Abstract
High temperature (HT) is a major environmental stress limiting oversummer production of nonheading Chinese cabbage (NHCC, Brassica campestris ssp. chinensis Makino). In the present study, the effects of HT on photosynthetic capacity, including light reaction and carbon assimilation, were completely investigated in two NHCC, ‘xd’ (heat-tolerant), and ‘sym’ (heat-susceptible). The two genotypes showed significant differences in plant morphology, photosynthetic capacity, and photosynthate metabolism (carboassimilation). HT caused a decrease in photosynthesis, chlorophyll contents, and photochemical activity in NHCC. However, these main photosynthetic-related parameters, including net photosynthetic rate (PN), maximal photochemical efficiency of PSII (Fv/Fm), and total chlorophyll content in ‘xd’, were significantly higher than those of ‘sym’ plants. The antioxidant contents and antioxidative enzyme activities of ascorbic acid-reduced glutathione cycle in the chloroplast of ‘xd’ were significantly higher than those of ‘sym’. Microscopic analyses revealed that HT affected the structure of photosynthetic apparatus and membrane integrity to a different extent, whereas ‘xd’ could maintain a better integrated chloroplast shape and thylakoid. Inhibited light reaction also hampered carbon assimilation, resulting in a decline of carboxylation efficiency and imbalance of carbohydrate metabolism. However, larger declined extents in these data were presented in ‘sym’ (heat-susceptible) than ‘xd’ (heat-tolerant). The heat-tolerant genotype ‘xd’ had a better capacity for self-protection by improved light reaction and carbon assimilation responding to HT stress.
Highlights
High temperature (HT) is a major environmental stress limiting oversummer production of nonheading Chinese cabbage (NHCC, Brassica campestris ssp. chinensis Makino)
Stem diameter, fresh, and dry weights of shoot in heat-susceptible genotype ‘sym’ (HT-sym) were significantly decreased by 32.08%, 19.19%, and 23.08%, respectively, whereas an increase in plant height was by 19.85% (Table 1)
In heat-tolerant genotype ‘xd’ (HT-xd), the stem diameter, fresh, and dry weights of shoot were decreased by 19.47%, 18.90%, and 12.77%, respectively, and the plant height was unaffected under HT
Summary
BREEDING, CULTIVARS, ROOTSTOCKS, AND GERMPLASM RESOURCES of leaves after dark adaptation, respectively; Fo#, Fs, and Fm# are the minimal, steadystate, and maximal fluorescence level in illuminated leaves, respectively These fluorescence parameters were used to calculate the following: 1) the actual photochemical efficiency of PSII, VPSII = (Fm# – Fs)/Fm#; 2) the photochemical fluorescence quenching coefficient, qP = (Fm# – Fs)/(Fm# – Fo#); 3) nonphotochemical quenching coefficient, qN = (Fm – Fm#)/(Fm# – Fo#); and 4) the maximal photochemical efficiency of PSII, Fv/Fm = (Fm – Fo)/Fm. Isolation of chloroplast. Reaction mixture contained 50 mM K-P buffer (pH 7.8), 0.2 mM EDTA, 10 mM GSSG, 2.4 mM nicotinamide adenine dinucleotide phosphate (NADPH), and chloroplast suspension (0.1 mg/mL Chl). Reaction mixture contained 50 mM Hepes-KOH buffer (pH 7.6), 1 mM NADPH, 5 mM AsA, and 0.14 unit of ascorbate oxidase and chloroplast suspension (0.1 mg/mL Chl). The data were statistical analyzed with SAS software (SAS Institure, Cary, NC) and Duncan’s multiple range test at the P < 0.05 level of significance
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