The effect of irrigation and crop load on stem water potential and apple fruit size

The effect of irrigation rate under various crop loads on the fruit size of apple (Malus domestica Borkh cv. Golden Delicious) was investigated in three field experiments in 1993-1995. During the first two years the field experiments evaluated the effects of various crop loads on yield, fruit size and midday stem water potential under 40% deficit irrigation. In 1995, the effects of five irrigation levels (0.42-1.06 of USDA Class A evaporation pan) and four crop loads (100-450 fruits per tree) were studied in a factorial experiment. Midday stem water potential increased with irrigation level and decreased with crop load in 1993 and at the lowest irrigation level in 1995. Daily fruit growth rate decreased with midday stem water potential in 1993 and at the lowest irrigation level in 1993. The effect of crop load on fruit growth rate was associated with limited soil water availability. A reduction in yield and average fruit size were associated with midday stem water potentials lower than -1.3 MPa. Taking an additional 0.1 MPa as a safety factor, -1.2 MPa could serve as a reasonable threshold for irrigation control in the orchard.