The effects of a variable water supply on the water use, growth and yi
eld of two crisphead and one romaine (i.e., Cos) lettuce cultivar were
examined in a field experiment using a line source sprinkler system t
hat produced a range of water regimes that occur in growers fields. Fo
ur 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 (0.87FC); (3
) 30% below FC (0.70FC); and (4) 55% below FC (0.45*FC. A linear prod
uction function for dry matter accumulation and fresh weight vs. crop
evapotranspiration (ET(c)) was determined for lettuce during this peri
od, 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 appli
cation, 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 tre
atment, 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 redu
ced. Instantaneous rates of leaf photosynthesis and leaf water potenti
al showed no response to the irrigation treatments in this study, desp
ite differences in biomass production. Evaporation was determined to b
e the major component of ET(c) for 45 of the 63 days of the growing se
ason. 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 applica
tions.