Effects of Low Light on Photosynthetic Characteristics of Different Position Leaves of Cucumber Seedlings

【Objective】The aim of this study was to investigate the effects of low light on the photosynthetic characteristics of different position leaves of seedlings in two cucumber (Cucumis sativus L.) varieties and to try to offer some academic base for the cucumber production in solar greenhouse in winter.【Method】Shade-susceptible ‘Jinyan 2’ and shade-tolerant ‘Deltastar’ were planted in the growth chamber. The gas exchange parameters and the chlorophyll a fluorescence parameters of different position leaves in seedlings were measured by the LI-6400 Portable Photosynthesis System and the PAM-2100 Chlorophyll Fluorometer under the low light [75-85 μmol·m-2·s-1, 25℃ (day)/18℃(night)] and control (500-600 μmol·m-2·s-1). 【Result】After treated for 13 days under the low light, the net photosynthesis rate (Pn), stomatal conductance (Gs), stomatal limitation value (Ls), transpiration rate (Tr) and water use efficency (WUE) of different position leaves (from 3rd to 5th) were basically declined and the intercellular CO2 concentration (Ci) was basically increased, especially obvious in middle position leaves in the two cultivars. The principal factorwhich resulted in the decrease of the Pn was the non-stomatal restriction. Under the low light, the photosystem Ⅱ (PSⅡ) actual photochemical efficiency (ΦPSⅡ), the photochemical reaction rate (Prate), the linear whole-chain electron transport rate (J) and the partitioning proportion for photochemical reaction of light energy absorbed by PSⅡ (P) of middle and up position leaves (from 3rd to 5th) of seedlings were declined. At the same time, the imbalance of partitioning of excitation energy between PSⅠ and PSⅡ (β/α-1) was increased. The photochemical reactions of PSⅡ of middle position leaves of seedlings were strongly restrained by the low light. The photosynthetic and fluorescent parameters, such as Pn, ΦPSⅡ, J and P etc., were basically decreased from the bottom to the top leaves in ‘Jinyan 2’ under the low light, but the β/α-1 was increased. Nevertheless, the changing of these parameters just mentioned in ‘Deltastar’ was similar to its control under the low light. Namely, the photosynthetic efficiency and photochemistry activity were stronger in middle position leaves in ‘Deltastar’.【Conclusion】The photosynthetic assimilation and photochemistry activity of PSⅡ of different position leaves are restrained in seedlings of cucumbers under the low light, especially obvious in middle and up position leaves. There are some differences in the effects of low light on photosynthetic characteristics of different position leaves of seedlings in two different tolerance-low-light cucumbers. The capacity of sustaining photosynthesis of whole plant in ‘Jinyan 2’ is weaker than the one in ‘Deltastar’ under the low light.