Influence of the water treatment on the xylem anatomy and functionality of current year shoots of olive trees

From May to October 2006 we applied two water treatments in an olive orchard in south Spain with 38-year-old ‘Manzanilla’ trees at 7x5 m spacing: a) rainfed, with rainfall as the only source of water supply; b) FAO, in which the trees were drip-irrigated daily from May to September, to replace the crop water needs. The aim of this work was to evaluate the influence of the water treatments on both the anatomical characteristics and the hydraulic performance of the xylem of current-year shoots. In November, 3 cm long segments were taken at 5 cm from the base of current-year shoots of each treatment. Part of these samples was used for hydraulic conductivity (Kh, g m MPa-1 s-1) measurements with a hydraulic conductivity meter designed and built in our group. The rest was cryo-fixed in liquid nitrogen for observation by cryo-scanning electron microscopy (cryo-SEM), which allowed us to determine vessel density (vessels mm-2) and vessel distribution per diameter classes, at 2 µm diameter intervals. The xylem water potential (Ψxylem) of each sample was also measured, at shoot sampling. The anatomical analysis showed no differences between treatments in vessel density. From the amount of vessel, rainfed trees showed narrower vessels than FAO trees. This may explain the lack of differences in Kh found between the two studied treatments. In fact, the incidence of cavitation is smaller in vessels of reduced diameter. In addition, any reduction in diameter implies a reduced collective pit area between vessels, which reduces the incidence of embolism due to air-seeding. Therefore, the greater number of vessels with reduced diameter induced by the greater water stress suffered by the rainfed trees may have been the major reason for maintaining similar hydraulic conductivities in those trees than in drip-irrigated olive trees.