Effects of partial root-zone drying irrigation on yield, fruit quality, and water-use efficiency in processing tomato

Previous partial root-zone drying (PRD) experiments on tomato, carried out under controlled conditions, have occasionally shown significant positive results in terms of increased water-use efficiency and fruit quality. These results raised expectations for the transfer of PRD to commercial-scale tomato growing. However, PRD experiments in the open field are completely lacking for processing tomato varieties. An experiment was conducted, in two consecutive growing seasons, to assess the potential of PRD compared to deficit irrigation (DI) or full irrigation (FULL), on two varieties of processing tomato, using drip irrigation. Both the DI and PRD treatments delivered a 50% recovery of crop evapotranspiration (ETc), whereas the FULL treatment delivered 100% of ETc. Soil moisture measurements highlighted the difficulty in attaining the desired alternate wet and dry pattern in different parts of the root-zone prescribed by PRD, as well as the considerable water stress experienced under PRD or DI. On average, over the 2 years, marketable yields were significantly reduced, by 56% under DI and by 52% under PRD, compared to FULL irrigation. Yield decreases were caused by reductions in fruit fresh weight (FW), rather than in the number of fruit per plant. DI and PRD improved fruit quality in terms of total soluble solids contents (TSSC) as well as titratable acidity (TA) and juice pH, but not lycopene concentrations or fruit colour. Water-use efficiency (WUE), calculated as the ratio of marketable yield to actual evapotranspiration (ETa; calculated using the water balance) was not significantly different among treatments. However, when computed as the ratio of above-ground dry weight (biomass) to ETa, WUE was higher, on average, in the PRD (2.1 kg m(-3)) and DI (1.9 kg m(-3)) treatments than in the FULL treatment (1.4 kg m(-3)).