Effects of relative humidity and nutrient supply on growth and nutrient uptake in greenhouse tomato production

• Elevated CO2 concentration with higher air humidity improved tomato growth.
• Higher air humidity promoted water use efficiency in low EC.
• Higher air humidity decreased nutrient content in leaves in low EC.
• Supplying higher EC nutrient solution did not lead to full recovery of Ca contents.

Higher relative humidity under elevated CO2 conditions increases stomatal conductance and is expected to promote photosynthesis and growth of tomato. However, yield increase under higher relative humidity has been often reported to be unstable, depending on the growing conditions. Therefore, we investigated the effect of relative humidity and nutrient supply on growth and nutrient uptake under elevated CO2 conditions in greenhouse tomato production. In two greenhouses, we grew tomato hydroponically at two electrical conductivity (EC) levels (low EC treatments: 0.8–1.2 dS m−1, high EC treatments: 1.6–1.9 dS m−1). In one greenhouse, we installed a humidification system. The other was designated as a control. The dry weight (DW) per plant tended to increase with humidification though without significant differences. The leaf area per plant was not affected by humidification, but the high EC treatment increased the leaf area. The average water uptake in the low EC with mist decreased compared with that without mist. Our results suggested that water use efficiency was increased by higher humidity, whereas the nutrient content of leaves was suppressed by mist in the low EC. In both the treatments, supplying higher EC levels of nutrient solutions increased N, K and P contents but did not increased Ca or Mg contents as well as in low EC without mist. These results suggest that supplying high EC nutrient solutions cannot increase Ca and Mg contents sufficiently in leaves and stems. To stably increase yield of tomato by humidification under elevated CO2 conditions, it is important to monitor the transpiration rate and carefully control relative humidity.