Photosynthetic response of Cannabis sativa L., an important medicinal plant, to elevated levels of CO2.

The effect of elevated CO2 concentrations (545 and 700 μmol mol(-1)) on gas exchange and stomatal response of four high Δ(9)-THC yielding varieties of Cannabis sativa (HPM, K2, MX and W1) was studied to assess their response to the rising atmospheric CO2 concentration. In general, elevated CO2 concentration (700 μmol mol(-1)) significantly (p < 0.05) stimulated net photosynthesis (P N), water use efficiency (WUE) and internal CO2 concentration (C i), and suppressed transpiration (E) and stomatal conductance (g s) as compared to the ambient CO2 concentration (390 μmol mol(-1)) in all the varieties whereas, the effect of 545 μmol mol(-1) CO2 concentration was found insignificant (p < 0.05) on these parameters in most of the cases. No significant changes (p < 0.05) in the ratio of internal to the ambient CO2 concentration (C i/C a) was observed in these varieties under both the elevated CO2 concentrations (545 and 700 μmol mol(-1)). An average increase of about 48 %, 45 %, 44 % and 38 % in P N and, about 177 %, 157 %, 191 % and 182 % in WUE was observed due to elevated CO2 (700 μmol mol(-1)) as compared to ambient CO2 concentration in HPM, K2, MX and W1 varieties, respectively. The higher WUE under elevated CO2 conditions in Cannabis sativa, primarily because of decreased stomatal conductance and subsequently the transpiration rate, may enable this species to survive under expected harsh greenhouse effects including elevated CO2 concentration and drought conditions. The higher P N, WUE and nearly constant C i/C a ratio under elevated CO2 concentrations in this species reflect a close coordination between its stomatal and mesophyll functions.