THE RATE OF DEVELOPMENT OF WATER DEFICITS AFFECTS SAXIFRAGA-CERNUA LEAF RESPIRATION

We allowed plant water deficits to develop at two different rates foll owing the cessation of watering in order to investigate the effects of water stress on cytochrome pathway and alternative pathway respiratio n in the leaves of the arctic herb Saxifraga cernua. Plants were pretr eated by growth in either a commercial organic (GO) mixture or a vermi culite-perlite CVP) mixture, which allowed the complete development of water deficits in 19 and 8 days, respectively. The rate of water pote ntial reduction was approximately 0.11 MPa day(-1) in the leaves of CO plants, compared to a reduction of 0.21 MPa day(-1) in leaves of VP p lants. Osmotic adjustment occurred to a greater extent in leaves of CO plants and corresponded with an increase in ethanol-soluble sugars. I n leaves of CO plants, cytochrome pathway activity gradually declined from that of control rates until day 11, and then declined more rapidl y. In contrast, cytochrome pathway activity significantly increased in response to water deficits in leaves of VP plants. In leaves of both CO and VP plants, alternative pathway activity declined as water stres s progressed. Relatively severe water deficits reduced alternative pat hway capacity in leaves of both CO and VP plants. We also investigated the effect of previous exposure to water deficits on leaf respiration . In plants that had previously experienced three cycles of water stre ss, the increase in cytochrome pathway activity during the fourth wate r stress cycle was small compared to the increase observed in leaves o f plants experiencing water stress for the first time. These results s uggest that cytochrome pathway activity is differentially sensitive to the rate of development of plant water deficits and that respiratory responses to acute water stress are not necessarily similar to the res ponses to chronic water stress.