Abstract:

Using three Actinidia arguta cultivars as experimental materials，the relative electrical conductivity（REC），gas exchange and chlorophyll fluorescence parameters，physio-biochemical indexes and field growth performances，heat injury index under artificial-manipulated and ambient high temperature stresses were measured. The heat tolerance was evaluated using fuzzy membership functions over multiple indictor varaibles. The results showed that the relationships between REC and temperature in three tested cultivars conformed well with the Logistic equation. The variation in REC of all A. arguta cultivars with artificial-manipulated increasing temperature followed a S shaped curve. The semi-lethal temperature（LT₅₀）for‘Longcheng 2’‘Minda’and‘Kangtian’were estimated to be 44.60，41.92 and 39.69 ℃，respectively. The failure time of‘Longcheng 2’and‘Minda’under their LT₅₀ were 85 and 76 min，respectively. REC，chlorophyll fluorescence parameters，malondialdehyde（MDA）content and free proline（Pro）content of the three cultivars showed little adjustemnts at < 35 ℃，while photosynthesis and physiological metabolism were drastically mediated at > 40 ℃，resulting in increased membrane peroxidation degree and decreased photosynthetic efficiency. Meanwhile，maximum photochemical efficiency（F_v/F_m），actual photochemical efficiency（Φ_PSⅡ），electron transport rate（ETR），photochemical quenching（q_P），total chlorophyll content[Chl（a + b）]，net photosynthetic rate（P_n），water use efficiency （WUE），transpiration rate（T_r）and Pro of‘Longcheng 2’were the highest among three cultivars，and significantly higher than those of‘Kangtian’. On the contrary，‘Kangtian’was the highest with respect to the yield for dissipation by down-regulation（Y_NPQ），the yield of other non-photochemical losses（Y_NO），the relative limitation of photosynthesis（L_PTF），MDA and REC，indicating that its’ PSⅡ reaction center had been most damaged by high temperature. In the growing season，field-grown‘Longcheng 2’had faster growth in shoot length，base diameter，leaf thickness and higher relative chlorophyll content（SPAD），and its growth rate was significantly higher than that of the other two cultivars，with slighter leaf damage and the lowest heat injury index under high temperature. LT₅₀ was positively correlated with Φ_PSⅡ，ETR and q_P（P < 0.05），and negatively correlated with L_PTF and MDA（P < 0.01）. Heat injury index was negatively correlated with P_n，Chl（a + b），WUE，F_v/F_m，Pro，Φ_PSⅡ and q_P（P < 0.01），but positively correlated with REC and MDA（P < 0.01）. Simutenously，the heat injury index was negatively correlated with growth rate and LT₅₀（P < 0.01），and the LT₅₀ was positively correlated with the growth rate（P < 0.01）. The rank of heat tolerance in terms of semi-lethal temperature was consistent with the results of a comprehensive evaluation based on physiological indexes and heat injury index. These results indicate that REC，photosynthetic parameters，physio-biochemical indexes and heat injury index can be used to indicate heat tolerance of A. arguta cultivars. Altogether，the rank of heat tolerance was determined as follow：‘Longcheng 2’（strong heat tolerance）>‘Minda’（medium heat tolerance）>‘Kangtian’（weak heat tolerance）.

Key words: Actinidia arguta, semi-lethal temperature, photosynthesis, physiological mechanism, heat tolerance