Water Stress and Clipping Management Effects on Guineagrass: II. Photosynthesis and Water Relations

Abstract Growth and biomass production and allocation in guineagrass are affected by clipping and water stress, but little is known about the mechanisms responsible. This study was undertaken to determine the short‐term interacting effects of clipping management and water stress on photosynthesis and water relations of guineagrass (Panicum maximum Jacq.). The experiments were conducted outdoors in pots in Maracaibo, Venezuela. After seedling emergence and establishment, plants were clipped at four frequencies and three heights for 3 mo, followed by exposure to water stress. After a stress cycle of 15 d, a group of plants was reirrigated for 21 d, to observe recovery. Photosynthesis and transpiration rates, stomatal conductance, leaf water potential, and water use efficiency (WUE) were measured. During the stress period, the most closely clipped plants had greater photosynthesis, but not increased stomatal conductance. Photosynthesis was also greater in more frequently clipped plants, and both photosynthesis and conductance were reduced by water stress. Upon rewatering, photosynthesis increased sharply. Leaf transpiration rates were not affected by clipping height or frequency during water stress. A linear decrease in WUE occurred as cutting height increased, but clipping frequency did not alter WUE. Water stress reduced leaf water potential, and this effect occurred sooner in plants clipped less frequently and to greater heights. After reirrigation, highest photosynthesis in closely clipped plants was associated with increased stomatal conductance; leaf transpiration rate, WUE, and water potential were not altered by clipping height or frequency. Partial defoliation may relieve water stress to some degree. The increase in photosynthesis caused by close clipping is not fully related to higher stomatal conductance as previously reported, so other factors must also be involved.