The effect of irrigation on radiation absorption, water use efficiency and yield of conventional and semi-leafless peas

The growth, water use and yield of conventional and semi-leafless peas were studied for two spring sowings under dryland and irrigated conditions on a Templeton silt loam soil at Lincoln during the 1981/82 season. Irrigation was applied weekly in amounts equal to the differences between the potential evapotranspiration and rainfall of the preceding week from either the vegetative , flowering or pod-swelling phase until maturity. For the first (23 October) sowing, dry seed yield increased from 2100 kg ha⁻¹ without irrigation to 4360 kg ha⁻¹ with irrigation throughout growth. Equivalent figures for the second (16 December) sowing were 1150 and 2940 kg ha⁻¹. Intermediate yields were obtained when the period of irrigation was restricted. Neither the differences in seed yield between cultivars nor the cultivar x irrigation interaction were significant. Irrigation increased pods per plant, peas per pod, mean seed weight and vine length, but there were significant cultivar x irrigation interactions in these components of yield. Irrigation increased the total dry matter production and harvest index by about 60% and 2% respectively for the first sowing; 70% and 10% in the second sowing. The increase in total dry matter production with irrigation was due to a 40% increase in growth efficiency (energy content of crop/solar energy absorbed during growth) and a 10% increase in the amounts of radiation absorbed in the first sowing; 60% and 20% respectively in the second sowing. Evapotranspiration integrated over the whole growing season approximated potential, corrected for cover and soil evaporation. However, over smaller time intervals it overestimated potential in the fully irrigated treatments but the error was large (20%). Both cultivars had an average water use efficiency (WUE) of about 25 kg ha⁻¹mm⁻¹ at both times of sowing. The WUE of irrigated treatments averaged 25 kg ha⁻¹mm⁻¹ for the first sowing and 20 kg ha⁻¹mm⁻¹ for the second sowing; there were no significant differences between irrigation treatments. The WUE of the unirrigated treatments was about 40% greater than that of the irrigated treatments in the first sowing and about 95% greater in the second sowing. In this dry season, therefore, there was no advantage to the semi-leafless cultivar either in yield or WUE. Irrigation applied during any phase, except pod growth for the first sowing promoted growth and yield. The results of the present experiment, therefore, do not support the idea of moisture sensitive periods during which yield of the crop is particularly susceptible to drought. Penman's (1971) model provides a potentially useful description of the response of the yield of peas to irrigation. The maximum response of yield to irrigation applied when needed was about 14 kg ha⁻¹mm⁻¹ for peas in Canterbury, but a more typical response could be about 8 kg ha⁻¹ mm⁻¹.