A thermocouple psychrometer that measures water potentials of intact leaves
was used to study the water potentials at whidh leaves grow. Water potentials and water
ulp,take during recovery from water deficits were measured simultaneously with leaves of
sunflower (Helianthus annuus L.), tomato (Lycopersicon esculentum Mill.), papaya (Carica
papaya L.), and A butilon 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 constanit 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 obitain 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 growith. 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 onlv when leaf water potenitials were above -3.5 bars.
Sunflower leaves therefore require a minimuim 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.