Seasonal and Diurnal Changes in the Water Contents and Water Deficits of Bartlett Pear Leaves

It is general knowledge that the water content of (leciduous leaves usually decreases as the season advances. It has also been noted that the water deficit fluctuations of such leaves are often more pronounced early in the growing season when these parts are young and succulent than later when they are older and harder. The seasonal changes in the actual weights of the water and dry matter components of leaves, however, are less commonly understood. Leaf samples are comparatively easy to pick, dry and weigh. For this reason, the water content of leaves is often used to indicate water changes within plants. Knight (5) found slight diurnal variations in water contents of leaves of the plants he tested in the area of southeastern England. Livingston and Brown (7) found large diurnal changes in the water contents of the desert plants with which they worked. Miller (9) made frequent periodic water content determinations on the leaves of corn and sorghums in his studies of the water relations of these crop plants. MIaximov (8), Stanescu (12), Kramer (6), and Wilson, Boggess, and Kramer (14) have also reported on the fluctuations in the water contents of plants. Stocker (13) devised a measurement of water deficits by which he related the amount of water needed to bring a leaf to full saturation, or zero deficit, to the total amount of water in the leaf at saturation. The resulting ratio figure was multiplied by 100 and expressed as a percentage. Stocker used such a measurement to obtain a comparison of the water deficits existing in the leaves taken at different periods of the day from plants growing in widely separated climatic zones. Oppenheimer and Mendel (10), Halma (3, 4), Compton (1), and Runyon (11) have reported on the use of similar measurements to determine changes in the water deficits of plants. During the past five years the Department of Horticulture of the State College of Washington has conducted a study of certain water relationships of Bartlett pear trees. A portion of this study included the measurement of the seasonal and diurnal fluctuations in the leaf water contents and water deficits and the weight changes of the leaf constituents responsible for these fluctuations.