Misting of Tomato Plants Improves Leaf Water Status but not Leaf Growth

Field and laboratory experiments were carried out to determine whether modification of the aerial environment in a hot climate would improve the leaf growth rate and ultimately the yield of well- irrigated processing tomatoes, Lycopersicon esculentum Mill. cv. UC82B. In the field, the transpiration rate was modified by frequent applications of a fine mist. Misting reduced the diurnal fall in leaf water potential by up to 0.5 MPa, but had no effect on weekly vegetative growth or fruit yield, compared to a control treatment which was not misted but had accurately managed drip irrigation. To investigate these processes further, we studied the relationship between leaf water status and leaf growth in the laboratory using equipment in which a pot is placed inside a pressure chamber, with the leaves enclosed in a cuvette. Immediately following a misting event, both the hydrostatic pressure of the leaf xylem and the leaf expansion rate increased. The increase in leaf expansion rate was sustained for about 10 min and then fell below the pre-misted rate so that the net effect of a misting event on leaf expansion was small or nil. When an elevated leaf water status was sustained by pressurising the soil for over an hour, there was only a transient increase in leaf growth, and leaf growth stopped after the pressure was removed. Both in the laboratory and field, the overall leaf or vegetative growth was not sensitive to rapid fluctuations in leaf water status. The implication is that, if sufficient attention is paid to managing the root zone of crops, the more expensive and less efficient modification of the shoot environment is unnecessary.