EFFECT OF DIURNAL FLUCTUATIONS IN HUMIDITY ON THE ACCUMULATION OF NUTRIENTS IN THE LEAVES OF TOMATO (LYCOPERSICON-ESCULENTUM)

Tomato plants were grown at four humidities (vapour pressure deficits of 0.15, 0.25, 0.43 and 0.65 kPa) that were maintained continuously for four weeks. The Ca and Mg contents of the upper leaves declined progressively with increasing humidity. In the main experiment, a high (0.21 kPa) and low humidity (0.47-0.55 kPa) during the day were tested in factorial combination with a high (0.16 kPa) and low humidity (0.45-0.50 kPa) at night. Leaves were divided into terminal and basal leaflets, the rest of the lamina and the petiole. High humidity at night reduced the Ca, Mg, P and K contents (%) of all portions of the leaf lamina whilst high humidity during the day reduced only the Ca and K contents. The N content was not affected by humidity. The Ca and K contents of the petioles were reduced by high humidity during the day and at night. The changes in the Ca, Mg and K contents of the tissue in response to high humidity were greater in the terminal leaflet than in the basal leaflet or in the rest of the lamina, suggesting that the terminal leaflet was suitable for monitoring the nutrient status of the crop. Yellowing of the terminal leaflet at high humidity appeared to be associated with deficiencies of both Ca and K rather than with Ca only.