Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)






The occurrence of compensatory photosynthesis was examined in the field during the spring for all foliage elements on two Agropyron bunchgrass species that differ in their evolutionary history of grazing pressure. Compensatory photosynthesis did occur in many individual foliage elements during at least part of their ontogeny. For both species, compensatory photosynthesis was related primarily to delayed leaf senescence and increased soluble protein concentrations, but not to an improvement in the water status of clipped plants. Photosynthetic water use efficiency and photosynthetic rates per unit soluble protein of foliage on partially defoliated plants were not increased following the clipping treatments. Light and temperature dependencies of gas exchange measurements were usually very similar between the two Agropyron species. However, gas exchange rates per unit foliage area of leaves exserted late in the spring on~ spicatum plants were significantly different from those on A. desertorum plants when these leaves were senescing. To determine the ecological significance of these differences between species for light and temperature dependencies, the average carbon gain and water loss rate per tiller were estimated. The differences between species for carbon gain and water loss rates per tiller in this environment were substantially less than the individual leaf gas exchange differences between species. Photosynthetic activity and survival of leaves were also determined during the fall, winter, and early spring for the two Agropyron species in the field. A large proportion of the leaves of both species survived the winter. Photosynthetic rates of both species declined as air temperature dropped during the fall, were slightly positive during the winter between periods of snow cover, and increased during the early spring. Even though there is potential for photosynthesis during a winter with intermittent snow cover, total plant saccharide pools were barely maintained over such a winter. Although A. desertorum and A. spicatum were exposed to different levels of grazing pressure during their evolutionary history, the phenology, water status, and gas exchange rates of foliage and tillers were very similar both for undefoliated as well as partially defoliated plants. Therefore, we conclude that compensatory photosynthesis does not appear to be an important ecological component of herbivory tolerance for these species.