Study On Injury Mechanisms Of The Combined Cold, Poor Light And Salt Stresses In Pepper Seedlings

According to the common problem on combined cold, poor light and salt stresses in solar greenhouse vegetable production, pepper (Capsicum annuum L.) was selected as experimental material in this study to make a deep research on the effects of simulative cold, poor light and salt stress, individually and in combination, on the growth, photosynthetic characteristics, chlorophyll fluorescence parameter and reactive oxygen species metabolism in pepper seedlings, and investigate the injury mechanisms. The aim of study was to provide the evaluation indexes that reflected tolerance to combined cold, poor light and salt stresses in pepper cultivars.1. Evaluating the salt tolerance of pepper cultivars by multivariate analysis of variance, factor analysis and cluster analysis to classify ten cultivars into three categories, including salt sensitive cultivars, moderately salt sensitive cultivars and salt tolerant cultivars. Zhongjiao No.4 that was selected as material in this paper was belonged to salt sensitive cultivar.2. Compared to control, leaf plastochron index (PI), plant height and root-shoot ratio decreased significantly under the treatments of individual stresses and combined cold, poor light and salt stresses, among them, the inhibitory degree of combined stress of cold and salt was the most serious. Individual poor light stress and combined poor light and salt stresses increased Chla, Chlb and Chla+b content, decreased Chla/b ratio, and individual poor light stress made leaf area increased and specific leaf weight decreased in pepper seedlings, but other individual stresses and combined stresses were opposite change. And net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) obviously decreased under each stress treatment, descent range of which under combined stresses was more than under individual stresses, among them combined cold and salt stresses was the most influential stress. The limiting factor of different stresses leading to Pn decreasing on pepper seedlings was also different. Besides apparent quantum yield of photosynthesis (AQY) under individual poor light stress and CO₂ compensation point (CCP) under combined cold and poor light stresses increased, light compensation point (LCP), light saturation point (LSP), AOY, CCP, CO₂ saturation point (CSP), carboxylation efficiency (CE), Pn at light saturation point and maximum RuBP regeneration rate under other individual stresses and combined stresses especially significantly declined in pepper seedlings.3. The combined cold and salt stresses obviously decreased maximal PSâ…¡efficiency (Fv/Fm) of leaves in pepper seedlings, but other treatments had the less range in Fv/Fm decrease. And each treatment decreased actual PSâ…¡efficiency (ΦPSâ…¡), PSâ…¡potential activity, PSâ…¡effective photochemical quantum efficiency (Fv'/Fm'), photosynthetic active fluorescence quenching (qP) and electron transport rate (ETR) in different extent. Meanwhile, after every stress treatment, allocation of photons absorbed by PSâ…¡antenna to the photochemical reaction (Prate) decreased, but thermal energy dissipation rate (Drate) and reaction centre energy dissipation (Ex) increased. Drate was the main way of superfluous light allocation. The combined cold and salt stresses, cold and poor light and salt stresses had great influence on the above indexes.4. For the reactive oxygen species metabolism, compared to control, superoxide dismutase (SOD), glutathione peroxidase (GPX) and gultathione reductase (GR) activity increased under each stress treatment in different degree, but individual cold, poor light, salt stress and combined cold and poor light stresses, poor light and salt stresses resulted in a decrease of catalase (CAT) and a increase of ascorbate peroxidase (APX) activity, combined cold and salt stresses, cold and poor light and salt stresses had the opposite change. Therefore, starting reactive oxygen scavenging ways was not identical under different stress treatment. And the rate of O₂·^- production, H₂O₂ content and MDA content increased in different degree under different stress treatments, among them combined cold and salt stresses had the most important influence, combined cold and poor light and salt stresses ranked secondly.5. Analyzing the relationship of each stress factor showed the correlation between cold and poor light, cold and salt, poor light and salt belonged to cross-synergism. The correlation between combined cold and salt stresses and individual poor light stress belonged to cross-adaptation, namely poor light relief the injury on pepper seedlings from cold and salt stress. There were interactive effects on different indexes between two stress factors or among three stress factors.6. Conducting factor analysis for physiological indexes under the combined stresses to determine the main influence indexes, that were selected as first and important indexes for envaluating sress tolerant cultivars. Pn might be selected as main index by factor analysis for envaluating cold and poor light tolerant cultivar, leaf area and yield were as assistant index, the result indicated that Zhongjiao No.5, Zhongjiao No.11 among sweet pepper varieties, and Zhongjiao No.6, Zhongjiao No.10 among hot pepper varieties were better tolerant varieties in natural living through winter solar greenhouse.