Interactive effects of environmental stresses on photosynthesis

by Griffin, Jason Jay.

GRIFFIN, JASON JAY. Interactive Effects of Environmental Stresses on Photosynthesis. (Under the direction of Dr. Thomas G. Ranney.) Plants are frequently exposed to a variety of environmental stresses that occur separately or in combination. The independent, additive, and interactive effects of these stresses, and the ability of a plant to resist these stresses, can be important factors in plant growth, distribution, and survival. Cercis canadensis L. (eastern redbud) has a broad range that includes diverse ecotypes. In this study eastern redbud and Cercis canadensis var. mexicana (Rose) M. Hopkins (mexican redbud) were examined for their tolerance to high temperature and drought stress. In another study, taxa of Illicium L. (flowering anise) were evaluated for their differential tolerance to high irradiance and underlying photosynthetic characteristics of different taxa grown in full-sun and 50% shade. Thermotolerance and drought tolerance of eastern redbud and mexican redbud were studied by growing containerized plants under high temperatures and increasing drought. Although both ecotypes responded similarly, the mexican redbud maintained higher stomatal conductance and photosynthesis than eastern redbud as the root substrate dried. The mexican redbud also maintained greater instantaneous water use efficiency (WUE). At the optimum temperature for photosynthesis of all taxa (37 °C), mexican redbud had a greater maximum rate of photosynthesis than both watered and droughtstressed eastern redbud. The maximum rate of photosynthesis was greatest for watered plants, and the mexican redbud maintained a higher rate of assimilation than the eastern redbud. Tissue osmotic potential was more negative in the eastern redbud, but was unaffected by drought stress in either ecotype. Soluble carbohydrate concentration was also higher in the eastern ecotype, perhaps contributing to differences in osmotic potential. In both ecotypes, pinitol was the major carbohydrate and its concentration was considerably higher in the drought-stressed plants. Total polyol accumulation was greater in the drought-stressed plants as well. Both ecotypes proved to be very tolerant of high temperatures and drought. In addition to drought and heat stress, irradiance stress is another commonly encountered environmental stress. Species of Illicium are mostly understory species that are not well adapted to high irradiance. To investigate how light affects Illicium, 11 commercially available taxa were grown in full sun or under 50% shade. Light-tolerance was evaluated by measuring light-saturated photosynthetic capacity (Amax), dark-adapted quantum efficiency of photosystem II (Fv/Fm), and relative chlorophyll concentration. Amax indicated that three of the 11 taxa maintained similar rates of photosynthesis when grown in full-sun as when grown in 50% shade. All other taxa in full-sun experienced a significant reduction in Amax. Fv/Fm was similar between sun and shade plants for the same three taxa that were able to maintain Amax,. Chlorophyll concentration was not significantly reduced in these taxa either, whereas the other taxa did experience a significant reduction. In fact, chlorophyll concentration was higher in I. parviflorum ‘Forest Green’ grown in full sun, which is trait typical of sun tolerant species. A deeper examination of I. parviflorum ‘Forest Green’ (high-light tolerance) and I. floridanum (low-light tolerance) demonstrated that I. parviflorum ‘Forest Green’ had a considerably higher Amax, a higher light saturation point, greater potential photosynthetic capacity, reduced susceptibility to photoinhibition as indicated by superior PSII efficiency following light exposure, greater capacity for thermal de-excitation as indicated by a higher rate of non-photochemical quenching (NPQ) under full sun, greater apparent electron transport rate (ETR) at mid-day, and higher concentrations of the free-radical scavenger myo-inositol. All of these factors potentially contribute to a greater capacity to utilize light energy for carbon fixation while minimizing photodamage.