Genetic variation in N-use efficiency and associated traits in Indian wheat cultivars

Nitrogen (N) fertilizer represents a significant cost for the grower and may also have environmental impacts through nitrate leaching and N⁠2O (a greenhouse gas) emissions associated with denitrification. The objectives of this study were to quantify the genetic variability in N-use efficiency (NUE) in Indian spring wheat cultivars and identify traits for improved NUE for application in breeding. Twenty eight bread wheat cultivars and two durum wheat cultivars were tested in field experiments in two years in Maharashtra, India. Detailed growth analysis was conducted at anthesis and harvest including dry matter (DM) and N partitioning. Senescence of the flag leaf was assessed from a visual score every 3–4 days from anthesis to complete flag-leaf senescence and fitted against thermal time to estimate the onset and end of post-anthesis senescence. Grain yield (GY) was reduced under low N (LN) by an average of 1.59t ha⁠−1 (−28%). Significant N×genotype level interaction was observed for grain yield and NUE. Above-ground N uptake at harvest was reduced from 162kg N ha⁠−1 under high N (HN) to 85kg N ha −1 under low N (LN) conditions, while N-utilization efficiency (grain DM yield per unit crop N uptake at harvest; NUtE) increased from 32.7 to 44.6kg DM kg −1 N. Genetic variation in GY under LN related mainly to variation in N uptake at harvest rather than NUtE; and the N×genotype effect for GY was mainly explained by the interaction for N uptake at harvest. Averaging across years, the linear regression of onset of flag-leaf senescence on GY amongst cultivars was significant under both HN (R2 0.16. p<0.05) and LN (R2 0.21, p<0.05) conditions. Onset of flag-leaf senescence was positively associated with N uptake at anthesis under HN (R2 0.34, p<0.001) and LN (R2 0.22, p<0.01) conditions. Flag-leaf senescence timing was not associated with post-anthesis N uptake. It is concluded that increased N accumulation at anthesis was correlated with flag-leaf senescence timing and that N accumulation at anthesis is an important trait for enhancing grain yield and NUE of wheat grown under low to moderate N supply in India

Mass screening of rice mutant populations at low CO2 for identification of lowered photorespiration and respiration rates

Introduction: Identifying rice (Oryza sativa) germplasm with improved efficiency of primary metabolism is of utmost importance in order to increase yields. One such approach can be attained through screening genetically diverse populations under altered environmental conditions. Growth or treatment under low carbon dioxide (CO2) concentrations can be used as a means of revealing altered leaf photorespiration, respiration and other metabolic variants. Methods: We developed a pipeline for very high throughput treatment of gamma- and ethyl methanesulfonate- (EMS) induced mutant populations of IR64 rice seedlings at very low CO2 for 7 days. 1050 seedlings per batch at 5th leaf stage were exposed to 60 ppm CO2 for the first day and 30 ppm for the remaining three days. Following this, putative candidates were identified by measuring chlorophyll depletion using SPAD. Screening results showed a distinct difference between the mutants and the WTs. Results and discussion: The mean chlorophyll loss in WTs ranged from 65% to 11% respectively, whereas in the mutant lines chlorophyll loss ranged from 0 to 100%, suggesting considerable phenotypic variation. Rice mutants with a reduced chlorophyll reduction (<10%) were identified as ‘Chlorophyll retention mutants’ (CRMs) under low CO2 stress. In total, 1909 mutant lines (14,000 seedlings) were screened for chlorophyll content under 30 ppm CO2, with 26 lines selected for detailed screening. These 26 putative candidates were self-seeded to produce an M5 generation, used to determine the genetic control of the altered response to low CO2. Gas exchange of light and CO2 response revealed that there were significant variations among photosynthetic properties in two selected rice mutants. The CO2 compensation points in the absence of photorespiration and leaf respiration rates were lower than the WTs and anatomical analyses showed that CRM 29 had improved mesophyll cell area. We propose that this approach is useful for generating new material for breeding rice with improved primary metabolism

Chlorophyll fluorescence-based high-throughput phenotyping facilitates the genetic dissection of photosynthetic heat tolerance in African (Oryza glaberrima) and Asian (Oryza sativa) rice.

Acknowledgements We are grateful to the University of Nottingham glasshouse staff for their assistance with general plant maintenance. We acknowledge the insight of two anonymous reviews whose comments greatly improved this manuscript. JR and JNF were supported by the Palaeobenchmarking Resilient Agriculture Systems (PalaeoRAS) project funded by the Future Food Beacon of the University of Nottingham.Peer reviewedPostprin

Rice plants overexpressing OsEPF1 show reduced stomatal density and increased root cortical aerenchyma formation

Stomata are adjustable pores in the aerial epidermis of plants. The role of stomata is usually described in terms of the trade-off between CO2 uptake and water loss. Little consideration has been given to their interaction with below-ground development or diffusion of other gases. We overexpressed the rice EPIDERMAL PATTERNING FACTOR1 (OsEPF1) to produce rice plants with reduced stomatal densities, resulting in lowered leaf stomatal conductance and enhanced water use efficiency. Surprisingly, we found that root cortical aerenchyma (RCA) is formed constitutively in OsEPF1OE lines regardless of tissue age and position. Aerenchyma is tissue containing air-spaces that can develop in the plant root during stressful conditions, e.g. oxygen deficiency when it functions to increase O2 diffusion from shoot to root. The relationship with stomata is unknown. We conclude that RCA development and stomatal development are linked by two possible mechanisms: first that reduced stomatal conductance inhibits the diffusion of oxygen to the root, creating an oxygen deficit and stimulating the formation of RCA, second that an unknown EPF signalling pathway may be involved. Our observations have fundamental implications for the understanding of whole plant gas diffusion and root-to-shoot signalling events

Short‐term nitrogen‐induced modulation of phospho enol pyruvate carboxylase in tobacco and maize leaves

International audienceUntransformed maize and tobacco plants and tobacco plants constitutively expressing nitrate reductase were grown with sufficient NO−3 to support maximal growth. Four days prior to treatment the tobacco plants were deprived of nitrogen. Excised maize leaves and tobacco leaf discs were fed with either 40 mM KNO 3 or 40 mM KCl (control) in the light. Phospho enol pyruvate (PEP) carboxylase (Case) activity was measured at 0.3 mM and 3 mM PEP. The light‐ induced increase in PEPCase Vmax was greater in maize than tobacco. Furthermore light decreased malate sensitivity in maize (which was N‐replete) but not in N‐deficient tobacco. NO−3 treatment increased PEPCase Vmax values in both species and decreased the sensitivity to inhibition by malate, but effects of NO−3 were much more pronounced in tobacco than maize. PEPCase kinase activity was, however, greater in maize leaves fed NO−3 than in the Cl − ‐treated controls, suggesting that it is responsive to leaf nitrogen supply. A correlation between foliar glutamine content and PEPCase activity was observed. It is concluded that PEPCase is sensitive to N metabolites which favour increased flow through the anapleurotic pathway in both C 3 and C 4 plants