Managing Fruit Soluble Solids with Late-season Deficit Irrigation in Drip-irrigated Processing Tomato Production

Fruit soluble solids concentration (SSC) is an important quality factor for toma- toes (Lycopersicon esculentum Mill.) grown for processing. The use of drip irrigation often results in undesirably low SSC. The effects of late-season irrigation management on fruit yield and SSC was investigated in a series of drip-irrigated fi eld trials in California from 2000-04. The effects of irrigation cutoff or defi cit irrigation implemented 40 to 50 days preharvest (the period corresponding to the initiation of fruit ripening) were compared to a standard grower practice of irrigation cutoff 20 days preharvest. Irrigation cutoff 40 to 50 days preharvest increased SSC but resulted in substantial yield loss, with signifi - cantly reduced brix yield (Mg fruit solids ha -1 ). By contrast, defi cit irrigation signifi cantly increased SSC compared to the standard practice, with no signifi cant loss of brix yield. In three commercial fi elds the effect of defi cit irrigation on fruit SSC was investigated. Fruits were sampled on three dates: 1) 4 to 5 weeks preharvest, early-ripening, pink-stage fruit only, 2) about 1 week preharvest, both late-ripening, pink-stage fruit and early-ripening fruit now fully ripe, and 3) commercial harvest, composite of early- and late-maturing fruit. SSC increased in response to soil moisture stress induced by defi cit irrigation, with late-maturing fruit as much as 1.6 °brix higher than fruit maturing before signifi cant soil moisture stress. However, once a fruit reached the pink stage of maturity, its SSC was not affected by subsequent soil moisture stress. An additional fi ve commercial fi eld trials were conducted to compare growers' irrigation practices with greater degrees of defi cit irrigation. In each fi eld the grower's defi cit irrigation regime was compared to a reduced treatment receiving 25% to 50% less water over the fi nal 4 to 7 weeks before harvest. Across fi elds, applying 20% to 60% of reference evapotranspiration (ET o ) over the fruit ripening period resulted in acceptable SSC without signifi cant brix yield reduction. We conclude that defi cit irrigation initiated during early fruit ripening provides a fl exible tool for SSC management. Brix monitoring of earliest ripening fruit can help classify fi elds as to the severity of irrigation defi cit required to reach desirable SSC at harvest. The use of drip irrigation in California processing tomato production is increasing rapidly, with >12,000 ha drip-irrigated in 2004. Drip irrigation minimizes plant water stress during growth and development by allowing frequent, small-volume irrigations (Hartz, 1996), and offers the potential to effi ciently deliver nutrients (Hartz and Hochmuth, 1996). Conversion to drip irrigation has typically in- creased processing tomato fruit yield by 20% or more compared to furrow irrigation, the industry standard. However, some processors remain skeptical of drip irrigation because fruit soluble solids concentration (SSC) has gener- ally been lower than that achieved with furrow irrigation; high SSC improves both paste yield per unit of fresh fruit and overall processing effi ciency. Linden (2004) reported that each 0.1 °brix decrease in SSC costs California processors about $1.30/Mg of raw product in additional processing and handling.