Limitations to carrot (Daucus carota L.) productivity when grown with reduced rates of frequent irrigation on a free-draining, sandy soil

To determine the influence of reduced irrigation input, carrot (Daucus carota L. cv. Ivor) plants were grown on a coarse-textured, sandy soil during summer near Perth, Western Australia. Irrigation was applied daily with sprinklers at 5 rates ranging from 47 to 151% replacement of daily pan evaporation (Ep). A reduction in irrigation from the industry standard of 151% Ep to 124 or 97% Ep resulted in a 17% increase in water use efficiency (WUE) when calculated as total root yield/water applied. However, the percentage of total roots that were marketable declined with reduction in irrigation, being 73, 56, and 63% at 151, 124, and 97% Ep, respectively. This minimised any increase in WUE when calculated on a marketable yield basis. Further reductions in irrigation to 47 and 73% Ep resulted in large reductions in yield, WUE, and the percentage of marketable roots. Leaf biomass was not significantly different between the 151 and 124% Ep treatments and was reduced by 55 and 85% at 73 and 47% Ep, respectively. Soil water tension (psi(s)) varied from 4 to 80 kPa among the irrigation treatments. Pre-dawn leaf water potential (psi(1)) was most sensitive to psi(s) over psi(s) range of 0-20 kPa and the influence of psi(s) on psi(1) was small (400 kPa) compared with the diurnal variations in psi(1) of up to 1400 kPa, which occurred independently of significant changes in bulk soil psi(s). There was no significant effect of irrigation on the osmotic potential of leaf sap and there was no evidence of osmotic adjustment. For all irrigation treatments, photosynthesis (Pn) peaked in the early morning (0830 hours) at rates of 17-20 mumol CO2/m(2).s, after which Pn decreased to a transient minimum value at midday. The diurnal decline in Pn was positively correlated with psi(1) for all treatments, and minimum values of Pn varied from 12.5 to 7.8 mumol CO2/m(2).s for plants grown with 151 and 47% of Ep, respectively. When grown under frequently applied, suboptimal irrigation, carrot productivity is predominately limited by reduced leaf growth rates that limit leaf area to match transpiration requirements with water availability. Furthermore, even under conditions of water deficit, the aerial environment imposes a greater constraint to Pn than soil water availability.