CONTROL OF PLANT WATER DEFICITS USING THE SNOW AND TINGEY SYSTEM AND THEIR INFLUENCE ON THE WATER RELATIONS AND GROWTH OF SUNFLOWER

A system, designed by Snow and Tingey (1985) for 'subjecting plants to reproducible water stress levels for extended periods of time', is considered. Modifications are also outlined which enable water table heights to be maintained without the need for complex float chambers.

Sunflower plants (Helianthus annuus L. cv. Frankasol) were grown using the system and these were either 'well-watered' or subjected to water deficits. The temporal development of water deficits was closely monitored by regular psychrometric measurements of leaf water potential. Diurnal stomatal behaviour, mid-day abaxial stomatal conductance, and photosynthetic assimilation rates were also determined throughout the experiment, with growth analysis at the end.

A reduction in stomatal conductance occurred within 2-4 d after the onset of a restriction in water supply. Data from both viscous flow and diffusion porometry suggested that stomatal closure apparently began without a fall in bulk leaf water potential. Leaf water potentials of plants with a restricted supply of water did, however, subsequently decline during the early part of the experiment reaching values as low as -0.99 +/- 0.07 MPa after 14d. No further reduction in bulk water potential was observed after a further 5 d, suggesting that a steady-state had been reached. Corresponding values of leaf water potential for well-watered plants were about -0.60 +/- 0.04 MPa.

Biomass determinations indicated the potential for quantifying the effects of water deficits, of controlled magnitudes, on rates of leaf production and expansion. However, the possibility of physical limitations of root development - imposed both by the plant container and also by the imposition of restricted water supplies-must be carefully considered when planning experiments with this system.