Root-zone temperature effects on photosynthesis, 14C-photoassimilate partitioning and growth of temperate lettuce (Lactuca sativa cv. 'Panama') in the tropics

The effect of root growth temperature on maximal photosynthetic CO2 assimilation (P (max)), carbohydrate content, C-14-photoassimilate partitioning, growth, and root morphology of lettuce was studied after transfer of the root system from cool root-zone temperature (C-RZT) of 20 A degrees C to hot ambient-RZT (A-RZT) and vice versa. Four days after RZT transfer, P (max) and leaf total soluble sugar content were highest and lowest, respectively, in C-RZT and A-RZT plants. P (max) and total leaf soluble sugar content were much lower in plants transferred from C-to A-RZT (C -> A-RZT) than in C-RZT plants. However, these two parameters were much higher in plants transferred from A-to C-RZT (A -> C-RZT) than in A-RZT plants. A-RZT and C -> A-RZT plants had higher root total soluble sugar content than A -> C-RZT and C-RZT plants. Leaf total insoluble sugar content was similar in leaves of all plants while it was the highest in the roots of C-RZT plants. Developing leaves of C-RZT plants had higher C-14-photoassimilate content than A-RZT plants. The A -> C-RZT plants also had higher C-14-photoassimilate content in their developing leaves than A-RZT plants. However, more C-14-photoassimilates were translocated to the roots of A-RZT and C -> A-RZT plants, but they were mainly used for root thickening than for its elongation. Increases in leaf area, shoot and root fresh mass were slower in C -> A-RZT than in C-RZT plants. Conversely, A -> C-RZT plants had higher increases in these parameters than A-RZT plants. Lower root/shoot ratio (R/S) in C-RZT than in A-RZT plants confirmed that more photoassimilates were channelled to the shoots than to the roots of C-RZT plants. Roots of C-RZT plants had greater total length with a greater number of tips and surface area, and smaller average diameter as compared to A-RZT plants. In C -> A-RZT plants, there was root thickening but the increases in its length, tip number and surface area decreased. The reverse was observed for A -> C-RZT plants. These results further supported the idea that newly fixed photoassimilates contributed more to root thickening than to root elongation in A-RZT and C -> A-RZT plants.