Irrigation and crop load influence fruit size and water relations in field-grown 'Spadona' pear

Interrelations between water potential and fruit size, crop load, and stoma tal conductance were studied in drip-irrigated 'Spadona' pear (Pyrus commun is L) grafted on quince C (Cydonia oblonga L.) rootstock and growing in a s emi-arid zone. Five irrigation rates were applied in the main fruit growth phase: rates of 0.25, 0.40, 0.60, 0.80, and 1.00 of "Class A" pan evaporati on rate. The crop in each irrigation treatment was adjusted to four levels (200 to 1200 fruit per tree) by hand thinning at the beginning of June 1999 . The crop was harvested on 1 Aug. 1999, and fruit size was determined by m eans of a commercial sorting machine. Soil, stem, and leaf water potentials and stomatal conductance were measured during the season. Crop yield was h ighly correlated with stem and soil water potentials. The highest midday st em water potential was lower than values commonly reported for nonstressed trees, and was accompanied by high soil water potential, indicating that th e maximal water absorption rate of the root system under those particular s oil conditions was limited. Stomatal conductance was highly correlated with leaf water potential (r(2) = 0.54), but a much better correlation was foun d with stem water potential (r(2) = 0.80). Stomatal conductance decreased a t stem water potentials less than -2.1 MPa. Both stem water potential and s tomatal conductance were unaffected by crop load under a wide range of irri gation rates.