A wiring diagram to integrate physiological traits of wheat yield potential

As crop yields are pushed closer to biophysical limits, achieving yield gains becomes increasingly challenging and will require more insight into deterministic pathways to yields. Here, we propose a wiring diagram as a platform to illustrate the interrelationships of the physiological traits that impact wheat yield potential and to serve as a decision support tool for crop scientists. The wiring diagram is based on the premise that crop yield is a function of photosynthesis (source), the investment of assimilates into reproductive organs (sinks) and the underlying processes that enable expression of both. By illustrating these linkages as coded wires, the wiring diagram can show connections among traits that may not have been apparent, and can inform new research hypotheses and guide crosses designed to accumulate beneficial traits and alleles in breeding. The wiring diagram can also serve to create an ever-richer common point of reference for refining crop models in the future.

Mitigating tradeoffs in plant breeding

Tradeoffs among plant traits help maintain relative fitness under unpredictable conditions and maximize reproductive success. However, modifying tradeoffs is a breeding challenge since many genes of minor effect are involved. The intensive crosstalk and fine-tuning between growth and defense responsive phytohormones via transcription factors optimizes growth, reproduction, and stress tolerance. There are regulating genes in grain crops that deploy diverse functions to overcome tradeoffs, e.g., miR-156-IPA1 regulates crosstalk between growth and defense to achieve high disease resistance and yield, while OsALDH2B1 loss of function causes imbalance among defense, growth, and reproduction in rice. GNI-A1 regulates seed number and weight in wheat by suppressing distal florets and altering assimilate distribution of proximal seeds in spikelets. Knocking out ABA-induced transcription repressors (AITRs) enhances abiotic stress adaptation without fitness cost in Arabidopsis. Deploying AITRs homologs in grain crops may facilitate breeding. This knowledge suggests overcoming tradeoffs through breeding may expose new ones.

Improving global integration of crop research

Field laboratories in realistic crop environments are needed

Awns reduce grain number to increase grain size and harvestable yield in irrigated and rainfed spring wheat

Genotypic variation in ear morphology is linked to differences in photosynthetic potential to influence grain yield in winter cereals. Awns contribute to photosynthesis, particularly under water-limited conditions when canopy assimilation is restricted. We assessed performance of up to 45 backcross-derived, awned-awnletted NILs representing four diverse genetic backgrounds in 25 irrigated or rainfed, and droughted environments in Australia and Mexico. Mean environment grain yields were wide-ranging (1.38-7.93 t ha(-1)) with vegetative and maturity biomass, plant height, anthesis date, spike number, and harvest index all similar (P >0.05) for awned and awnletted NILs. Overall, grain yields of awned-awnletted sister-NILs were equivalent, irrespective of yield potential and genetic background. Awnletted wheats produced significantly more grains per unit area (+4%) and per spike (+5%) reflecting more fertile spikelets and grains in tertiary florets. Increases in grain number were compensated for by significant reductions in grain size (-5%) and increased frequency (+0.8%) of small, shrivelled grains ('screenings') to reduce seed-lot quality of awnletted NILs. Post-anthesis canopies of awnletted NILs were marginally warmer over all environments (+0.27 °C) but were not different and were sometimes cooler than awned NILs at cooler air temperatures. Awns develop early and represented up to 40% of total spikelet biomass prior to ear emergence. We hypothesize that the allocation of assimilate to large and rapidly developing awns decreases spikelet number and floret fertility to reduce grain number, particularly in distal florets. Individual grain size is increased to reduce screenings and to increase test weight and milling quality, particularly in droughted environments. Despite the average reduction in grain size, awnless lines could be identified that combined higher grain yield with larger grain size, increased grain protein concentration, and reduced screenings.

Genetic characterization of the wheat association mapping initiative (WAMI) panel for dissection of complex traits in spring wheat

The wheat association mapping initiative is appropriate for gene discovery without the confounding effects of phenology and plant height. The wheat association mapping initiative (WAMI) population is a set of 287 diverse advanced wheat lines with a narrow range of variation for days to heading (DH) and plant height (PH). This study aimed to characterize the WAMI and showed that this diverse panel has a favorable genetic background in which stress adaptive traits and their alleles contributing to final yield can be identified with reduced confounding major gene effects through genome-wide association studies (GWAS). Using single nucleotide polymorphism (SNP) markers, we observed lower gene diversity on the D genome, compared with the other genomes. Population structure was primarily related to the distribution of the 1B.1R rye translocation. The narrow range of variation for DH and PH in the WAMI population still entailed segregation for a few markers associated with the former traits, while Rht genes were associated with grain yield (GY). Genotype by environment (G × E) interaction for GY was primarily explained by Rht-B1, Vrn-A1 and markers on chromosomes 2D and 3A when running GWAS with genotype scores from the G × E biplot. The use of PC scores from the G × E biplot seems a promising tool to determine genes and markers associated with complex interactions across environments. The WAMI panel lends itself to GWAS for complex trait dissection by avoiding the confounding effects of DH and PH which were reduced to a minimum (using Rht-B1 and Vrn-A1 scores as covariables), with significant associations with GY on chromosomes 2D, 3A and 3B.

