Effects of low night temperature on pigments, chl a fluorescence and energy allocation in two bitter gourd (Momordica charantia L.) genotypes

Using two different inbred lines of Momordica charantia (bitter gourd), Y-106-5 and Z-1-4, the cell membrane stability, leaf water potential, pigment contents and the chlorophyll a fluorescence were investigated with different low night temperature (LNT) treatments over a 7 day time period and the sequent a 7 day recovery. Under LNT treatments, electrolyte leakage increased in both inbred lines and it increased more significantly in Y-106-5 plants than that in Z-1-4. The content of Chl b and total Chl decreased, while the Chl a/b ratio increased in stressed plants of the two lines. Almost all LNT treatments induced little change in Chl a content in Z-1-4 whereas obvious decreases in 5 and 8°C treated Y-106-5 plants were observed. Chilling changed the water status of plants and induced decreases of leaf water potential (LWP) in 5 and 8°C treated plants. LNT treatments also resulted in changes in the chlorophyll fluorescence parameters in bitter gourd leaves. The potential PSII activity (Fv/Fo) was reduced obviously by LNT stress and showed more sensitive to LNT than the maximum quantum efficiency of PSII primary photochemistry (Fv/Fm). The efficiency of open PSII centers \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(F_{\rm v}^{\prime}/F_{\rm m}^{\prime})$$\end{document} exhibited a slight decrease whereas the photochemical quenching efficient (qP) was affected more seriously by LNT stress in both two inbred lines. The allocation of energy was rearranged by LNT stress. The light fraction used for PSII photochemistry (P) was reduced, while that used for heat dissipation (D) and the third fraction of absorbed light defines excess energy (E) increased due to the chilling stress. The impacts of LNT stress on bitter gourd generally increased with the number of LNT chilling and the severe night chilling. Plants were little affected by 12°C night chilling and the most acute damage was found in 5°C night chilling treatments. A 7 day recovery mitigated the adverse effects of LNT for both lines and almost all LNT treated plants restored to control levels except 5°C night chilling treated Y-106-5 plants. The two lines have a variance in tolerance to LNT stress and display obvious differences of phenotypes under extreme conditions.