EFFECTS OF ROOT-ZONE TEMPERATURE ON THE ROOT DEVELOPMENT AND NUTRIENT UPTAKE OFLACTUCA SATIVAL. “PANAMA” GROWN IN AN AEROPONIC SYSTEM IN THE TROPICS

Lactuca sativa L. cv. Panama is a temperate plant, but can be grown in the tropics by only subjecting its roots to 20°C while its aerial portions are exposed to the hot, fluctuating temperatures in the greenhouse. This study showed that in Lactuca sativa L. cv. Panama grown at 20°C root-zone temperature (RZT) in the tropics, the roots were longer with a greater number of root tips and total root surface area, and smaller average root diameter as compared with those of ambient RZT (A-RZT) plants. In plants transferred from 20°C to A-RZT (20°C ⇒ A-RZT) when they were 3 weeks old, rate of increase in root length was decreased as was root tip number and surface area; root thickening was increased. The reverse was observed for plants transferred from A-RZT to 20°C-RZT (A ⇒ 20°C-RZT). Mineral nutrients such as NO3 −, nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), copper (Cu), iron (Fe), magnesium (Mg), manganese (Mn), and zinc (Zn) present in the plant shoot and root tissues were also determined. Generally, it was found that 20°C-RZT plants had higher leaf N and P concentrations on the basis of per unit dry weight compared with plants grown at A-RZT. Results also showed that total shoot and root NO3 −, K, Ca, Cu, Fe, Mg, Mn, and Zn accumulation of 20°C-RZT plants were more than A-RZT plants. 20°C ⇒ A-RZT plants suffered from a reduction of total mineral accumulation, and Ar ⇒ 20°C-RZT plants increased in total mineral accumulation. Relationships between RZT, root growth, and mineral composition of the experimental plants are discussed.