STUDIES UPON THE COPPER FUNGICIDES XI. ABSORPTION OF SOLUBLE COPPER IN RELATION TO STAGE OF LEAF GROWTH AND TO LEAF INJURY

A study of the absorption of soluble copper by apple leaves was undertaken with a view to obtaining a better understanding of the mechanism of copper spray damage. The ability of young apple leaves to absorb copper was found to be relatively high and this may be due to the large number of stomata and hairs present. As the leaves grew more mature this capacity fell, rose again to a maximum in July or August and then fell again as the leaves approached senescence. It is suggested that these changes may be due to seasonal changes in leaf metabolism, which probably affects nutrient absorption by leaves as in roots. It has been observed in the field that apple leaves are often more susceptible to copper spray damage in the autumn. Since absorption is then at a low level it is concluded that the rate of absorption is not the only factor affecting damage to leaves caused by copper sprays. The presence of injuries in the leaves has two effects: first, it causes an increased excretion of copper-dissolving substances (Arman & Wain, 1958) and, secondly, it increases the rate at which soluble copper is absorbed. A combination of these two factors may bring about localized spray damage. However, the increase in the absorption of soluble copper caused by the presence of extensive injuries in the leaves was sufficiently small for the conclusion to be drawn that injuries do not normally constitute a major route for the entry of copper into the leaf and it is probable that spray damage can occur in the absence of injuries.