Effects of high temperature stress on photosynthesis and gene expression of different grape cultivars

Abstract:【Objective】Grape is one of the most important fruit tree species in the world. Solar green-house cultivation can realize the promotion and delay of ripening of grape berries, but sometimes it also can cause high-temperature damage, resulting in reduction of the quality and yield of the berries and even the death of the plants. Therefore, evaluation of the physiological response of grapes to high-tem-perature stress (HTS) and selection of heat-resistant grapes suitable for greenhouse cultivation are of great significance for the breeding of heat-resistant varieties.【Methods】In this experiment, five grape cultivars; Xinyu (Xy), Miguang (Mg), Summer Black (Sb), Beihong (Bh) and Flame seedless (Fs) were used as experimental materials. Photosynthetic parameters and chlorophyll fluorescence parameters were measured on the 0, 2, 4 and 6 d after the treatments of high temperature (day 45±2 ℃/ night 22±2 ℃) and the control temperature (day 42±2 ℃/ night 22±2 ℃) in the solar greenhouse.【Results】As the number of days of HTS increased, the total chlorophyll content (Chl), transpiration rate (Tr), net pho-tosynthetic rate (Pn), Intercellular CO2 concentration (Ci) and stomatal conductance (Gs) firstly showed an upward trend and then a downward trend. Initial fluorescence (F0), non-photochemical quenching co-efficient (CNPQ) and photochemical quenching coefficient (CqP) firstly showed a decreasing trend and then an increasing trend. The maximum fluorescence yield (Fm), maximum energy conversion efficien-cy (Fv/Fm), maximum fluorescence under light (Fm’), actual photochemical efficiency (ΦPSⅡ), poten-tial photochemical activity (Fv/F0) showed a continuous downward trend. The rapid rise or fall of each parameter mostly occurred on the second to sixth days when the high-temperature continued. On the 4th day of HTS, the Cal content of Xy, Mg, Sb, Bh and Fs increased by 74.8%, 80.5%, 85.0%, 51.4% and 120% compared with 0 d. The Fs total chlorophyll content (Chl) was 1%､18%､18%､19% higher than those of Xy, Mg, Sb and Bh. The changes in chlorophyll a (Chl a), chlorophyll b (Chl b) and carotenoids (Car) were consistent with the total chlorophyll content, and the content of Xy and Fs were higher. Pn was a direct manifestation of the function of the photosynthetic system. When the Pn of the five grape caltivars decreased due to HTS, the Gs, Tr and Ci decreased significantly too, implying that the decrease of Pn would be caused by the limiting factors of stoma. The Pn of Xy was 11%, 29%, 3% higher than those of Sb, Bh, and Fs, but lower than Mg. This result indicated that the photosynthesis of Sb and Bh was effected by the high temperature worse than other cultivars. The F0, CNPQ and CqP values of Bh were always higher. Among of them, F0 of Bh was 1.62,1.25,1.28, and 1.20 times as much as those of Xy, Mg, Sb and Fs, indicating that the PSII optical system of Bh had the highest degree of damage. Un-der high-temperature treatment; Fm, Fv/Fm, Fm’, ΦPSⅡ, Fv/F0 had the least decrease in Fs, followed by Xy and Bh had the highest decline, indicating that Xy and Fs had strong heat resistance, and Bh had poor heat resistance. According to the order of the principal component scores, the heat resistance F from large to small was: Xy>Fs>Sb>Mgh>Bh. The RNA-seq was conducted on the leaves of Xy and Bh after one week of HTS, P value and log2FC were used to screen the DEGs, with the conditions of pvalue ≤ 0.05 and |log2 fold change| ≥1. It was found that 32 genes related to heat shock protein (Hsp) and heat shock transcription factor (Hsf) were differentially expressed. Among them, 7 differential genes were ex-pressesed in Xy, and 4 differential genes were specifically expressed in Bh. Hsp20, Hsp70, Hsp90,Hsp101 in grape leaves were up-regulated by heat stress. In this experiment, the heat shock transcription factor HsfA2 was up-regulated in two cultivars, indicating that HsfA2 might also play an important regu-latory role in the heat tolerance of grapes. At the same time, the B and C types of Hsf were also differen-tially expressed.【Conclusion】The high-temperature stress would cause obvious damage to grapes and reduce their photosynthesis and even cause reversible or irreversible damage to plants of grape. The physiological indicators such as Chl a, Chl, Pn, F0, Fv/Fm, Fv/F0 could be used as the main indicators for the heat resistance evaluation. With the increase of the time of the high-temperature stress, the 5 culti-vars tested showed significant differences in the tolerance, to the high-temperature stress. The long-term high-temperature treatment destroyed the PSII activity of grape leaves and decreased the net photosyn-thetic rate of the 5 grape varieties due to stomata limiting factors. A comprehensive evaluation of the high-temperature tolerance of the five grape cultivars was proposed. Compared with Mg and Sb, Xy and Fs had stronger resistance to HTS and were more able to adapt to high-temperature environments, Bh had the lowest heat resistance. Using log10 (FPKM) value for clustering, it was found that Hsp and Hsf of Xy and Bh were highly expressed after one week of HTS treatment, while the expression levels of different grape cultivars were different. The expression level of Xy was higher than that of heat-sensitive variety Bh.