Radically rethinking agriculture for the 21st century

Population growth, arable land and fresh water limits, and climate change have profound implications for the ability of agriculture to meet this century's demands for food, feed, fiber, and fuel while reducing the environmental impact of their production. Success depends on the acceptance and use of contemporary molecular techniques, as well as the increasing development of farming systems that use saline water and integrate nutrient flows.

Association between canopy reflectance indices and yield and physiological traits in bread wheat under drought and well-irrigated conditions

Spectral reflectance (SR) indices [NDVI (R900 – R680/R900 + R680); GNDVI (R780 – R550/R780 + R550); and water index, WI (R900/R970)]; and 6 chlorophyll indices (R740/R720, NDI = R750 – R705/R750 + R705, R780 – R710/R780 – R680, R850 – R710/R850 – R680, mND = R750 – R705/R750 + R705 – 2R445, and mSR = R750 – R445/R705 – R445) were measured with a FieldSpec spectroradiometer (Analytical Spectral Devices, Boulder, CO) on bread wheat (Triticum aestivum L.) genotypes adapted to irrigated and drought conditions to establish their relationship with yield in field-grown plots. Bread wheat genotypes from the International Maize and Wheat Improvement Center (CIMMYT) were used for this study in 3 experiments: 8 genotypes in a trial representing historical progress in yield potential, and 3 pairs of near-isolines for Lr19, both of which were grown under well-watered conditions; and the third experiment included 20 drought tolerant advanced genotypes grown under moisture stress. These were grown during the 2000 and 2001 spring cycles in a temperate, high radiation environment in Obregón, NW México. The 9 SR indices were determined during grain filling along with canopy temperature depression (CTD), flag leaf photosynthetic rate, and chlorophyll estimates using a SPAD meter. The relationship of SR indices with grain yield and biomass fitted best with a linear model. NDVI and GNDVI showed positive relationships with grain yield and biomass under well-irrigated conditions (r = 0.35–0.92), whereas NDVI showed a stronger association with yield under drought conditions (r = 0.54). The 6 chlorophyll indices showed significant association with yield and biomass of wheat genotypes grown under well-irrigated conditions (r = 0.39–0.90). The association between chlorophyll indices and chlorophyll estimates was correlated (r = 0.38–0.92), as was the case for photosynthetic rate (r = 0.36–0.75). WI showed a significant relationship with grain yield in wheat genotypes grown under drought stress conditions (r = 0.60) as well as with grain yield and biomass under well-irrigated conditions (r = 0.52–0.91). The relationship between WI and CTD was significant (P ≤ 0.05) in both environments (r = 0.44–0.84). In conclusion, the SR showed potential for identifying higher-yielding genotypes in a breeding program under dry or irrigated conditions, as well as for estimating some physiological parameters.

Plant breeding and drought in C3 cereals: what should we breed for?

Drought is the main abiotic constraint on cereal yield. Analysing physiological determinants of yield responses to water may help in breeding for higher yield and stability under drought conditions. The traits to select (either for stress escape, avoidance or tolerance) and the framework where breeding for drought stress is addressed will depend on the level and timing of stress in the targeted area. If the stress is severe, breeding under stress-free conditions may be unsuccessful and traits that confer survival may become a priority. However, selecting for yield itself under stress-alleviated conditions appears to produce superior cultivars, not only for optimum environments, but also for those characterized by frequent mild and moderate stress conditions. This implies that broad avoidance/tolerance to mild-moderate stresses is given by constitutive traits also expressed under stress-free conditions. In this paper, we focus on physiological traits that contribute to improved productivity under mild-moderate drought. Increased crop performance may be achieved through improvements in water use, water-use efficiency and harvest index. The first factor is relevant when soil water remains available at maturity or when deep-rooted genotypes access water in the soil profile that is not normally available; the two latter conditions become more important when all available water is exhausted by the end of the crop cycle. Independent of the mechanism operating, a canopy able to use more water than another would have more open stomata and therefore higher canopy temperature depression, and 13C discrimination (delta13C) in plant matter. The same traits would also seem to be relevant when breeding for hot, irrigated environments. Where additional water is not available to the crop, higher water-use efficiency (WUE) appears to be an alternative strategy to improve crop performance. In this context delta13C constitutes a simple but reliable measure of WUE. However, in contrast to lines performing better because of increased access to water, lines producing greater biomass due to superior WUE will have lower delta13C values. WUE may be modified not only through a decrease in stomatal conductance, but also through an increase in photosynthetic capacity. Harvest index is strongly reduced by terminal drought (i.e. drought during grain filling). Thus, phenological traits increasing the relative amount of water used during grain filling, or adjusting the crop cycle to the seasonal pattern of rainfall may be useful. Augmenting the contribution of carbohydrate reserves accumulated during vegetative growth to grain filling may also be worthwhile in harsh environmcnts. Alternatively, extending the duration of stem elongation without changing the timing of anthesis would increase the number of grains per spike and the harvest index without changing the amount of water utilized by the crop.

