Effects of temperature regime and water deficit on leaf physiology and biomass productivity
Temperature regime showed significant effects on gas exchange variables including light saturated photosynthetic rate (Asat; P <0.001), stomatal conductance (gs, P <0.001) and the ratio of intercellular to ambient CO2 (Ci/Ca; P <0.001) (Figure 1 and 2; Table 1 and S1). Compared with plants grown under cool daytime temperatures, Asat, gs and Ci/Ca increased by 31.5%, 105.6% and 10.5%, respectively, under warm day growth temperatures. In addition, rates of leaf dark respiration (Rn) were significantly increased by growth temperature (P <0.001), with Rn being 30.3% higher in warm grown plants than in cool grown plants, although this pattern was less clear in the well-watered treatment (Figure 1, Table S1). On the other hand, water deficit treatment did not alter most gas exchange variables, except for Rn, which was decreased by 11% in response to water deficit stress, on average. A significant interaction was detected for temperature and water treatment on leaf level carbon balance represented by the difference between Asat and Rn (Asat-Rn; P <0.001). However, the variation of leaf carbon balance across treatments was primarily driven by growth temperature rather than water availability, given that Asat-Rnincreased by 10.7% in warm grown plants compared with cool grown plants, but decreased by 2% in the well-watered treatment relative to water deficit treatment, on average.
Warmer growth temperature had significant effects on the optimum temperature of photosynthesis (Topt; P =0.02) (Figure 3, Table 2). Topt of plants grown under 28/22oC was higher than Topt of the other temperature treatments (ca . +1oC); nonetheless, the variation in Topt across temperature treatments was relatively minor. There was a significant interaction between growth temperature and water treatment on photosynthetic rate at Topt (Aopt; P =0.03). Growth temperature was the main determinant of the variation across treatments given that the difference induced by the water treatment was small for all temperature treatments.
Both temperature regime and water treatment showed significant main effects on the number of days required for plant height to reach 50% of its maximum height (H50; P <0.01 for both treatments); H50 was higher at cool temperatures and in the well-watered treatment on average (Figure 4; Table 1 and S2). Growth temperature and water regime showed significant interactive effects on the dry mass of vegetative organs (P <0.01), as well as total aboveground dry mass (P <0.05). Although the pattern of variation across treatment combinations was less evident, both variables were generally decreased by elevated growth temperature and water deficit stress. In addition, growth temperature and water independently affected the fruit biomass (P <0.001 for both treatments), with values lower under the warm temperature regimes (-15.6%) and water deficit treatment (-11.8%) compared with cool temperature regimes and well-watered treatment, respectively (Table S2).