Physiological mechanisms controlling stress tolerance in maize

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Genetic yield improvement of maize in North America of about 1% per year during the past 70 years has been associated directly with increased stress tolerance. We have shown that increased stress tolerance in newer maize hybrids manifests itself under stressful conditions in enhanced dry matter partitioning to the ear and kernels during the period bracketing silking and in the enhanced ability of leaves to both stay green and to maintain high leaf carbon exchange rates, which is termed functional stay green. The physiological mechanisms that underlie the response of these two traits to stress can be quantified by examining maize genotypes that are equal in every respect other than the traits of interest (i.e., near isogenic lines). The objectives of the proposed research program are (1) to evaluate effective methodologies to assess effects of stress on both kernel-formation dynamics and functional stay green among large numbers of genotypes, (2) to identify QTLs for these traits by applying the effective procedures to QTL mapping populations, and (3) to quantify the physiological mechanisms underlying kernel-formation dynamics and functional stay green using recombinant inbred lines (RILs) at the extreme ends of the spectrum for their response to stress. Results of studies under Objective #1 are important because the current procedures to evaluate either trait are very complex and cannot be done practically for large number of genotypes (i.e., measuring growth rate of individual plants within a population for kernel-formation dynamics and measuring leaf carbon exchange rate [CER] throughout the grain-filling period within the canopy). Results of the studies under the second objective will identify RILs that can be used for the studies under the third objective. Studies on the physiological mechanisms underlying the stress tolerance response (i.e., Objective #3) will be carried out in our field hydroponic system in order to have maximum control over environmental variables and to enable in-depth investigations of dry matter accumulation and partitioning (including roots) and of leaf CER and fluorescence parameters throughout the life cycle. Stresses examined will include water, nitrogen and shade stress.