Identifying physiological processes limiting genetic improvement of ear fertility in wheat

Wheat (Triticum aestivum L.) is one of the three most important cereal crop grown globally and there is a gap between yield production and world demand. Previous studies on wheat have generally shown traits influencing the capacity of the grains to store assimilate (sink) to be better correlated with yield than traits influencing potential assimilate production (source). Therefore, strategies to improve ear fertility, defined as the number of grains per ear, are one of the most relevant features in the development of new cultivars and genetic improvement of yield potential. In the present study, novel large-ear phenotype (long rachis) is investigated as a trait to increase the number of grains per year, thus grains m", with the aim of identifying physiological processes limiting genetic improvement of ear fertility in wheat. Two advanced lines (NLI and NL2) developed by the International Centre for Maize and Wheat Improvement (CIMMYT) with novel ear morphology and one CIMMYT cultivar with conventional ear morphology Bacanora referred to as the parental spring wheat genotypes were characterized. In addition, 69 doubled-haploid (DR) lines from a cross between NL2 x Rialto (UK-bred cultivar) were used to investigate the physiological basis of improved ear fertility and yield potential in the novel material. Three field experiments were carried out at the CIMMYT experimental station in Cd. Obregon (Mexico) on the parental genotypes (2003/04, 2004/05 and 2005/06) while two field experiments were carried out for the NL2 x Rialto population (2004/05 and 2005/06). An additional experiment consisting of a subset of 15 lines of the NL2 x Rialto population was carried out in 2005/06. Two controlled-environment experiments were carried out at Sutton Bonington, University of Nottingham in 2004 and 2005 to investigate the developmental basis of the large-ear phenotype on the three parental genotypes. A post-anthesis (GS 61+ 14d) degraining treatment was imposed in field experiments examining the parental genotypes in 2004, 2005 and 2006 while a similar degraining treatment was carried out for the subset of 15 DR lines of the NL2 x Rialto population in 2006. A range of physiological traits related to ear fertility were measured on the parental genotypes and the DR lines including rachis length, spikelets ear"1, developmental stages, green area, radiation interception, radiation use-efficiency (RUE), dry matter production and partitioning, stem water soluble carbohydrate reserves, potential grain weight, grain weight and combine yield. In the growth-room experiments, the rate and duration of spikelet primordial production of the main shoots were measured. Present results showed that in the novel genotypes, longer rachis increased spikelets ear" and also grains ear-i. Thus, the novel genotypes showed greater ear fertility by having more grains ear" than the benchmark cultivar Bacanora. Grains m-2 was not actually increased in novel genotypes; indeed lower grains m-2 was found in NL2 compared to Bacanora. Heavier grains were found in the novel genotypes which had greater potential and final grain weight compared to Bacanora. The NL2 genotype possesses a tiller inhibition gene (tin) on chromosome 1A and this genotype had fewer ears m-2 than the other genotypes, and this trait also contributed to the lower grains m-2 ofNL2. The grains-to-ear DM ratio at GS 61 in NL2 was markedly lower compared to other genotypes. Results of the growth-room experiments showed that there was a developmental basis for the higher spikelets ear" observed in NLl and NL2 than Bacanora. A longer thermal duration from floral initiation to terminal spikelet was associated with a higher spikelet number in NL1 (27) and NL2 (29) compared to Bacanora (23). Since the growth-room experiments were carried out under long photoperiod (16 hrs) results also suggested that the large-ear phenotype may have been associated with the effects of 'earliness per se' genes. Grain weight of the parental genotypes did not respond differently to degraining. Averaging across parental genotypes and years, responses to degraining in the field experiments showed that although assimilate supply per grain was potentially increased by 100%, average grain weight was only increased by 15%. These findings indicated that grain yield was mainly limited by post-anthesis sink size in these experiments. In the subset of 15 DH lines experiment, there were different responses of the lines to degraining and the lines which showed larger responses of individual grain weight to degraining had lower grain weight in control intact ears. Results of the DH experiments showed that rachis length was positively correlated with spikelets ear", grains ear" and grains m-2 among the lines. There was also a positive phenotypic correlation between the grains-to-ear DM ratio at GS 61 and grains m-2 amongst the lines. However, the large-ear phenotype (long rachis, high spikelets ear") was not associated with greater grain yield due to a trade-off between grains m-2 and individual grain weight. The physiological mechanisms potentially explaining this trade off are analyzed. Harvest biomass was positively correlated with grain yield amongst the DH lines. So traits to improve biomass whilst maintaining harvest index may be important for future breeding. Present results showed a positive correlation between pre-anthesis RUE and harvest biomass amongst the subset of 15 lines of the NL2 x Rialto DH population. It is suggested that breeders might select for higher RUE (via high specific leaf weight) to improve grains m-2 and yield potential in future years

