ABSTRACT Background Marginal water sources, such as wastewater, have been proposed as the basis for hydroponic nutrient solutions to improve the resource efficiency of greenhouse‐based tomato production. Such water sources are often low in plant‐available nutrient concentrations, and uptake and use efficiency of these nutrients may vary widely among tomato varieties. Water and nutrient uptake are strongly influenced by environmental factors, particularly temperature and air humidity (summarized by the term VPD vapor pressure deficit). Aim In‐depth understanding of the interactions between environmental conditions (VPD) and variety‐specific nutrient acquisition is required to optimize the use of marginal water in hydroponic tomato production. Methods In a greenhouse experiment, we investigated water use, nutrient uptake, plant growth, and biomass partitioning in four tomato varieties (“Saluoso,” “Sweeterno,” “Reddery,” and “Moneymaker”) over 30 days under two VPDs (2.1 kPa and 0.3 kPa). Plants were grown in weakly concentrated nutrient solutions (20 mg L −1 N, 5.1 mg L −1 P) using deep‐water hydroponics. Results Evapotranspiration, nutrient uptake, and dry matter accumulation were significantly higher under high VPD (2.1 kPa) compared with low VPD (0.3 kPa), with strong varietal differences. In “Reddery,” biomass and leaf area were higher by factors of 2.0 and 3.0, respectively, under high VPD (2.1 kPa), whereas in “Moneymaker,” they were higher by factors of 1.2 and 1.5. Cumulative N and PO 4 –P uptake was significantly higher under high VPD, with “Saluoso” and “Sweeterno” showing up to twice the total uptake of “Moneymaker” and “Reddery.” Under low VPD (0.3 kPa), transpiration rates as well as calcium and potassium concentrations in leaves were reduced. Conclusions As a consequence, using marginal water sources in high VPD environments requires careful optimization of nutrient solution management, as well as the selection of suitable crop varieties and appropriate cultivation systems able to maintain nutrient uptake and thus productivity under increased transpiration.
Detering et al. (Thu,) studied this question.