The effect of three rootstocks on water use, canopy conductance and hydraulic parameters of apple trees and predicting canopy from hydraulic conductance

This study investigated the effect of the dwarfing M9, semi-dwarfing MM106 and local Hashabi rootstocks on the water use, canopy conductance (G(c)) and hydraulic conductance (k) of apple orchards with the same scion, Golden Delicious. The average summer leaf area index (LAI) was 2-4, 2.7 and 1.7 for M9, MM106 and Hashabi, respectively. Irrigation in 1997 was less than water use until June, and excessive afterwards. In 1998, irrigation was doubled, and was excessive throughout the season. Sap flow (J) in June-August 1998 totalled 476, 682 and 606 mm (or 0.60, 0.86 and 0.76 of class A pan evaporation) for M9, MM106 and Hashabi, respectively. Maximum sap velocity in the three rootstocks (approximately 70 cm h(-1)) occurred in the outer 30-60% of the stem, and its decrease with depth was greater in M9 than in the other rootstocks. Midday G(c) during both summers was least for M9, intermediate for Hashabi and greatest for MM106. The k value of M9 and MM106 for the soil to stem, stem to leaves and soil to leaves pathways were determined from daily courses of water potential of leaves, Psi(1), stem, Psi(stem) and J. Specific k (k(s), i.e. relative to stem sapwood area) did not significantly differ between the two rootstocks for soil to stem and soil to leaf pathways, but leaf specific k (k(1)) was greater for MM106 soil to stem (71% greater) and soil to leaf (63%) pathways, respectively. The inverse slopes of the relationships between midday canopy resistance (R-c) and vapour pressure deficit (D) for MM106 was 1.75 of that for M9, and the ratio of their Huber values, i.e. the ratio of sapwood to leaf area, was 1.6. These findings indicate that differences in water use are attributable to differences in k(1), and not to differences in wood properties (k(s)). Application of a model relating R-c to orchard area specific k (k(g)) showed that the slope of the relationships between midday R-c and D for the 1998 data could be predicted using common values of k(s) (0.134 kg m(-2) s(-1) MPa-1) and midday Psi(1) (-1.34 MPa) for the three rootstocks. The implications of these findings, and the similarities in the differences between rootstocks of G(c), k(1), k(g) and Huber values, are discussed with respect to rootstock water use and irrigation.