Effects of Shading on Photosynthetic Characteristics and Xanthophyll Cycle of Summer Maize in the Field

Abstract

A field experiment was conducted to study the effects of shading on photosynthetic characteristic and xanthophyll cycle of summer maize. One non-shading treatment was used as control and three shading treatments with a shading degree of 60% were designed in growing period ranging from tasseling to maturity (S1), from jointing to tasseling stage (S2) and whole growing period (S3), respectively. Two cultivars Zhengdan 958 and Zhenjie 2 were used as experimental materials. Results showed that grain yield decreased significantly under shading condition. Effects of different shading treatments on grain yield showed S3 > S1 > S2, and there was a yield reduction of 96.87% and 90.78% for Zhengdan 958 and Zhenjie 2 in S3. Compared with CK, photo- synthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), chlorophyll content decreased and the intercellular CO2 con- centration (Ci) increased at first and then decreased, with the rame tendency for two maize cultivars. The stomatal factor and the non-stomata factor had double impacts decreasing photosynthesis. The actual photo-chemical efficiency (ΦPSII), the xanthophyll cycle pool (A+Z+V) and the de-epoxidation extent of xanthophyll cycle (A+Z)/(A+Z+V) increased, and the maximal photo- chemical efficiency (Fv/Fm) and non-photochemical quenching (NPQ) increased. In other words, in a long time shading, the dis- tribution of sunlight captured by leaves changed and the start of the xanthophyll cycle promoted the heat dissipation of excess light energy. Meanwhile, the ratio of the energy of photosynthetic electron transport to the absorbed light reduced. Finally, the (A+Z)/(A+Z+V) and NPQ increased rapidly in early days after ceasing shading, showing that leaves adapted shading are sensitive to normal illumination in initial stage of light recovery and the enhancement of xanthophyll cycle restrains the damage of photo- synthetic structure by sunlight.