Robotics and autonomous systems for net-zero agriculture

Purpose of ReviewThe paper discusses how robotics and autonomous systems (RAS) are being deployed to decarbonise agricultural production. The climate emergency cannot be ameliorated without dramatic reductions in greenhouse gas emis-sions across the agri-food sector. This review outlines the transformational role for robotics in the agri-food system and considers where research and focus might be prioritised.Recent FindingsAgri-robotic systems provide multiple emerging opportunities that facilitate the transition towards net zero agriculture. Five focus themes were identified where robotics could impact sustainable food production systems to (1) increase nitrogen use efficiency, (2) accelerate plant breeding, (3) deliver regenerative agriculture, (4) electrify robotic vehicles, (5) reduce food waste.SummaryRAS technologies create opportunities to (i) optimise the use of inputs such as fertiliser, seeds, and fuel/energy; (ii) reduce the environmental impact on soil and other natural resources; (iii) improve the efficiency and precision of agri-cultural processes and equipment; (iv) enhance farmers’ decisions to improve crop care and reduce farm waste. Further and scaled research and technology development are needed to exploit these opportunities

Acclimation of leaf photosynthesis and respiration to warming in field-grown wheat

Climate change and future warming will significantly affect crop yield. The capacity of crops to dynamically adjust physiological processes (i.e. acclimate) to warming might improve overall performance. Understanding and quantifying the degree of acclimation in field crops could ensure better parameterization of crop and Earth System models and predictions of crop performance. We hypothesized that for field-grown wheat, when measured at a common temperature (25°C), crops grown under warmer conditions would exhibit acclimation, leading to enhanced crop performance and yield. Acclimation was defined as: (i) decreased rates of net photosynthesis at 25°C (A25) coupled with lower maximum carboxylation capacity (Vcmax25); (ii) reduced leaf dark respiration at 25°C (both in terms of O2 consumption, Rdark_O225; and CO2 efflux, Rdark_CO225); and (iii) lower Rdark_CO225:Vcmax25. Field experiments were conducted over two seasons with 20 wheat genotypes, sown at three different planting dates, to test these hypotheses. Leaf-level CO2 based traits (A25, Rdark_CO225, and Vcmax25) did not show the classic acclimation responses that we hypothesized; by contrast, the hypothesized changes in Rdark_O2 were observed. These findings have implications for predictive crop models that assume similar temperature response among these physiological processes, and for predictions of crop performance in a future warmer world

Linear discriminant analysis reveals differences in root architecture in wheat seedlings by nitrogen uptake efficiency

Root architecture impacts water and nutrient uptake efficiency. Identifying exactly which root architectural properties influence these agronomic traits can prove challenging. In this paper approximately 300 wheat plants were divided into four groups using two binary classifications, high vs. low nitrogen uptake efficiency (NUpE), and high vs. low nitrate in medium. The root system architecture for each wheat plant was captured using 16 quantitative variables. The multivariate analysis tool, linear discriminant analysis, was used to construct composite variables, each a linear combination of the original variables, such that the score of the wheat plants on the new variables showed the maximum between-group variability. The results show that the distribution of root system architecture traits differ between low and high NUpE wheat plants and, less strongly, between low NUpE wheat plants grown on low vs. high nitrate media

Wheat photosystem II heat tolerance: evidence for genotype‐by‐environment interactions

