Effects of soil temperature and elevated atmospheric CO2 concentration on gas exchange, in vivo carboxylation and chlorophyll fluorescence in jack pine and white birch seedlings

One-year-old jack pine (Pinus banksiana Lamb.) and current-year white birch (Betula papyrifera Marsh.) seedlings were grown in ambient (360 ppm) or twice ambient (720 ppm) atmospheric CO2 concentration ([CO2]) and at three soil temperatures (Tsoil = 7, 17 and 27 °C initially, increased to 10, 20 and 30 °C two months later, respectively) in a greenhouse for 4 months. In situ foliar gas exchange, in vivo carboxylation characteristics and chlorophyll fluorescence were measured after 2.5 and 4 months of treatment. Low Tsoil suppressed net photosynthetic rate (Pn), stomatal conductance (gs) and transpiration rate (E) in jack pine in both CO2 treatments and gs and E in white birch in ambient [CO2], but enhanced instantaneous water-use efficiency (IWUE) in both species after 2.5 months of treatment. Treatment effects on gs and E remained significant throughout the 4-month study. Low Tsoil reduced maximal carboxylation rate (Vcmax) and PAR-saturated electron transport rate (Jmax) in jack pine in elevated [CO2] after 2.5 months of treatment, but not after 4 months of treatment. Low Tsoil increased actual photochemical efficiency of photosystem II (PSII) in the light (ΔF/Fm′) in jack pine, but decreased ΔF/Fm′ in white birch after 4 months of treatment. In response to low Tsoil, photosynthetic linear electron transport to carboxylation (Jc) decreased in jack pine after 2.5 months and in white birch after 4 months of treatment. Low Tsoil increased the ratio of the photosynthetic linear electron transport to oxygenation (Jo) to the total photosynthetic linear electron transport rate through PSII (Jo/JT) in both species after 2.5 months of treatment, but the effects became statistically insignificant in white birch after 4 months of treatment. High Tsoil decreased foliar N concentration in white birch. Elevated [CO2] increased Pn, IWUE and Jc but decreased Jo/JT in both species at both measurement times except Jc in white birch after 2.5 months of treatment. Elevated [CO2] also decreased gs and E in white birch at high Tsoil, Vcmax in both species and triose phosphate utilization in white birch at low Tsoil after 4 months of treatment, and ΔF/Fm′ in white birch after 2.5 months of treatment. Elevated [CO2] also increased foliar N concentration in both species. Low Tsoil caused no permanent damage to PSII in either species, but jack pine responded and acclimated to low Tsoil more quickly than white birch. Photosynthetic down-regulation and a decrease in photosynthetic electron transport to photorespiration occurred in both species in response to elevated [CO2].