Root zone parameters controlling plant growth in soilless culture

Plant roots in soilless culture are exposed daily to large variations in root temperature while deep penetrating roots of soil grown plants can escape extreme hot and cold root temperatures. Root temperature control the root resistance to water flow. At the same water potential gradients, the rate of water flow through the stem in tomato was increased by 250% when the root temperature changed from 12 to 20°C at constant light radiation and air humidity and temperature of the canopy. When roots were cooled from 20 to 12°C the water flux dropped immediately to the same value observed in plants that their roots were kept constantly at 12°C. The presence of ammonium in the root zone solution is beneficial when the root temperatures are low. Ammonium becomes detrimental at high root temperatures. The differences in sensitivity between plants to ammonium toxicity are due to the differences in the sugar concentration in the root. Ammonium is metabolized completely in the root and needs sugar to produce soluble amino acids and prevent ammonia toxicity to the root cell's cytoplasm. The upward flux of nitrate, phosphate, and potassium is very much reduced with low root temperatures. Maintenance of optimum root temperature, specific for each plant cultivar, is the main factor in plant production under soilless culture conditions.