DIURNAL CHANGES IN STEM DIAMETER DEPEND UPON VARIATIONS IN WATER-CONTENT - DIRECT EVIDENCE IN PEACH-TREES

Three to five-year-old peach trees (Prunus persica (L.) Batsch cv. 'Maycrest') grafted on P. 'Damas 1869' grown in a sand trench were removed in the spring and grown hydroponically for several months. The system comprised two balances continuously recording the mass of the nutrient solution and that of the tree, so as to estimate transpiration and water uptake rates separately. Diurnal variation in plant water content (transpiration minus water uptake) was observed, with rapidly decreasing values when the solar radiation increased, while the reverse occurred when radiation decreased. Changes in stem diameter were continuously recorded using linear variable differential transducers. Data collected over several days of contrasting climatic conditions revealed that rapid changes in the stem diameter occurred throughout the day and were closely related to plant water content. A lag-time not exceeding 10 min was found between changes in stem diameter and plant water content. These results are discussed in relation to the use of micromorphometric methods to control irrigation in fruit trees. Moreover, we give values for the water stored in the shoots which may contribute to the transpiration stream.