Recovery of Photosynthesis in Sunflower after a Period of Low Leaf Water Potential

Abstract

Photosynthesis was studied in sunflower plants subjected to 1 to 2 days of desiccation and then permitted to recover. The leaf water potential to which leaves returned after rewatering was dependent on the severity of desiccation and the evapora-tive conditions. Under moderately evaporative conditions, leaf water potential returned to predesiccation levels after 3 to 5 hours when desiccation was slight. Leaf water potentials remained below predesiccation levels for several days after re-watering when leaf water potentials decreased to-13 to-19 bars during desiccation. Leaf water potential showed no sign of recovery when leaf water potentials decreased to-20 bars or below during desiccation. The lack of full recovery of leaf water potential was attributable to increased resistance to water transport in the roots and stem. The resistance ultimately became large enough to result in death of the leaves because net water loss continued even after the soil had been rewatered. Measurements of photosynthesis were made at high light intensities, where stomatal aperture often affects photosynthe-sis, and at low light intensities, where the photochemical activity of the leaves limits photosynthesis. Providing leaf water potentials remained above-12 bars during the desiccation period and returned to predesiccation levels during recovery, photosynthesis under both low and high light paralleled the recovery in leaf water potential after rewatering. After desiccation to leaf water potentials below-12 bars, recovery was incomplete under high light and could be attributed to lack of full stomatal opening. Lack of full opening persisted for 3 days and showed no sign of eventual recovery even though leaf water potentials recovered fully. Under low light, however, recovery in photochemical activity was complete within 15 hours after desiccation to leaf water potentials as low as-17 bars. Plants often recover only partially from desiccation after they have been rewatered. There is evidence that following a period of desiccation, rates of photosynthesis under high light (2, 11, 21, 25) and transpiration (11, 17, 21) may remain lower, resistance to water movement through the root system may be larger (17, 18), and stomata may open less in light than do those of nondesiccated control plants (12). If these reactions are not simply expressions of acelerated senescence, they could be caused by leaf water potentials that do not rise to the levels 'Supported by Grant B-036-I11 from the Office of Water Resources Research, Department of Interior. present before desiccation. For example, increased root resistance could result in low leaf water potentials, and incomplete stomatal opening might then be attributable to low water potentials rather than to an after-effect of desiccation acting on the stomatal mechanism itself. There are few measurements (7) of the degree to which leaf water potential recovers after a period of desiccation. The present work describes experiments which test whether the incomplete recovery of photosynthesis is associated with leaf water potentials that remain low after rewatering or whether some aspect of the photosynthetic process is inhibited irreversibly by desiccation. MATERIALS AND METHODS Sunflower (Helianthus annuus L. var. Russian Mammoth) plants were grown from seed in a controlled environment chamber (day temperature, 29 ± 0.5 C; night temperature, 23 + 0.5 C; relative humidity, 70 + 5%; light intensity, 0.19 cal cm-2 min-1 (fluorescent); photoperiod, 14 hr). In one experiment , soybean (Glycine max L. [Merr.] var. Harosoy) plants were also used and were grown under the same conditions. When the sunflower and soybean plants were approximately 45 cm tall, low leaf water potentials were produced by withholding water from the soil for varying lengths of time. In some instances, soil was gently washed from roots, and low leaf water potentials were produced by placing the root system in water at 10 C. Except when stated otherwise, recovery was initiated by rewatering the soil or, when roots were chilled, by warming the root system. Leaf water potential was measured by the isopiestic technique with thermocouple pyschrometers which could accommodate either intact leaves (6) or excised leaf tissue (4, 10). Several hours before measurements were made, the experimental tissue was rinsed in distilled water and permitted to dry in order to remove surface contaminants. The walls of the psychrometer chambers were coated with melted and re-solidified petrolatum to reduce adsorption of water vapor (5), and all measurements were corrected for heat of respiration (3). In intact leaf experiments which required continuous determination of leaf water potentials, an initial isopiestic measurement (4, 10) was made but water was placed on the thermo-couple junction for subsequent determinations. The data from the isopiestic measurement provided a correction factor for the diffusive resistance of the tissue which remained the same for any particular tissue (6) and could be used to correct the determinations with water on the thermocouple. The rates of net photosynthesis and transpiration were measured by a method previously described (9) in shoots of intact plants in an assimilation chamber (air temperature, 25 ± 0.25 C; leaf temperature, within 0.6 C of air temperature; relative humidity, 77 + 2%; light intensity, 1.6 cal cm-2 min-' [in-candescent, filtered through 10 cm of water] unless otherwise specified; net radiation, approximately 0.28 of incident light 816