High temperature stress inhibits photosynthesis and threatens wheat production. One measure of photosynthetic heat tolerance is Tcrit – the critical temperature at which incipient damage to photosystem II (PSII) occurs. This trait could be improved in wheat by exploiting genetic variation and genotype-by-environment interactions (GEI). Flag leaf Tcrit of 54 wheat genotypes was evaluated in 12 thermal environments over 3 years in Australia, and analysed using linear mixed models to assess GEI effects. Nine of the 12 environments had significant genetic effects and highly variable broad-sense heritability (H2 ranged from 0.15 to 0.75). Tcrit GEI was variable, with 55.6% of the genetic variance across environments accounted for by the factor analytic model. Mean daily growth temperature in the month preceding anthesis was the most influential environmental driver of Tcrit GEI, suggesting biochemical, physiological and structural adjustments to temperature requiring different durations to manifest. These changes help protect or repair PSII upon exposure to heat stress, and may improve carbon assimilation under high temperature. To support breeding efforts to improve wheat performance under high temperature, we identified genotypes superior to commercial cultivars commonly grown by farmers, and demonstrated potential for developing genotypes with greater photosynthetic heat tolerance

Leaf photosynthesis and associations with grain yield, biomass and nitrogen-use efficiency in landraces, synthetic-derived lines and cultivars in wheat

Future genetic progress in wheat grain yield will depend on increasing above-ground biomass and this must be achieved without commensurate increases in N fertilizer inputs to minimise environmental impacts. Our objective was to quantify variation in grain yield, above-ground biomass and N-use efficiency (NUE) and associated traits in a panel of diverse hexaploid wheat germplasm comprising: (i) landraces from the AE Watkins collection, (ii) synthetic-derived hexaploid lines in a cv. Paragon spring wheat background and (iii) UK modern cultivars including cv. Paragon under low N and high N conditions. A field experiment was carried out in two seasons examining 15 genotypes (five landraces, five synthetic-derived (SD) hexaploid lines and five UK modern cultivars) under low N and high N conditions at Nottingham University farm, UK. Machine-harvested grain yield, above-ground biomass and NUE were measured. Physiological traits were assessed including flag-leaf light-saturated photosynthetic rate (Amax) and relative chlorophyll content (SPAD) under HN conditions; and flag-leaf senescence duration and rate and Normalized Difference Vegetative Index (NDVI) under LN and HN conditions. Under HN conditions, the modern cultivars overall produced higher grain yield than the SD lines (+9.7%) and the landraces (+60.4%); and the modern cultivars and SD lines also produced higher biomass than the landraces (30.3% and 28.4%, respectively). Under LN conditions, reduction in grain yield and biomass compared to HN conditions was least for the landraces (−1% and −8.6%, respectively), intermediate for the SD lines (−7.4 and −10.2%, respectively) and highest for the modern cultivars (−9.3 and −24.6%, respectively). As a result, the SD lines had higher biomass (+17%) than the modern cultivars under LN conditions. Under HN conditions the synthetic derivatives (23.8 μmol m−2 s−1) and modern cultivars (241.1 μmol m−2 s−1) had higher pre-anthesis Amax than the landraces (19.7 μmol m−2 s−1) (P < 0.001). Pre-anthesis Amax was strongly positively linearly associated with above-ground biomass (R2 = 0.63, P < 0.001) and grain yield (R2 = 0.75, P < 0.001) amongst the 15 genotypes. Flag-leaf Amax was also positively linearly associated with flag-leaf relative chlorophyll content at anthesis (R2 = 0.74; P < 0.001). Comparing the SD lines to the recurrent parent Paragon, under HN conditions one line (SD 22) had higher pre-anthesis flag-leaf Amax than Paragon (P < 0.05). Under LN conditions one line (SD 24, +27%) had higher yield than Paragon (P < 0.05) and two lines (SD 24 and SD 38, +32% and +31%, respectively) had more biomass than Paragon (P < 0.05). Our results indicated that introgressing traits from synthetic-derived wheat and landraces into UK modern wheat germplasm offers scope to raise above-ground biomass and grain yield in moderate-to-low N availability environments

