Water,Osmotic and Pressure Potential Relationships in Apple Leaves

Water (ψ_w) and osmotic (ψ_s) potentials were measured in leaves of irrigated and unirrigated Cox’s Orange Pippin apple trees on MM. 104 rootstock during the summer seasons of 1969 and 1970. Turgor pressure potential (ψ_ρ) was assessed from the difference between and ψ_s.

Intercept values for ψ_s derived from regressions of ψ_s on ψ_w reflected an increasing intracellular osmotic concentration throughout the season and an increasing ability of the trees to withstand environmental stress in respect to turgor. In 1970 ψ_s decreased about 5 bars from July to September.

The slope relating ψ_s and ψ_w remained nearly constant after June and reflected the property of the leaf cells to expand or contract with increasing or decreasing ψ_w. Since the slope was similar throughout both seasons, a general relationship between ψ_w, ψ_s and ψ_p was evaluated, from which it was deduced that a diurnal decrease in ψ_w resulted from the sum of the depressions in ψ₃ and ψ_ρ in the ratio of 3 : 4.

The rate at which ψ_w decreased with increasing evaporation demand appeared to be under stomatal control; the latter was maximal when ψ_ν equalled the value of ψ_s at full turgor. The value of ψ_p at this point was about 8 bars. Further depression of ψ_w led to yellowing and dropping of the basal leaves of shoots and spurs—an effect only present in the unirrigated trees.

Under moderate conditions of evaporation demand the effect of irrigation on ψ_w appeared to be negligible while the soil moisture tension within any part of the root zone remained within the tensiometer measurement range. When soil moisture stress became greater than this, irrigation maintained ψ_w at between 5.3 and 6.3 bars higher than it was in the leaves of the unirrigated trees. Subsequently, the unirrigated trees defoliated completely several weeks earlier than the irrigated trees.