Effect of oxygen supply and temperature at the root on the physiology of kiwifruit vines

Abstract

SummaryA severe leaf disorder currently affects kiwifruit vines (Actinidia deliciosa var. deliciosa) in New Zealand and a number of other countries. The symptoms, which develop in mid‐summer, include interveinal chlorosis, development of necrolic areas on the leaf and eventual leaf death. Field observations strongly indicated the involvement of the root system. The present study investigated the extent to which temperature and oxygen supply at the root influenced the physiology of kiwifruit vines.Maximum root growth occurred at 20 °C, while temperatures in excess of 25 °C reduced growth. Shoot growth was also reduced by root temperatures above 20 °C. No obvious leaf symptoms developed on the vines affected by the high temperatures. Below 20 °C there was a marked reduction in growth with little or no growth occurring at 10 °C. While soil temperature can be expected to influence root growth per se, it would appear unlikely that it directly controls the seasonal pattern of root growth or greatly influences the physiology of the leaf.This study showed kiwifruit vines are not well adapted to tolerate low concentrations of oxygen in their root zone. Growth was substantially reduced when the supply of oxygen to the roots was reduced. The extent of the reduction was proportional to the number of hours per day the vines were without aeration. Root growth was particularly affected. Distinctive leaf symptoms appeared on all oxygen‐stressed vines and included the sudden appearance of large necrotic areas on the older leaves, sometimes preceded by an interveinal chlorosis. Measurement of the porosity of the roots indicated that the average air space between the tightly packed cells accounted for 2% or less of the total root volume. There was no obvious change in root porosity with decreasing oxygen supply or difference between the solution grown vines and vines from the field.Decreasing the supply of oxygen to the roots resulted in rapid reduction in the stomatal conductance of the leaves and the virtual elimination of the diurnal pattern. Associated with the decrease m the stomatal conductance was a marked increase in the maximum daily temperature of the leaves compared to the control vines. The difference between the two treatments increased with time. An estimate of the rate of consumption of oxygen per unit length of root (9.1 × 10−10 mol m−1 s −1) suggests that a mature kiwifruit vine could deplete the oxygen supply in a well aerated soil to an anoxic condition in about 5 hours. The relatively high consumption of oxygen coupled with the low porosity of the roots ensures that even short periods of low concentrations of oxygen in the root zone will seriously affect the physiology and growth of kiwifruit vines. It was concluded that poor aeration of the root system is likely to be a major factor in the incidence of the leaf disorder currently affecting kiwifruit vines in a number of countries.