Physiological responses of wax apple to low temperatures and its cold resistance

Abstract: 【Objective】Wax apple [Syzygium samarangense (BI.)Merr. et Perry] is a typical tropical fruitwith poor cold resistance. Temperature is the most important factor for transplanting wax apple from thesouth to the north in China. Systematic study of cold resistance of wax apple is rare. The physiological re-sponses of wax apple to low temperatures were studied to provide a theoretical basis for introduction,coldresistance breeding,and transplanting of wax apple from the south to the north regions.【Methods】One-year-old cutting seedlings of‘Nongke 1'(also called "Tud Ting Jiang") wax apple were placed in incuba-tors for low temperature treatments. The experiment included six temperature treatments at 10,7,4,1,-2 and-5 ℃ using incubators for a duration of 3,6,9 or 12 h in darkness in each temperature treat-ment. There were totally 24 treatments,each with three replicates consisting of one seedling each. Matureleaves were collected after 3,6,9 and 12 h of exposure to the low temperatures and divided in to two por-tions. One of them was used to determine the relative electric conductivity(REC),the ratio of bound waterto free water(BW/FW),free proline(Pro) content,and water soluble sugar(WSS) content; the other por-tion was frozen in liquid nitrogen and stored at-80 ℃ until quantitative determination of malondialde-hyde(MDA) and the activities of superoxide dismutase(SOD),catalase(CAT) and peroxidase(POD).Measurements of REC were carried out using Yang Huageng's method and the semi-lethal temperature(LT50) was determined using Zhu Genhai's computational method. The ratio of BW/FW and contents ofPro and WSS were measured according to Li Lin's methods. The activities of SOD,CAT and POD,andthe content of MDA were determined following instructions of their orresponding assay kits,commerciallysupplied by Nanjing Jiancheng Biological Research Institute.【Result】The REC value increased as thetemperature dropped. The REC value increased significantly from 23%-35% to 86%-96% when tempera-ture dropped from 1 to -2 ℃. The regressed logistic equation of Y=91.055/(1+0.592 6e0.434 3x)(R=0.952 3*)was established between the REC values and temperatures in‘Nongke No. 1'. We also calculated the LT50o‘fNongke 1',which was^-1.2 ℃. MDA content increased with the temperature decrease,and its peakvalue was 54.43 nmol·g^-1under at-5 ℃ for 12 h. In addition,MDA content in leaves was significantlyhigher in the treatment at-5 ℃ than at-2 ℃ in all the exposure durations,and was significantly higherthan at 10,7,4 ℃ and 1 ℃. Therefore,wax apple displayed cold injury under-2 ℃. The change patternof SOD activity with exposure duration was different in various low temperatures. It increased with time at10 ℃,while at 7,4,1,-2 and -5 ℃ it displayed a reverse trend. Moreover,the stress response thresh-old temperature was 4 ℃,and the peak value(38.55 U·mg^-1prot) was found in the samples treated for 3 hat this temperature. Both CAT and POD activities shown three change trends with exposure duration invarious low temperatures. At 10 ℃,CAT activity decreased first and then increased. However,at 7,4,1 and -5 ℃,it rose first and then decreased. Different from the above,CAT continuously decreasedat- 2 ℃. The peak values of CAT activity at 1 ℃ for 6h and 9 h were 112.49 U·mg^-1.prot and 109.93 U·mg^-1.prot respectively,which were the highest values among all treatments. POD activity decreased in the ear-ly period and then increased at 10 ℃,which was opposite to those at 7 and 4 ℃. The enzyme activity de-creased continuously at 1,-2 and -5 ℃. The highest value of POD activity was 153.46 U·mg^-1 prot at4 ℃ for 9 h. It was concluded that the stress response threshold temperature of CAT and POD was 1 ℃.The changes in BW/FW ratio with time were different in various low temperature conditions. Under thetemperatures above 1℃,the ratio of BW/FW continuously increased,it decreased first and then in-creased at 1 ℃. Interestingly,at-2 and-5 ℃,the ratio of BW/FW increased and then decreased. The ra-tio of BW/FW increased with temperature decrease for durations of 3,6 and 9 h,but it increased whentreatment duration was 12 h. The value of BW/FW at-2 ℃ for 9 h reached a max of 20.9%. Pro contentshowed a downward trend with time in various low temperatures. Pro content in low temperature treat-ments with 12 h exposure duration was significantly lower than those with 3 h. The highest Pro content(0.139 μg·g^-1) was found in the treatment of-2 ℃ for 3 h,and the lowest(0.129 μg·g^-1) in that at -5 ℃ for 12 h. The change trend of Pro content with temperature decrease followed a decrease-increase-de-crease pattern,decreasing from 7 to 4 ℃,increasing from 4 to 1 ℃ and decreasing again as temperaturefurther dropped. The change trend of WSS content with time differed among treatments. The WSS con-tents decreased at first and then increased at 10 and 7 ℃,increased initially and then decreased at 4 and -5 ℃,showed a decreasing-increasing-decreasing pattern at 1 ℃,and decreased continuouslyat-2 ℃. The WSS content presented a decrease-increase-decrease trend with the decrease in tempera-ture,however the beginning temperatures of up and down were different. WSS contents that in samplestreated at 10,1 and-2 ℃ were higher than those at 7,4 and-5 ℃. The highest value was 22.27 mg·g^-1in the treatment of 1 ℃ for 9 h and the lowest value was 20.65 mg·g^-1at-5 ℃ for 12 h. The results of cor-relation analysis showed that REC was always significantly correlated with MDA. REC was significantlyand negatively correlated with CAT and SOD. No significant correlation was found between REC and PODunder low temperatures for 3 h. However,REC was significantly and negatively correlated with POD inlow temperature treatments with exposure duration of 6,9 and 12 h. In treatments with durations of 3,6and 9 h,REC was significantly correlated with BW/FW ratio but not with Pro and WSS contents. More-over,REC was strongly and negatively correlated with BW/FW,Pro and WSS in the treatments with a du-ration of 12 h.【Conclusion】The LT50 of‘Nongke 1'wax apple was^-1.2 ℃. Under chilling stress,mem-brane lipid peroxidization exacerbated; MDA accumulated;and electrolyte leakage increased in leaves,with osmotic adjustment that involved the increase in BW,Pro and WSS,which displayed different de-grees of protective effects in different stages under chilling stress. CAT and POD were the main protectiveenzymes.