University of Illinois at Urbana-Champaign. Water Resources Center
Abstract
Two of the largest problems in irrigation planning are 1) the measurement of crop moisture status and 2) the interpretation of the measurements in terms of irrigation need. This research program was devoted to both these problems and involved the development of a rapid, simple method for measuring crop moisture status that could be used on the individual farm, and the determination of the physiological response of corn and soybean to low moisture availability, As a result, a small, inexpensive pressure chamber was designed, field-tested, and shown to give reasonably accurate values for the moisture status of corn and soybeans. The response of leaf enlargement, photosynthesis, and dark respiration were measured under laboratory conditions in corn, soybean, and sunflower as moisture availability decreased. These data then were used to simulate these processes during a drought that occurred in the field. The simulation showed that, during vegetative development in corn and soybean, leaf enlargement was much more sensitive than photosynthesis or dark respiration to drought. Consequently, irrigation could be based on the maintenance of leaf enlargement alone. Specifically, if pressures measured with the pressure chamber were 30 psi or less at sundown and 2 hours after sunup in leaves normally exposed to full sun, leaf growth, photosynthesis, and dark respiration would be maximal. This relationship did not hold during reproductive growth, however. Chloroplast photosynthesis was studied and was shown to be limiting photosynthesis in sunflower at low moisture availabilities, Effects on the cloroplasts appeared to be largely in the "light” reactions to photosynthesis, where photosystem II, photosystem I, cyclic photophosphorylation, and noncyclic photophosphorylation were inhibited severely. Irrigation after a period of desiccation often brought about complete recovery of the chloroplasts but only incomplete recovery of the stomata, which had been closed prior to irrigation. Studies of liquid water transport showed that soybean had a higher resistance to water movement than did corn or sunflower and that the high resistance was associated primarily with the roots. The root resistance could be kept reasonably low by infecting the roots with mycorrhizal fungi. This work indicates that the moisture status of crops can be simply measured and interpreted at the practical level, and suggests means for irrigation planning which should maximize the efficiency of water use in corn and soybean growing vegetatively.U.S. Geological SurveyU.S. Department of the InteriorOpe