Predicting dark respiration rates of wheat leaves from hyperspectral reflectance

Greater availability of leaf dark respiration (R dark) data could facilitate breeding efforts to raise crop yield and improve global carbon cycle modelling. However, the availability of R dark data is limited because it is cumbersome, time consuming, or destructive to measure. We report a non‐destructive and high‐throughput method of estimating R dark from leaf hyperspectral reflectance data that was derived from leaf R dark measured by a destructive high‐throughput oxygen consumption technique. We generated a large dataset of leaf R dark for wheat (1380 samples) from 90 genotypes, multiple growth stages, and growth conditions to generate models for R dark. Leaf R dark (per unit leaf area, fresh mass, dry mass or nitrogen, N) varied 7‐ to 15‐fold among individual plants, whereas traits known to scale with R dark, leaf N, and leaf mass per area (LMA) only varied twofold to fivefold. Our models predicted leaf R dark, N, and LMA with r 2 values of 0.50–0.63, 0.91, and 0.75, respectively, and relative bias of 17–18% for R dark and 7–12% for N and LMA. Our results suggest that hyperspectral model prediction of wheat leaf R dark is largely independent of leaf N and LMA. Potential drivers of hyperspectral signatures of R dark are discussed

Nitrogen partitioning and remobilization in relation to leaf senescence, grain yield and grain nitrogen concentration in wheat cultivars

Our objective was to investigate the determinants of genetic variation in N accumulation, N partitioning and N remobilization to the grain post-flowering and associations with flag-leaf senescence, grain yield and grain N% in 16 wheat cultivars grown under high N (HN) and low N (LN) conditions in the UK and France. Overall, cultivars ranged in leaf lamina N accumulation at anthesis from 5.32 to 8.03 g N m−2 at HN and from 2.69 to 3.62 g N m−2 at LN, and for the stem-and leaf-sheath from 5.45 to 7.25 g N m−2 at HN and from 2.55 to 3.41 g N m−2 at LN (P < 0.001). Cultivars ranged in N partitioning index (proportion of above-ground N in the crop component) at anthesis for the leaf lamina from 0.37 to 0.42 at HN and 0.34 to 0.40 at LN; and for the stem-and leaf-sheath from 0.39 to 0.43 at HN and from 0.35 to 0.41 at LN (P < 0.001). The amount of leaf lamina N remobilized post-anthesis was negatively associated with the duration of post-anthesis flag-leaf senescence amongst cultivars in all experiments under HN. In general, it was difficult to separate genetic differences in lamina N remobilization from those in lamina N accumulation at anthesis. Genetic variation in grain yield and grain N% (through N dilution effects) appeared to be mainly influenced by pre-anthesis N accumulation rather than post-anthesis N remobilization under high N conditions and under milder N stress (Sutton Bonington LN). Where N stress was increased (Clermont Ferrand LN), there was some evidence that lamina N remobilization was a determinant of genetic variation in grain N% although not of grain yield. Our results suggested that selection for lamina N accumulation at anthesis and lamina N remobilization post-anthesis may have value in breeding programmes aimed at optimizing senescence duration and improving grain yield, N-use efficiency and grain N% of wheat

Phenotyping pipeline reveals major seedling root growth QTL in hexaploid wheat

Seedling root traits of wheat (Triticum aestivum L.) have been shown to be important for efficient establishment and linked to mature plant traits such as height and yield. A root phenotyping pipeline, consisting of a germination paper-based screen combined with image segmentation and analysis software, was developed and used to characterize seedling traits in 94 doubled haploid progeny derived from a cross between the winter wheat cultivars Rialto and Savannah. Field experiments were conducted to measure mature plant height, grain yield, and nitrogen (N) uptake in three sites over 2 years. In total, 29 quantitative trait loci (QTLs) for seedling root traits were identified. Two QTLs for grain yield and N uptake co-localize with root QTLs on chromosomes 2B and 7D, respectively. Of the 29 root QTLs identified, 11 were found to co-localize on 6D, with four of these achieving highly significant logarithm of odds scores (>20). These results suggest the presence of a major-effect gene regulating seedling root vigour/growth on chromosome 6D