The Journal of Horticultural Science & Biotechnology
Vol. 48 No: 2
Water, Osmotic and Pressure Potential Relationships in Apple Leaves
J.E. GOODE and K.H. HIGGS
Water (ψw) and osmotic (ψs) 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 (ψp) was assessed from the difference between ψw and ψs.
Intercept values for ψs derived from regressions of ψs on ψw 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 ψs decreased about 5 bars from July to September.
The slope relating ψs and ψw remained nearly constant after June and reflected the property of the leaf cells to expand or contract with increasing or decreasing ψw. Since the slope was similar throughout both seasons, a general relationship between ψw, ψs and ψp was evaluated, from which it was deduced that a diurnal decrease in ψw resulted from the sum of the depressions in ψs and ψp in the ratio of 3:4.
The rate at which ψw decreased with increasing evaporation demand appeared to be under stomatal control; the latter was maximal when ψw equalled the value of ψs at full turgor. The value of ψp at this point was about 8 bars. Further depression of ψw 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 ψw 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 ψw 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.
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