The Eco-physiological Mechanisms Of Photosynthetic Capacity Improvement And Yield Increase Of Maize Hybrids Released In Different Years

Changes in physiological attributes related to genetic improvement for grain-yield were studied for six Chinese maize (Zea mays L.) hybrids that widely grown in North China during the past 50 years to analyze the eco-physiological mechanisms that lead to the differences in yield and photosynthetic capacity between the newer and older hybrids. Nitrogen deficiency and water stress were imposed to characterize the different stress tolerance ability between the newer and older maize hybrids and explore the possible eco-physiological mechanisms. We found that the newer cultivars could quickly enlarge leaf area before flowering and created more kernels at the lag phases of grain filling. After flowering, they could maintain Psat of leaves at different positions longer than the older hybrids. The post-anthesis decline of Psat in the older hybrids was associated with a reduction in the chlorophyll content and the soluble protein content. The newer hybrids and the older ones have different diurnal photochemical efficiency. A midday depression in photosynthetic rate occurred in all hybrids, which might be caused by serious photoinhibition. Then the newer hybrids have improved photosynthetic capacity: higher photosynthetic rate, larger green leaf area and longer leaf longevity. The newer hybrids not only could establish larger source and sink than the old ones, but also have the ability to optimally partition assimilations into leaf, shoot, root and grain. We also found that the decreased Psat due to N shortage in the older hybrids was associated with the reductions in non-stomatal limitation as can be seen by the decreased PEPCase activities, the drop of chlorophyll and soluble protein concentration. N-deficient also accelerated the leaf senescence rate in the older hybrids. The greater resistance of the newer hybrids to water stress was primary due to their greater capability to maintain high leaf water potential during water stress, which ensures better preservation of the function of the photosynthetic apparatus and rapid recovery of photosynthetic capacity upon re-hydration. Under N deficiency or water stress, the newer hybrids could maintain high plant weight and grain weight, which is achieved by maintaining high photosynthetic rate and green leaf area longer than the older ones after flowering.