EFFECT OF WATER STRESS ON STOMATAL AND PHOTOSYNTHETIC RESPONSES IN SUBALPINE PLANTS TO CLOUD PATTERNS

Daily cloudcover is a frequent and persistent environmental feature of subalpine habitats. Response patterns in net photosynthesis (A), stomatal conductance to water vapor diffusion (g), transpiration (E), and water use efficiency (WUE = A/E) to intermittent cloudcover were evaluated for two subalpine species to determine if their responses supported the hypothesis that species that undergo water stress during sunlit intervals may have more rapid and greater stomatal responses to irradiance fluctuations than species under little apparent water stress. In Helianthella quinquenervis, a herbaceous perennial with midday xylem pressure potential <-2.0 MPa, responses in both A and g to simulated changes in natural irradiance occurred almost simultaneously and were relatively rapid. Although A was reduced by 22% during periods of shade compared with full sunlight periods, E was similarly reduced, and thus, no reduction in WUE occurred. In contrast, the riparian herb Senecio triangularis was under little apparent water stress and g was relatively insensitive to fluctuations in irradiance. As a result, periods of cloudcover led to a smaller reduction in A of only 15% relative to full sunlight with little concurrent reduction in E, leading to a corresponding 12% decrease in WUE compared with steady state values under full sunlight. These data suggest that species that experience apparent water stress during sunlit intervals may maintain higher WUE during intermittent cloudcover through water conservation at the expense of carbon gain. In contrast, species in which water stress does not occur during sunlit intervals can maximize carbon gain at the expense of reduced WUE. The degree of competition for water and sunlight in a habitat may determine the particular physiological response pattern expressed during natural cloudcover regimes. BROKEN OR intermittent cloudcover is a consistent feature of the summer growing season in the subalpine zone of the central and northern Rocky Mountains (Salisbury et al., 1968; Caldwell, 1968; Young and Smith, 1983; Olyphant, 1984; Knapp and Smith, 1987). Thus, subalpine plants are subjected to numerous, large fluctuations in incident solar irradiance over a day. For example, Salisbury et al. (1968) documented that sun/shade transition cycles due to clouds occurred over 50 times per day in the Colorado Rocky Mountains. Young and Smith (1983) also reported that clear days were rare in the subalpine zone of southeastern Wyoming and that half the days during the growing season were cloudy at least 40% of the total daylight period. Very little information exists regarding the physiological responses of plants to natural fluctuations in sunlight during cloudcover. Recently, Knapp and Smith (1987) measured the effect of simulated, natural cloudcover on net photosynthesis (A), stomatal conductance to water vapor diffusion (g), transpiration (E) and I Received for publication 12 May 1987; revision accepted 25 August 1987. Research supported by NSF Grant BSR-8700690. 2 Current address: Division of Biology, Kansas State University, Manhattan, KS 66506. water use efficiency (WUE = A/E) in several subalpine species. In general, responses in A were rapid and closely followed changes in solar irradiance, while changes in g were considerably slower during both shaded and sunlit periods. As a result, substantial differences in daily carbon gain, water loss and WUE occurred among the species (Knapp and Smith, 1987). A hypothesis generated from this initial study was that plants that experience water stress during sunlit intervals would have a more sensitive response pattern in g to fluctuations in irradiance (more closely tracking responses in A), thus resulting in greater water conservation (lower E) and WUE, but perhaps a reduction in A. Conversely, plants under little water stress may maintain higher g during shaded periods, enabling A to remain higher and return to maximum levels more rapidly during sunlit periods. In this case, carbon gain is favored at the expense of water conservation. The purpose ofthe present study was to compare physiological response patterns in a more mesic species, Senecio triangularis, with a species that experienced significant apparent water stress, Helianthella quinquenervis, to determine if these responses were consistent with the above hypothesis. Further documentation of the natural summer cloudcover regimes that