Relationship among photosynthesis, ribulose-1,5-bisphosphate carboxylase (Rubisco) and water relations of the subtropical vegetable Chinese broccoli grown in the tropics by manipulation of root-zone temperature

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Photosynthesis, initial Rubisco activity, Rubisco protein, total soluble protein and water relations were studied in subtropical Chinese broccoli ( Brassica alboglabra) grown aeroponically in a tropical (Singapore) greenhouse. Aerial parts of the plant were maintained at hot ambient (A) temperature, but with their roots exposed to two different root-zone temperatures (RZTs): a constant 25°C-RZT and a diurnally fluctuating ambient temperature (25–40°C). Plants grown at 25°C-RZT exhibited greater non-stomatal and stomatal limitation of photosynthetic rates (light- and CO 2-saturated O 2 evolution, P max and light-saturated CO 2 assimilation, A sat). Light saturated stomatal conductance ( g s sat) was higher in 25°C-RZT than A-RZT plants. Initial Rubisco activity and Rubisco protein was significantly lower in A-RZT plants than 25°C-RZT. The total soluble protein per unit leaf area showed a marked decrease in plants grown at A-RZT compared to those grown at 25°C-RZT. Predawn and midday leaf water potential ( Ψ leaf) were higher in 25°C-RZT than A-RZT plants. Reciprocal RZT transfer experiments were also conducted to study the relationship among photosynthesis, Rubisco and water relations. Immediate parallel decreases were observed in A sat, g s sat and Ψ leaf when plants were transferred from 25°C-RZT to A-RZT. However, significant increases in these parameters were observed after only 3 days of RZT transfer from ambient to 25°C. Decreases in non-stomatal limitation of photosynthesis, P max, initial Rubisco activity, Rubisco and total leaf soluble proteins were not detected until 4 or 5 days post-transfer, while decreases in total leaf soluble protein was observed 2 or 3 days after RZT transfer from 25°C to ambient temperature. When plants were transferred from A-RZT to 25°C-RZT, no significant changes in P max, initial Rubisco activity, Rubisco and total leaf soluble proteins were observed during the first 4 days of RZT transfer. These results indicate that the direction of RZT transfer determined the relative contributions of stomatal and non-stomatal limitations to photosynthesis and the importance of water stress.