Effects of Foliar-Applied L-Glutamic Acid on the Diurnal Variations of Leaf Gas Exchange and Chlorophyll Fluorescence Parameters in Hawthorn (Crataegus pinnatifida Bge.).

Diurnal variations of gas exchange and chlorophyll fluorescence (CF) parameters were measured in hawthorn (Crataegus pinnatifida Bge.) six days after foliar application of L glutamic acid (Glu) (0, 200, 400 and 800 mg L⁻¹) that possibly affect flavonoids metabolite. The net photosynthetic rate (An), carboxylation efficiency (CE), maximum carboxylation velocity of Rubisco (Vcmax), chlorophyll (Chl) content and stomatal limitation (Ls) were higher in 800 mg L⁻¹ exogenous glutamic acid treatment than in control. The application of 800 mg L⁻¹ Glu always resulted in a higher An, particularly at noon. Although a midday depression in An occurred in all the treatments, the depression was less obvious in the 800 mg L⁻¹ Glu treatment when compared to the control. The electron transport per active reaction center (ET₀/RC) was similar in all the treatments and the control, but the maximal fluorescence level (Fm), the maximal quantum yield of photosystem II (PSII) (Fv/Fm), the potential quantum yield of PSII (Fv/Fo), the maximal quantum yield of PSII after dark adaptation (ΦPo), as well as the probability that an absorbed photon will move an electron into the electron transport chain beyond QA (ΦEo) were significantly higher than the control with the application of 800 mg L⁻¹ Glu. A 800 mg L⁻¹ Glu application enhanced energy absorption per active reaction center (ABS/RC) and energy trapping per active reaction center (TR₀/RC) of PSII, which, consequently, increased energy transformation and electron conversion efficiency and decreased energy dissipation per active reaction center (DI₀/RC). These findings suggest that the application of Glu could improve photosynthetic efficiency and the energy capture of antenna pigments for photochemical electron transport, as well as reduce photoinhibition.