Interactions between root restriction, irrigation and rootstock treatmentson 'Queen Cox' apple trees: Effects of soil and plant water relations

The purpose of this research was to determine if established, orchard-grown apple trees that were confined in root-restricting membranes received adeq uate water for growth when irrigated. Previous data had shown positive effe cts of root restriction on reducing shoot extension growth in apple. Soil m atric potential (Psi(sm)). leaf stomatal conductance (g) and leaf water pot entials (Psi(1)) were measured over daily cycles during the period of maxim um tree water use (July and August). Measurements were also made of the Psi (sm) and fruit size throughout the growing season. Daily and seasonal Psi(s m) showed that the soil within the root restricting membranes (+R) when irr igated (+I) remained closed to field capacity (<100 HPa), which was not the case for unirrigated soil within the membranes (-I +R). Psi(1) measured be fore dawn, showed that similar levels of drought stress were evident betwee n irrigated and restricted (+I +R) and unrestricted (+I -R and -I -R) trees . The Psi(sm), and Psi(1) for trees with roots within restricting membranes were significantly more negative, in the absence of irrigation (-I +R). Me asurement of g showed that root restricted trees were transpiring at simila r rates in the presence of irrigation (+I +R) as unrestricted trees (+I -R) with or without irrigation. Stomatal closure could not explain the increas ed Psi(1) observed for the restricted irrigated (+I +R) treatment compared with unrestricted (-R) trees. A reduced stomatal aperture was the most like ly explanation for the reduction in growth previously observed with the res tricted unirrigated trees. Fruit size was also affected by root restriction and the effect became greater as the roots became more restricted with tre e age. Similarly, there was also a negative effect, in one year, of root re striction on fruit size at harvest, even in the presence of irrigation. Dat a show that reductions in soil water availability, Psi(1) and g, for the ro ot-restricted trees (+I +R), were unlikely to be the causes for the previou sly observed reductions in shoot growth (tree size). These results imply th at other factors were in operation, among which root-synthesized chemical r egulators of shoot growth are the most likely candidates.