Avenues for genetic modification of radiation use efficiency in wheat

Radiation use efficiency (RUE) of a crop is a function of several interacting physiological phenomena, each of which can be tackled independently from the point of view of genetic improvement. Although wheat breeding has not raised RUE substantially, theoretical calculations suggest room for improvement. Selection for higher rates of leaf photosynthesis at saturating light intensities (Amax) has not resulted in improved RUE of crops, perhaps in part because most leaves in a canopy are not light-saturated. However, higher Amax may be observed as a pleiotropic effect of other yield-enhancing genes (e.g. genes for reduced height). Genetic transformation of Rubisco to double its specificity for CO2 would theoretically increase Amax by perhaps 20%, and some evidence suggests that photosynthesis at sub-saturating light intensities would also be improved. However, photo-protection may be jeopardized if capacity for oxygenase activity is impaired. Photosynthetic rate of the whole eanopy can be enhanced by manipulation of leaf angle, which is under relatively simple genetic control, and possibly by manipulating leaf-N distribution throughout the canopy. Genetic diversity for adaptation of lower canopy leaves (e.g. changes in chlorophyll a:b ratio) to reduced light intensity observed in some crops needs to be investigated in wheat. Improved RUE may be achieved by increasing sink demand (i.e. kernel number) if excess photosynthetic capacity exists during grain filling, as suggested by a number of studies in which source-sink balance was manipulated. Some evidence suggests that improved sink strength may be achieved by lengthening the duration of the period for juvenile spike growth. Balancing source- and sink-strength is a complex genetic challenge since a crop will change between source and sink limitation as conditions vary during the day, and with phenological stage. Improved RUE will be partly a function of a genotype's ability to buffer itself against changes in its environment to match the demand imposed by its development. Analysis of the physiological basis of genotype by environment interactions may indicate avenues for genetic improvement. The genetic control of photosynthetic regulation may be elucidated in the future through the application of genomics. However, given a lack of specific knowledge on the genetic basis of RUE, empirical selection is currently the most powerful tool for detecting favourable genetic interactions resulting from crosses between lines with superior photosynthetic traits and other high yielding characteristics. Selection for superior segregants can be accelerated using rapidly measured physiological selection traits, such as stomatal conductance or canopy temperature depression.

Photosynthesis at High Temperature in Tuber-Bearing Solanum Species : A Comparison between Accessions of Contrasting Heat Tolerance

Differences in the photosynthetic performance between pairs of heat tolerant (HT) and heat sensitive (HS) accessions of tuber-bearing Solanum species were measured at 40 degrees C, after treating plants at 40/30 degrees C. After 1 to 9 days of heat treatment, both HT and HS accessions showed progressive inhibitory effects, primarily decreased rates of CO(2) fixation, and loss of leaf chlorophyll. These effects were most pronounced in the HS accessions. Stomatal conductivity and internal CO(2) concentrations were lower for both accessions at 40 degrees C especially for the HS accessions, suggesting that at ambient CO(2) concentrations, stomatal conductance was limiting CO(2) availability at the higher temperature. In the HT accessions, stomatal limitations were largely attributed to differences in vapor pressure deficit between 25 degrees and 40 degrees C, while the HS accessions exhibited significant nonstomatal limitations. The young expanding leaves of the HS accession showed some HT characteristics, while the oldest leaves showed severe senescence symptoms after 9 days at 40/30 degrees C. The data suggest that differences in heat sensitivity between HT and HS accessions are the result of accelerated senescence, chlorophyll loss, reduced stomatal conductance, and inhibition of dark reactions at high temperature.