INTERACTION BETWEEN POTASSIUM CONCENTRATION AND ROOT-ZONE TEMPERATURE ON GROWTH AND PHOTOSYNTHESIS OF TEMPERATE LETTUCE GROWN IN THE TROPICS

Lactuca sativa L. plants were grown at three root-zone temperatures (RZTs): 25 degrees C, 30 degrees C and ambient RZT (A-RZT) on an aeroponic system. Three potassium (K) concentrations: - 25% (minus K), control (standard K), and + 25% (plus K) were supplied to plants at each RZT. Plants grown at the plus K and 25 degrees C-RZT had the highest productivity, largest root system and highest photosynthetic capacity. The minus K plants at 25 degrees C-RZT had the highest shoot soluble carbohydrate (SC) concentration, but they had the highest root SC concentration in the plus K plants at A-RZT. However, the highest starch concentration was found in both shoots and roots of the plus K plants at 25 degrees C-RZT. The plus K plants had the highest shoot K concentration at 25 degrees C-RZT, but they had the highest root K concentration at A-RZT. Highest proportion of absorbed K was partitioned to shoots when the plants were grown with the plus K at 25 degrees C-RZT.