During the dry summer of 1976, measurements were made of the photosynthesis, transpiration, respiration, and growth of irrigated (I) and non-irrigated (NI) spring barley growing in large field plots. Using a field enclosure, the photosynthesis of the irrigated barley was measured on 19 separate days during the latter two-thirds of the growing season when the plants were large enough to have measurable gas exchanges. The response of photosynthesis to water stress was determined from 3 days' comparative measurements on the I and NI crops, using both the field enclosure and, on 2 days only, a single-leaf photosynthesis chamber.

Water stress in the NI crop caused large decreases in yield: the dry-matter yields of grain and straw were respectively reduced by 19 and 27%; number of grains (but not grain mass) was also reduced. Field enclosure measurements, which were in good agreement both with values for canopy net photosynthesis derived from the leaf chamber measurements and with estimates of dry-matter production derived from plant weighings, indicated that the net CO2 uptake per unit leaf area was little affected by water stress. Results from the plant weighings and mensurations showed that, during the growing season, the main effect of water stress, mediated by the survival of fewer tillers and the premature senescence of leaves, was a reduction of leaf area (by 40%).