Different trellis systems on canopy microclimate and heat stress-responding physiology of Shine Muscat grape

Abstract:【Objective】With the ongoing impact of global warming, the frequency of extremely hot weather during summer have increased, and heat injury has become a significant disaster affecting grape production. Particularly in the southern regions, where grapes are primarily cultivated in controlled environments like greenhouse, and the relatively enclosed conditions intensify high-temperature stress (HTS). Therefore, studying how adult grapevines respond to HTS in natural surroundings is crucial. Assessing the physiological response of adult grapevines to HTS and choosing trellis systems that minimize HTS are vital for studying heat-resistant mechanisms and heat-resistant cultivation of grapes. 【Methods】With Shine Muscat grapevines as the experimental material, three trellis-system treatments of V-shaped (Vs), Flying Bird-shaped (Fs) and H-shaped (Hs) were implemented. Canopy temperature and humidity were collected from June to August. Subsequently, the levels of photosynthetic pigments, photosynthetic parameters, chlorophyll fluorescence parameters, and the expression of resistance genes[heat shock protein genes (Hsp), and heat shock transcription factor genes (Hsf), GLOS1] were assessed. Changes in leaf tissue structure of different tree shapes before and after high-temperature occurrence in the field were analyzed.【Results】The frequency and intensity of extreme temperature on the canopy during the hot months in summer were significantly lower in Fs and Vs compared to those in Hs. In August, the frequency of temperatures above 40 ℃ in Fs canopy decreased by 45.07% and 46.20% compared to those in Vs and Hs, respectively. Canopy humidity decreased gradually with the rising temperatures from June to August, with Fs having the highest humidity, followed by Vs, and the lowest in Hs. After 15 days of HTS, grapevine leaf thickness increased, with spongy tissue pores showing a notable increase. Compared to pre-high temperature, the thickness of palisade and spongy tissues of Fs changed less, while the thickness of spongy tissues of Vs increased by 43.79%, and the thickness of palisade and spongy tissues of Hs increased by 24.70% and 42.51%, respectively. As the number of days of HTS increased, the contents of chlorophyll a (Chl a), chlorophyll b (Chl b) and carotenoids (Car) in the leaves of all the three trellis systems first showed an upward trend and then a downward trend. Similarly, the net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) and intercellular CO2 concentration (Ci) also first showed an upward trend and then a downward trend. The decrease in Pn observed in the leaves of all the three trellis systems at the end of the stress period was accompanied by a significant decrease in Gs, Tr, and Ci, indicating that stomatal limiting factors were the primary reason for the decrease in Pn. Then, the results of correlation analysis showed that Pn was significantly positively correlated with photosynthetic pigment content and photochemical efficiency. However, no significant correlation was observed between Pn and Gs, Tr, and Ci, which suggests that non-stomatal limiting factors such as the reduction of photosynthetic pigment content and the impairment of the photosynthetic apparatus also caused the decrease in Pn. During the experiment, the chlorophyll and carotenoid contents were highest in Fs, followed by Vs, and lowest in Hs. The photosynthetic pigment contents of Fs were 6.74% and 49.87% higher than those of Vs and Hs after 6 days of HTS, respectively, and Pn of Fs was 14.21% and 76.22% higher than those of Vs and Hs after 15 days of HTS, respectively. Initial fluorescence (Fo) and non- photochemical quenching coefficient (NPQ) constantly increased. The maximum fluorescence yield (Fm) and photochemical quenching coefficient (qP) increased first and then decreased. The maximum energy conversion efficiency (Fv/Fm), potential photochemical activity (Fv/Fo) and actual photochemical efficiency (ΦPS Ⅱ ) remained stable in the early stages of HTS but decreased rapidly later. The ETR first decreased slightly followed by a slight increase, and then a rapid decrease after 6 days of HTS. During the period of HTS, the change range of the chlorophyll fluorescence parameters in Hs was greater, leading to significantly higher levels of Fo and NPQ compared to Vs and Fs (p＜0.05). However, the remaining fluorescence parameters of Hs were notably lower than those of Vs and Fs. No significant differences were observed between Vs and Fs, except for NPQ, which was significantly higher than that of Fs. In this experiment, HTS induced the expression of resistance genes in the leaves of all three trellis systems. The relative expression levels of VvHSP17.9 and VvHSP90 increased, but the other heat shock protein genes (VvHSP22, VvHSP70, VvHSP100, and VvHSP101) and three heat shock transcription factor genes (VvHSFA1, VvHSFA2, and VvHSFB1) along with GLOS1 showed a general trend of first increasing followed by a decreasing trend. Compared to Vs and Hs, Fs showed the highest up-regulation of the nine genes mentioned above except for VvHSP100 under HTS. The expression levels of VvHSP22, VvHSP101, VvHSFA1, VvHSFA2, VvHSFB1, and GLOS1 were significantly higher in Vs than those in Hs. Conversely, the expression levels of VvHSP17.9 and VvHSP70 were significantly lower in Vs than those in Hs. The results of correlation analysis indicated thatVvHSP70, VvHSP101, and VvHSFB1 were significantly positively correlated with Chl a, Chl b, Car, Pn, Gs, Ci, Fm, and qP. Conversely, VvHSP17.9, VvHSP90, VvHSFA1, and GOLS1 exhibited significant negative correlations with Fv/Fm, Fv/Fo, ΦPSII, and ETR. VvHSP22, VvHSP100 and VvHSFA2 showed no significant correlation with photosynthetic pigments, photosynthetic parameters, or chlorophyll fluorescence parameters. Under high-temperature treatment, the canopy temperature was lower and humidity higher in Fs, leading to a reduction in the combined stress of high temperature and drought. Additionally, Fs had fewer changes in leaf tissue structure, higher photosynthetic pigment content and photosynthetic rate, a more stable PSⅡ reaction center, and higher expression levels of resistance genes like VvHSP, VvHSF and GLOS1 compared to those of Vs and Hs.【Conclusion】The long-term high-temperature treatment destroyed the tissue structure of grape leaves, reduced the PSⅡ activity and inhibited photosynthesis of the three trellis systems. Compared with Vs and Hs, Fs had stronger resistance to HTS and was more adaptative to high-temperature environments. Hs had the lowest heat resistance.