Nitrogen accumulation and leaf gas exchange in apple trees grafted on M.26 EMLA, M.9 EMLA and Mark rootstocks under low soil temperature and drought stress

Two-year-old apple trees of cv Jonee grafted on Mark, M.9 EMLA and M.26 EMLA rootstocks were exposed for 10 days to low root temperature (8+/-1degreesC) and drought stress to evaluate, over a 3-day period, the accumulation of labeled nitrogen (N) in root and scion and leaf gas exchange rate under adverse soil environment. M.9 EMLA showed a similar and a higher percentage of N derived from fertilizer (NDFF) in the xylem sap than M.26 EMLA and Mark, respectively. Unlike drought stress, low root temperature was effective in reducing NDFF in the xylem sap, compared to control trees. While not affected by rootstock, the labeled N removed by trees in control soil was similar and higher (P=0.09) than in dry and low temperature ones, respectively. Labeled N increased from Day 1 to Day 3 in the xylem sap as well as in the whole tree. Three days after N supply, most of the labeled N was accumulated in the root and less than 16% was detected in the shoot. A linear correlation was observed between the percentage of NDFF in the xylem sap and the amount of labeled N detected in shoot both one and three days after fertilization. Carbon assimilation (A) and transpiration (E) rates were higher in control, intermediate in low temperature and were lowest in drought-stressed trees. M.9 EMLA showed the highest A under optimal soil conditions but the lowest under drought stress. Low root temperature was more effective than drought stress in depressing N absorption while drought stress was more effective than low root temperature in decreasing leaf gas exchange.