Effect of Sunlight on Sap Concentration of Citrus Leaves

Abstract

1. Increases in the sap concentration of leaves of Eureka lemon and Valencia orange occur when the leaves are exposed to direct sunlight. 2. The sap of Eureka lemon leaves on exposure to direct sunlight increases its concentration more rapidly than does that of Valencia orange leaves. 3. Increases in sap concentration due to the action of sunlight on the leaves are due entirely to photosynthetic products, the ash of the sap remaining the same under both conditions. 4. A greater percentage of the cross-sectional diameter of the Eureka lemon leaves is occupied by palisade tissue than is found to be the case with the Valencia orange leaves. This, together with the fact that more magnesium is found in the sap of Eureka lemon leaves than in that of Valencia orange leaves, and the more rapid response of leafy lemon cuttings to vegetative propagation, lends support to the view that greater increase in lemon leaf sap concentration in direct sunlight over that of orange leaf sap may be due to a more efficient photosynthetic system. 5. The great variation in sap concentration of citrus leaves brought about by direct sunlight indicates that large errors may result through indiscriminate sampling.