| The experiment was carried out in national maize engineering technology center (Shandong) and State Key Laboratory of Crop Biology from 2007 to 2008. We investigate the evolution of yield and physiology traits in china with maize hybrids and their parents which was popularized in production after 1960s, and analyze the physiology machanism of reponse to plant density of maize hybrids and their parents. This paper focused on photosynthetic carbon metabolism, study source-sink traits of maize hybrids and their parents released in different years. The main results as followed:1 Evolution Characteristics of Main Agronomic Traits with Genetic Improvement of Maize Hybrids and their Parents in ChinaCompared with 1980s and 1960s hybrids and their parents, the modern varieties had higher grain yield with the times evolution (P<0.05), accompanied with biomass and harvest index under two planting densities, indicating that the modern F1 hybrids had higher heterosis in their yields. The ear length of hybrids and their parents had less change, and greatly increased in ear diameter, kernel numbers per ear and 100-kernel weight with grain yield enhancing (P<0.05). Plant height and ear position height of hybrids was highest in 1980s, and those of their parents were heightened released in different years. The stem diameter increased, tassel branches reduced, and leaf orientation value (LOV) significantly enhanced of hybrids and their parents in different years (P<0.05). The individual leaf numbers, leaf area index(LAI), leaf area duration after anthesis and average net photosynthetic rate of 2000s hybrids and their parents were higher than those of the earlier varieties (P<0.05), especially at high density condition. The correlation and pathway analysis indicated that yield was positively correlated with ear diameter, kernel numbers per ear, 100-kernel weight, leaf area index and leaf orientation value (P<0.05), and 100-kernel weight, leaf area index and leaf orientation value of elite inbred lines showed more contribution in their yield.2 The grain filling traits and matter transportion of maize hybrids parents released in different yearsThe modern hybrids with higher grain yield had higher initiation potential(R0), and the appeared time of the highest rate delayed, the biggest production(Wmax) and the highest grain filling rate(Gmax) were higher than those of the earlier parents. Also, the modern hybrids parents maintained higher nutrient matter accumulation, stem-sheath matter exportation rate, stem-sheath matter contribution and higher nitrogen translocation efficiency. The modern hybrids parents maintained higher grain filling ability, which was closed correlated with stronger matter accumulation ability and higher nutrient matter translocation efficiency of modern maize parents.3 The evolution of photosynthesis efficiency in maize hybrids parents released in different yearsThe grain weight in the recently lines gradually increased released in different years, which was accompanied with the common enhancement of plant weight and harvest index. Also, the recently released lines had a better stay-green character, prolonging effective photosynthesis duration during leaf senescence owing to their higher chlorophyll content, soluble protein content, and larger leaf area at late growth stage. In recently released lines, photosynthetic rate (Pn), intercellular CO2 concentration (Ci) and stomatal conductance (Gs) showed higher values, and declined slowly at late growth stage obviously. Synchronously, Chlorophyll fluorescence parameters such as the maximal photochemical efficiency (Fv/Fm), the actual photochemical efficiency (ΦPSⅡ), photochemical quenching coefficient (qP) and non-photochemical quenching coefficient (NPQ) of recently released lines were higher than those of earlier released lines. We analyzed plant light energy partition indicated that more percentage energy partition was allocated to photo reaction center in recently released lines, but had a lower themal-dissipation of antenna pigment. These suggested recently released lines showed great advantages in photosynthetic performance, electron transfer, and light energy transform at the late growth stage, so the higher grain yield of recently released lines resulted from the common improvement of leaf physiological activity and photosynthetic efficiency in grain filling period.4 The photosynthetic diurnal Change traits and photosynthetic responses of maize hybrids parents released in different years. The modern maize hybrids parents maintained higher photosynthetic efficiency than those of earlier parents in diurnal Change, especially showed great advantages at noon. Three maize hybrids parents were not appeared photosynthetic midday depression. With the increase of light intensity, the modern parents showed higher photosynthetic potential. The modern hybrids parents had lower light compensation points and CO2 compensation points, and showed higher apparent quantum efficiency and carboxylic efficiency. These indicated that the modern hybrids parents maintained higher photosynthetic ability under low light intensity and low CO2 concentration.5 Effect of plant density on photosynthetic traits of maize hybrids and their parents released in different yearsThe modern hybrids and their parents maintained longer photosynthetica higher duration, and had higher leaf area index, chlorophyll content and soluble protein content, which delayed the process of senescence in maize plant. The modern hybrids and their parents had higher photosynthetic key enzymes activity, and showed higher photosynthetic efficiency, moreover, light energy utilization efficiency of the modern maize was higher than those of earlier maize, which could maintained the excellent structure and function of chloroplast components at the late of stage.6 Effect of source-sink change on yields and photosynthetic traits of maize hybrids and their parentsSource-sink change distinctly decreased grain yield and 100-grain weight of maize hybrids and their parents, and significantly at reduced sink treatment. The modern hybrids and their parents maintained higher chlorophyll content and soluble protein content, which prolonged the function of maize leaf and delayed the process of senescence in plant. The modern maize hybrids and their parents had higher photosynthetic efficiency and photochemistry efficiency, but not significant change by source-sink change, which was mainly caused by better chloroplast structure and more organelle numbers to sustain higher photochemistry efficiency.
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