Seasonal variation in xylem pressure of walnut trees: root and stem pressures

Measurements of air and soil temperatures and xylem pressure were made on 1 7-year-old orchard trees and on 5-year-old potted trees of walnut (Juglans regia L.). Cooling chambers were used to determine the relationships betwee n temperature and sugar concentration ([glucose] + [fructose] + [sucrose], GFS) and seasonal changes in xylem pressure development. Pressure transduce rs were attached to twigs of intact plants, root stumps and excised shoots while the potted trees were subjected to various temperature regimes in aut umn, winter and spring. Osmolarity and GFS of the xylem sap (apoplast) were measured before and after cooling or warming treatments. In autumn and spr ing, xylem pressures of up to 160 kPa were closely correlated with soil tem perature but were not correlated with GFS in xylem sap. High root pressures were associated with uptake of mineral nutrients from soil, especially nit rate. In autumn and spring, xylem pressures were detected in root stumps as well as in intact plants, but not in excised stems. In contrast, in winter , 83 % of the xylem sap osmolarity in both excised stems and intact plants could be accounted for by GFS, and both GFS and osmolarity were inversely p roportional to temperature. Plants kept at 1.5 degreesC developed positive xylem pressures up to 35 kPa, xylem sap osmolarities up to 260 mosmol l(-1) and GFS concentrations up to 70 g l(-1). Autumn and spring xylem pressures , which appeared to be of root origin, were about 55% of the theoretical pr essures predicted by osmolarity of the xylem sap. In contrast, winter press ures appeared to be of stem origin and were only 7% of the theoretical pres sures, perhaps because of a lower stem water content during winter.