A study on cold resistance in five Sapotaceae fruit trees

Abstract: 【Objective】Cold temperature is a key limit of northward expansion of the cultivation of trop-ical fruit trees. Chilling injure often causes yield reduction and even crop failure. Previous studies aboutSapotaceae fruit trees focused mainly on the description of commercial value and the prospects for devel-opment, but there have been few reports about the mechanisms of response to cold temperature whenthey were introduced northward. In this study. The semi-lethal temperature (LT50) , as well as the chang-es in activities of antioxidant enzymes and the contents of osmoregulation substances in 5 Sapotaceaespecies under cold stress were studied in order to provide a theoretical basis for introduction and plant-ing these species.【Methods】Two-year-old grafted seedlings of five Sapotaceae species including Ma-nilkara roxburghiana (Wight) Dubard, Manilkara zapota (L.) van Royen, Synsepalum dulcifcum Denill, Lucuma nervosa A. DC. and Chrysophyllum cainito L. were placed in incubators for low temperaturetreatments. With 25 ℃ as the control, the experiment set two temperature treatments. First 9 ℃ for 72 h, and then 3 ℃ for 24 h. Mature leaves were collected immediately after every treatments and divided in-to two parts. One was used to determine chlorophylls and relative electric conductivity (REC) ; the otherwas stored at-80 ℃ for the analyses of the parameters including contents of proline, soluble proteins, soluble sugars and malondialdehyde (MDA) , and the activities of superoxide dismutase (SOD) , peroxi-dase activity (POD) and catalase (CAT) . REC was determined following DENG Renju. LT50 was mea-sured according to LIU Shihong. Chlorophylls were measured using acetone-ethanol mixing method.POD, CAT, soluble sugars and MDA were determined following Zhang Zhiliang and Qu Weijing, whileSOD, proline and soluble proteins were analyzed using the methods of Li Hesheng's.【Results】The chlo-rophyll values of the 5 species kept decreasing as the temperature dropped. The range of decrease inchlorophylls was the largest in S. dulcifcum and lowest in M. roxburghiana and M. zapota. The value at3 ℃ in every specie was significantly lower than in the control. REC values in the 5 species increasedbut the rate of increase were not the same. The values in roxburghiana, M. zapota. and C. cainito at 9 ℃or at 3 ℃ were not significantly higher than their control values. A rapid rise was found in C. cainito at9 ℃ and in S. dulcifcum at 3 ℃. The REC value in M. roxburghiana or M. zapota was significantly low-er than the other 3 species. Regressed logistic equations between REC and temperature were established.The LT50 was calculated from the equations. The cold resistance based on LT50 was in the order of M. rox-burghiana (3.01) >M. zapota (3.32) >S. dulcifcum (3.36) >L. nervosa (3.80) >C. cainito (4.11) . Pro-line content increased constantly and was significantly higher at 9 ℃or at 3 ℃ than at the control temper-ature. The content and the range of increase in M. roxburghiana and M. zapota. were higer than in theother 3 species. The lowest proline content was found in C. cainito. Soluble protein content increasedfirst at 9 ℃ and then decreased gradually at 3 ℃. Except for M. zapota, the protein values in other 4 spe-cies were significantly higher than the control either at 9℃or at 3 ℃. Soluble sugar content at either 9 ℃or 3 ℃ in every specie was higher than in the control. It rose rapidly and reached the peak at 9 ℃ whenthe value in every specie significantly differed from each other. The highest value was in M. roxburghi-ana and the lowest value in C. cainito. MDA increased constantly with the drop of temperature. It wassignificantly higher at 3℃ than the control except for M. roxburghiana and M. zapota. The activities ofSOD, POD or CAT in all specie showed a similar pattern with the decrease in temperature. The valuerose first and reached its peak values at 9 ℃, and then declined at 3 ℃. M. roxburghiana and M. zapotamaintained significantly higher SOD and POD activities than the other three species at all the low tem-peratures. CAT activity in M. roxburghiana was distinctly higher than that in the other species.【Conclu-sion】Chlorophyll content in all specie was lower under chilling stress than in the control. The values ofMDA, REC, proline, soluble proteins and soluble sugars were higher under cold stress compared withthe control. On the whole, the change patterns of parameter were similar among the five species undercold stress. Based on the changes in osmoregulation substance, antioxidant enzymes and LT50, the cold re-sistance was in the order of M. roxburghiana>M. zapota>S. dulcifcum>L. nervosa>C. cainito. Thecold resistance of the two M. species were obviously higher than the other three. The indexes includingthe content and the increase range of proline, the contents of soluble proteins or soluble sugars, the activi-ties of SOD, POD and CAT might be used to measure the cold resistance of Sapotaceae fruit trees.