Root Morphology, Plant Growth, Nitrate Accumulation and Nitrogen Metabolism of Temperate Lettuce Grown in the Tropics with Elevated Root-Zone CO₂ at Different Root-Zone Temperatures

Abstract

This paper investigated the effects of root-zone (RZ) CO 2 concentration ([ CO 2 ]) on root morphology and growth, nitrate (NO 3 - ) uptake and assimilation of lettuce plants at different root-zone temperatures (RZT). Elevated RZ [CO 2 ] stimulated root development, root and shoot growth compared to ambient RZ [CO 2 ]. The greatest increase in root growth was observed in plants grown under elevated RZ [CO 2 ] of 50,000 ppm. However, RZ [CO 2 ] of 10,000 ppm was sufficient to achieve the maximal leaf area and shoot productivity. Lettuce plants exhibited faster shoot and root growth at 20°C-RZT than at ambient (A)-RZT. However, under elevated RZ [CO 2 ], the magnitude of increased growth was greater at A-RZT than at 20°C-RZT. Compared to RZ [CO 2 ] of 360 ppm, elevated RZ [CO 2 ] of 10,000 ppm increased NO 3 - accumulation and nitrate reductase activity (NRA) in both leaves and roots. NO 3 - concentrations of leaf and root were higher at 20°C-RZT than at A-RZT in all plants. NRA was higher in root than in leaf especially under A-RZT. The total reduced nitrogen (TRN) concentration was significantly higher in plants grown under elevated RZ [CO 2 ] of 10,000 ppm than under ambient RZ [CO 2 ] of 360 ppm with greater concentration in 20°C-RZT plants than in A-RZT plants. These results imply that elevated RZ [CO 2 ] significantly affected root morphology, root and shoot growth and N metabolism of temperate lettuce with greater impacts at A-RZT than at 20°C-RZT. These findings have practical significance to vegetable production by growing the vegetable crops at cool-RZT with elevated RZ [CO 2 ] to enhance its productivity.