Abstract: 【Objective】The daily cycle is a basic metabolic unit in maize growth period. Photosynthesis is one of the most sensitive physiological processes on the environmental change, and the photosynthetic carbon metabolism is the fundamental way for dry matter accumulation, the fixation and distribution of photosynthetic carbon under an environment with regular light and temperature changes determine the metabolic characteristics and environmental adaptability for a maize variety. In maize production, the environmental adaptability is different among varieties, and most of varieties have their appropriate regions for high-yielding and stable production. The aim of the experiment is to clarify the diurnal variation of photosynthetic rate, initial photosynthate concentration, and their responses under gradient soil drought stress for different maize hybrids, and give a reference for exploiting a new index in hybrid evaluation. 【Method】 The experiment was conducted with five maize hybrids (Zhengdan 958, Xianyu 335, Xundan 20, Nongda 108 and ND66) and four soil water regimes by setting the relative soil water content at 85% (adequate water supply control), 65%-70% (mild drought stress), 55%-60% (moderate drought stress) and 45%-50% (severe drought stress). Photosynthetic rate of ear leaf from 6:00 to 18:00 at 3 h intervals was determined, and sucrose concentration and starch concentration of ear leaf through day and night at 5 time points (0:00, 4:00, 10:00, 15:00 and 20:00) were synchronously determined at maize silking stage, and the contribution of unit photosynthetic rate to the increment of sucrose concentration (CISuC), contribution of unit photosynthetic rate to the increment of starch concentration (CIStC), and contribution to the ratio of sucrose increment/starch increment (CRSu/St) also were calculated.【Result】The maize photosynthesis presented a unimodal trend without “noon break” phenomenon in daytime, the rate peaked around noon under adequate water supply, but the peak decreased and appeared in advance as a result of drought stress, and it was consistent in different hybrids. The variation of sucrose, starch concentration in ear leaves presented a unimodal curve in the daily cycle, leaf sucrose concentration peak appeared earlier than that of starch but peak value was lower. The starch concentration decreased with stress aggravated, but the sucrose concentration was stable compared to that of starch. The increments of sucrose and starch concentration, and the ratio between them during the early period of photosynthesis in daytime were built relationships with photosynthetic accumulation during this period, respectively, and the CISuC, CIStC and CRSu/St of different maize hybrids under different water treatments were gotten. The CIStC of hybrid Nongda 108 was the highest and stable, and its CRSu/St was steadily lower than the other hybrids, so the hybrid belongs to the type of starch accumulation; the CRSu/St of hybrid Xianyu 335 was at the highest level, so Xianyu 335 benefits sucrose accumulation. 【Conclusion】 Under the conditions of this experiment, the contribution of unit photosynthetic rate to the ratio of sucrose increment/starch increment could be an index used for maize hybrid classification of photosynthetic carbon metabolism.

Key words: maize hybrid, photosynthetic carbon metabolism, diurnal variation, ratio of sucrose/starch, type of directivity to sucrose metabolism, type of directivity to starch metabolism, hybrid classification