply was also modified by injecting sucrose into the receptacle of sunflower capitula. Grain weight and oil content depended on both ISR and source‐sink ratio in a curvilinear manner. When sink size was decreased by grain excision, ISR failed to explain oil fatty acid composition, while source‐sink ratio appropriately described it. Sucrose injection significantly increased grain weight, oil content, and oleic acid percentage of shaded plants. It is concluded that effects of ISR on fatty acid composition are a consequence of changes in assimilate availability for grain oil synthesis. To explain these results a conceptual model is proposed: when assimilate supply limits grain growth and oil synthesis, mainly linoleic acid is synthesized. As the assimilate supply increases, oleic acid desaturation process gets saturated and oleic acid accumulates.