Effects of weak light and high temperature stress after anthesis on flag leaf chlorophyll fluorescence and grain fill of wheat

A field experiment with wheat showed that after anthesis, the photosystem II (PS II), photochemical efficiency (Fv/Fm) and photosynthetic rate (Pn) of flag leaf decreased substantially, its actual quantum yield of PS II electron transport (phiPS II) and photochemical quenching co-efficient (qP) declined slightly, while its non-photochemical quenching (NPQ) increased slightly three days after shading, compared with the control. Three days after high temperature treatment, the Fv/Fm, Pn, phiPS II and qP decreased significantly, while the NPQ increased slightly. During the course of recovery after the stresses, the parameters fluorescence and Pn which were stressed by shading and high temperature at prophase recovered slightly, while those stressed at metaphase decreased all along, indicating that the damage at prophase was reversible, but the stress at metaphase accelerated the caducity of flag leaves. The grain-filling processes of different treatments were modeled with Logistic equation, which showed that the decline of per kernel weight was mainly caused by the decrease of mean filling rate, maximal filling rate, and slow-increase period filling rate. Filling duration, occurrence time of maximal filling rate, duration of slight-increase period, and slight-increase period filling rate were less affected by weak light and high temperature.