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

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 degrees C-RZT and a diurnally fluctuating ambient temperature (25-40 degrees C). Plants grown at 25 degrees C-RZT exhibited greater non-stomatal and stomatal limitation of photosynthetic rates (light- and CO2-saturated O2 evolution, Pmax and light-saturated CO2 assimilation, Asat). Light saturated stomatal conductance (gs sat) was higher in 25 degrees C-RZT than A-RZT plants. Initial Rubisco activity and Rubisco protein was significantly lower in A-RZT plants than 25 degrees 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 degrees C-RZT. Predawn and midday leaf water potential (psileaf) were higher in 25 degrees 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 Asat, gs sat and Psileaf when plants were transferred from 25 degrees C-RZT to A-RZT. However, significant increases in these parameters were observed after only 3 days of RZT transfer from ambient to 25 degrees C. Decreases in non-stomatal limitation of photosynthesis, Pmax, 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 degrees C to ambient temperature. When plants were transferred from A-RZT to 25 degrees C-RZT, no significant changes in Pmax, 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.