Production and water use in lettuces under variable water supply

The effects of a variable water supply on the water use, growth and yield of two crisphead and one romaine (i.e., Cos) lettuce cultivar were examined in a field experiment using a line source sprinkler system that produced a range of water regimes that occur in growers fields. Four locations at increasing distances from the main line were monitored through the season (i.e., from thinning to harvest, 28-63 days after planting (DAP)). These locations at the end of the season corresponded to: (1) rewatering to field capacity (FC); (2) watering with a volume 13% below that required in the field capacity treatment (0.87*FC); (3) 30% below FC (0.70*FC); and (4) 55% below FC (0.45*FC. A linear production function for dry matter accumulation and fresh weight vs. crop evapotranspiration (ET(c)) was determined for lettuce during this period, giving a water use efficiency for dry matter of 1.86 g m(-2) mm(-1) and for fresh weight of 48 g m(-2) mm(-1). For lettuce irrigated to field capacity, ET(c) between thinning and harvest was 146 mm; maximum crop coefficients of 0.81-1.02 were obtained at maturity (55-63 DAP). For the three irrigation treatments receiving the largest water application, ET(c) was higher in the Cos culivar than in the two crisphead lettuce cultivars which had similar ET(c). Plant fresh weight was more sensitive than dry weight to reduction in water supply. In the FC treatment, root length density and soil water extraction were greatest in the top 0-45 cm, and decreased rapidly below 45 cm depth. Soil water extraction by roots increased at lower depths when irrigation was reduced. Instantaneous rates of leaf photosynthesis and leaf water potential showed no response to the irrigation treatments in this study, despite differences in biomass production. Evaporation was determined to be the major component of ET(c) for 45 of the 63 days of the growing season. The large loss of water by evaporation during mid-season and the apparent insensitivity of lettuce to the volume of irrigation during this period may provide an opportunity for reducing irrigation applications.