Abstract:Depending on the amount of light available during growth, plants possess the ability to react to the amount of available light during growth using two distinct growth-responses: the strong-light growth-response as found at high light quanta and the weak-light growth-response, which is seen in shade leaves and plants growing under low-light. This ability of plants and chloroplasts to adapt to light is a fundamental growth-response, which is associated with specific changes in the morphology, physiology, biochemistry and structure of leaves and chloroplants.
In wheat-cotton intercropping systems, cotton seedlings are shaded by wheat plants for about 40-45 days and then exposed to direct sunlight and high ambient temperatures and low humidity immediately after wheat plants are harvested. The growth of cotton plants declines for about 20 days following this high light exposure and then gradually acclimate, but the squaring and inflorescence period are delayed. In this study, the effects of shading (40% of natural light intensity) on photosynthesis and chlorofluorescence characteristics of cotton leaves were examined using chlorophyll fluorescence and gas exchange techniques. Compared with cotton seedlings grown in full sunlight, a lower photosynthetic rate, lower PSⅡ activity and lower efficiency of primary energy conversion as well as lower electron transport rate were found in plants grown under shaded conditions (40% sunlight). Light saturation point (LSP), light compensation point (LCP), CO2 saturation point (CSP) and CO2 compensation point (Γ) also were higher in leaves grown in full sunlight than under 40% sunlight. Upon sudden exposure from low to high photon flux density (PFD), the net photosynthetic rate (Pn), stomatal conductance (Gs), actual PSII efficiency (ΦPSⅡ) and non-photochemical quenching (NPQ) of full light leaves increased to maximal levels in a short period, whereas it took a much longer time for those of shaded leaves to reach maximal levels. The shaded cotton leaves were more susceptible to photoinhibition due to low CO2 assimilation and protective mechanisms, such as xanthophylls cycle-dependent dissipation of excessive energy.