Expansion and osmotic adjustment of strawberry fruit during water stress

Expansion of green-white and red fruit in control (watered) and water-stressed greenhouse-grown strawberry (Fragaria x ananassa Duch. 'Brighton') plants was monitored with pressure transducers. Expansion of green-white fruit in control plants was rapid, showing little diurnal variation; whereas in water-stressed plants, fruit expansion occurred only during dark periods and shrinkage during the day. Red fruit were mature and failed to show net expansion. The apoplastic water potential (psi aw), measured with in situ psychrometers in control plants was always higher in leaves than in green-white fruit. In stressed plants, psi aw of leaves was higher than that of green-white fruit only in the dark, corresponding to the period when these fruit expanded. To determine the ability of fruit to osmotically adjust, fruit were removed from control and water-stressed plants, and hydrated for 12 hours; then, solute potential at full turgor (psi s 100) was measured. Water-stressed green-white fruit showed osmotic adjustment with a psi s 100 that was 0.28 MPa lower than that of control fruit. Mature leaves of water-stressed plants showed a similar level of osmotic adjustment, whereas water stress did not have a significant effect on the psi s 100 of red fruit. Fruit also were severed to permit rapid dehydration, and fruit solute potential (psi s) was plotted against relative water content [RWC = (fresh mass - dry mass divided by fully turgid mass - dry mass) x 100]. Water-stressed, green-white fruit had a lower psi s for a given RWC than control fruit, further confirming the occurrence of osmotic adjustment in the stressed fruit tissue. The lack of a linear relationship between turgor pressure and RWC prevented the calculation of cell elasticity or volumetric elastic modulus. Osmotic adjustment resulted in about a 2.5-fold increase in glucose and sucrose levels in water-stressed green-white fruit. Although green-white fruit on water-stressed plants showed osmotic adjustment, it was not sufficient to maintain fruit expansion during the day.