Osmotic adjustment is a drought resistant trait that has been documented in the leaves and roots of immature cotton (Gossypium hirsutum L. cv Stoneville 506). However, the extent to which osmotic adjustment influences the water status and growth of mature field-grown plants is not known. Therefore, the water relations and growth of field-grown cotton were investigated for 8-wk old plants during the development of water stress, and for 8 days after rewatering. Specific objectives were 1) to characterize the osmotic potentials of leaves and root tips; 2) to assess leaf expansion and root elongation growth; and 3) to compare techniques and measurement errors for estimating solute accumulation from the Scholander pressure chamber, thermocouple psychrometers, and a vapor pressure osmometer.

A short-term water stress of 12 days was chosen as would be experienced between irrigation cycles in commercial production. Treatments consisted of a wellwatered control, a mild (S1) and a severe (S2) water deficit. Midday water potentials at peak stress for the control were -1.0 to -1.2 MPa, S1 was -1.3 to -1.6 MPa, and S2 was -1.7 to -2.1 MPa. All treatments were rewatered after 12 days and measurements continued during recovery.

Preliminary results suggest that both stress treatments influenced shoot and root growth. Pressure-volume (PV) and psychrometer data identified solute accumulation in the shoot which was greatest in young fully expanded leaves. Solute accumulation was rapidly lost upon rehydration. At peak stress pre-dawn osmotic potentials were -1.3 MPa (S1) and -1.1 MPa (control) from PV data, and -1.1 MPa (SI) and -1.0 MPa (control) from psychrometry. Young leaves had a lower modulus of elasticity than older leaves but short-term stress did not affect this significantly (modulus of tissue elasticity from PV data was 1.1 MPa for main-stem node 10 leaves, and 2.4 MPa for main-stem node 5 leaves). Apoplastic dilution of the symplasm was 12% for thermocouple psychrometry and osmometry as calculated by PV:psychrometer ratio for osmotic potential. Estimates from PV data alone indicated 16% of relative water content was apoplastic water for the first fully expanded leaf. Short-term stress did not affect the dilution significantly. Leaf expansion rates for S1 and S2 were similar although lower than the control during the stress, but equaled the control rate after rewatering and during recovery from stress. No significant difference was found in leaf expansion rates between S1 and S2 treatments. Root extension rates for S1 and S2 were lower than the control during stress. On recovery from stress, root extension rates for S1 and S2 were greater than the control, although S2 rates were lower than S1. Osmotic adjustment in actively growing root tips was not observed during stress by the techniques employed. Preliminary findings confirm that osmotic adjustment occurs in leaves of field-grown cotton. We suggest the mechanism may be involved in the comparative growth rates of leaves in stressed and control treatments on recovery from stress, and in the higher root extension rates from stressed plants compared to the control during recovery.