Response of sand‐grown tomato supplied with varying ratios of nitrate/ammonium to constant and variable root temperatures

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Abstract
Tomato (Lycopersicon esculentum Mill.) is commercially grown in a wide range of climatic conditions. There is a need to understand nutrient uptake by tomato at various climatic conditions in order to devise the specific nutrient supply to this important crop. Tomato seedlings, at the first flower blooming stage, were transplanted into 6‐L pots of washed sand. Four root temperature regimes (day/night, °C) 12/12, 16/08, 17/17, and 30/20, and four temperatures of the shoots 20/20, 25/25, 25/15, and 30/20 were controlled separately for the shoots and roots. All four growth cabinets had the same day length of 13 h and light intensity of 400 μmole/m2/s measured on top of the canopy, The relative humidity was 85%. The nutrient solutions contained 10 mM N in three ratios of nitrate:ammonium (NO3:NH4)—10:0, 8:2, and 6:4 each in three replications in each growth cabinet. The water consumption, fresh and dry weight, water soluble nutrient in the plant, exudate composition at the end of the experiment and total mineral composition in the shoots were determined. The transpiration ratio for the high root temperatures was 190 g H2O/g DM. Based on rate of water uptake, plant growth on the 16/08°C root regime was significantly higher than that of 12/12°C root regime, despite the fact that the average temperature on time was the same in both treatments. Low root temperatures decreased the concentrations of cations in the plant but had no effect on the balance between cations and anions (C‐A). Increasing NH4 proportion decreased C‐A in the plant. At root temperature of 12°C, potassium (K) and NO3 were present at high concentration in the exudate but soluble NO3 in the shoot was the lowest. The result suggests that nitrate reduced in the shoot produced organic anions that are left in the shoot and their charge balance that on K, due to slow root growth at low temperatures. Higher DM was produced by the NO3 only form at low variable root temperature than in the treatments that contained NH4. At low root temperature, the K selectivity against sodium (Na) was reduced. This suggest that, at low root temperatures, high K concentration in the nutrient solution might he needed.