夏玉米叶片水分变化与光合作用和土壤水分的关系

叶片是光合作用的重要器官,其含水量的变化必将影响光合作用,但关于叶片水分变化对光合作用的影响报道较少。以华北夏玉米为研究对象,利用三叶期不同水分梯度的持续干旱模拟试验资料,分析夏玉米叶片水分变化及其与叶片净光合速率和土壤水分的关系。结果表明:夏玉米叶片净光合速率对叶片水分变化的响应显著且呈二次曲线关系,叶片含水量约为70.30%时,叶片净光合速率为零;叶片含水量与土壤相对湿度呈非直角双曲线关系,叶片最大含水量约为85.14%。研究结果可为准确描述叶片水分变化对光合作用的影响及客观辨识夏玉米干旱的发生发展及监测预警提供参考。

Relationship of leaf water content with photosynthesis and soil water content in summer maize

Because leaves are the chief organs of photosynthesis, changes in leaf water content affect photosynthesis. Leaf water content can directly reflect crop growth and development, and indirectly reflect the degree of atmospheric drying, ability of soil to supply water, and drought tolerance of crops. Until now, soil water content and its effects on plant photosynthetic parameters have been investigated in numerous studies, but the influence of leaf water content on photosynthesis has not yet been reported. Estimation of this influence is essential for accurate simulation of photosynthesis. In this study, summer maize from north China, which suffers from frequent droughts, was used to determine the relationship of leaf water content with net photosynthetic rate (P_n) and soil water content, by performing water manipulation experiments using 3-leaf stage plants. On the basis of the average monthly natural precipitation for July, over 30 years from 1981 to 2010, in Baoding, Hebei Province, six watering treatments (7%, 20%, 40%, 60%, 80% and 100%) were used, with three replicates per treatment. The watering treatments with disposable irrigation were used to simulate the effects of consecutive droughts of different intensities on the photosynthetic characteristics of summer maize. The plants were not watered after July 2. The soil water content, fresh weight and dry weight of leaves, and net photosynthetic rate (on the same leaf on the same plant) were measured under sunny weather conditions every 1 to 2 weeks. The different watering intensities strongly affected the crop index of summer maize. The results indicated that the leaf water content of summer maize significantly affected the net photosynthetic rate, the two factors showed a quadratic curve relation under all weather conditions or developmental stages of plant growth, but with different curve coefficients. On an average, P_n reduced to zero when the leaf water content reached approximately 70.3%, this was considered as an important threshold in the photosynthetic physiological activity of leaves. It can provide the basis for dynamic monitoring of large-scale drought development and accurate assessment of drought degree in crops. Moreover, leaf water content showed a non-rectangular hyperbolic relationship with soil moisture, which differs from the linear relationship reported in the previous studies, because of the narrow water gradient. The leaf water content increased with increasing relative soil water content and then tended to plateau, which is called the leaf water holding capacity. According to the Michaelis-Menten equation, the maximum leaf water content was estimated to be 85.14%, using the double reciprocal Lineweaver-Burk plot. The leaf water holding capacity and the critical leaf water content are different for different crops; however, a consistency in the research methods and laws of response may provide a reference for understanding the effect of changes in leaf water content on photosynthesis. These results could contribute to objective identification of the occurrence and development of drought events in summer maize.