Relationship of Water Potential to Growth of Leaves

A thermocouple psychrometer that measures water potentials of intact leaves was used to study the water potentials at which leaves grow. Water potentials and water uptake during recovery from water deficits were measured simultaneously with leaves of sunflower (Helianthus annuus L.), tomato (Lycopersicon esculentum Mill.), papaya (Carica papaya L.), and Abutilon striatum Dickson. Recovery occurred in 2 phases. The first was associated with elimination of water deficits; the second with cell enlargement. The second phase was characterized by a steady rate of water uptake and a relatively constant leaf water potential. Enlargement was 70% irreversible and could be inhibited by puromycin and actinomycin D. During this time, leaves growing with their petioles in contact with pure water remained at a water potential of -1.5 to -2.5 bars regardless of the length of the experiment. It was not possible to obtain growing leaf tissue with a water potential of zero. It was concluded that leaves are not in equilibrium with the potential of the water which is absorbed during growth. The nonequilibrium is brought about by a resistance to water flow which requires a potential difference of 1.5 to 2.5 bars in order to supply water at the rate necessary for maximum growth.Leaf growth occurred in sunflower only when leaf water potentials were above -3.5 bars. Sunflower leaves therefore require a minimum turgor for enlargement, in this instance equivalent to a turgor of about 6.5 bars. The high water potentials required for growth favored rapid leaf growth at night and reduced growth during the day.