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"dry", watering the trees only when they closed their stomata. Both tree transpiration and leaf photosynthesis depend on the levels of water alimentation: transpiration reduction (fig 2) takes place as soon as the substrata dry out and net photosynthesis (table I) appears to be improved in the trees irrigated by the micromorphometric method under the conditions of low and medium light intensity, but limited in the trees subjected to "dry" treatment with a strong light intensity as compared with that observed in the trees which were irrigated daily. However, there are no significant differences in the water status and the stomatal resistance between the trees subjected to "daily irrigation" treatment and

le 18 -1-1988, accepté le 2 -12-1988) Résumé — Des travaux récents, Garnier (1985) et Huguet (1985), ont montré sur arbres fruitiers qu'il est possible d'utiliser la mesure des microvariations de diamètre des tiges pour piloter leur irrigation. Le (1985) and Huguet (1985) have recently used these techniques to monitor the internal water status of fruit trees and consequently to control orchard irrigation. The objective of our study was to improve our understanding of the daily variations in stem diameter of eggplant by simultaneously measuring stomatal resistance, net photosynthesis and the environmental conditions in a greenhouse. It appeared that stem diameter variations are an expression of the imbalance between water supply and climatic demand. This confirmed that stem diameter variation can be used as a criterion for the irrigation timing of vegetable crops. A linear relationship was found between the daily increase in stem diameter and daily piant photosynthesis before fruit-set net photosynthesis -stomatal resistance -eggplant