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

Abstract

The effect of root growth temperature on maximal photosynthetic CO2 assimilation (P max), carbohydrate content, 14C-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 °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 14C-photoassimilate content than A-RZT plants. The A→C-RZT plants also had higher 14C-photoassimilate content in their developing leaves than A-RZT plants. However, more 14C-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.