Regulation of growth and nutrient uptake under different transpiration regimes

To determine the extent to which air humidity affects the regulation of nutrient demand, an experiment with tomato plants was carried out under fully controlled climate conditions. Treatments consisted of three levels of relative air humidity (RH): 50%, 70% (control) and 95%, corresponding to 1.32, 0.79 and 0.13 kPa vapour pressure deficit (VPD), respectively. High humidity reduced the total plant dry matter, leaf dry weight, and total leaf area but no effect was found in leaf dry matter percentage. High humidity also increased the dry matter partitioning into the stems but reduced the dry matter partitioning into the leaves. Relative growth rate (RGR) was reduced by both high and low air humidity compared with the control with the stronger effect at 95%RH. Net assimilation rate (NAR) was more reduced than leaf area ratio (LAR) at low humidity, but at high humidity both parameters were affected to a similar extent. Water use efficiency (WUE) increased from 3.4 mgDM/ml H2O at 70%RH to 7.4 mgDM/ml H2O at 95%RH. The concentrations of N, K and Mg were not affected by humidity but concentrations of P, Ca and S in the plant were reduced at high humidity.