Effect of different photosynthetic photon flux densities (0, 500, 1000, 1500 and 2000 μmol m⁻²s⁻¹), temperatures (20, 25, 30, 35 and 40 °C) and CO₂concentrations (250, 350, 450, 550, 650 and 750 μmol mol⁻¹) on gas and water vapour exchange characteristics of Cannabis sativa L. were studied to determine the suitable and efficient environmental conditions for its indoor mass cultivation for pharmaceutical uses.

The rate of photosynthesis (P_N) and water use efficiency (WUE) of Cannabis sativa increased with photosynthetic photon flux densities (PPFD) at the lower temperatures (20–25 °C). At 30 °C, PN and WUE increased only up to 1500 μmol m⁻²s⁻¹ PPFD and decreased at higher light levels.

The maximum rate of photosynthesis (P_{N max}) was observed at 30 °C and under 1500 μmol m⁻²s⁻¹ PPFD. The rate of transpiration (E) responded positively to increased PPFD and temperature up to the highest levels tested (2000 μmol m⁻²s⁻¹ and 40 °C). Similar to E, leaf stomatal conductance (g_s) also increased with PPFD irrespective of temperature.

However, g_s increased with temperature up to 30 °C only. Temperature above 30 °C had an adverse effect on g_s in this species. Overall, high temperature and high PPFD showed an adverse effect on P_N and WUE.

A continuous decrease in intercellular CO₂ concentration (Ci) and therefore, in the ratio of intercellular CO₂ to ambient CO₂ concentration (Ci/Ca) was observed with the increase in temperature and PPFD. However, the decrease was less pronounced at light intensities above 1500 μmol m⁻²s⁻¹.

In view of these results, temperature and light optima for photosynthesis was concluded to be at 25–30 °C and ∼1500 μmol m⁻²s⁻¹ respectively. Furthermore, plants were also exposed to different concentrations of CO₂ (250, 350, 450, 550, 650 and 750 μmol mol⁻¹) under optimum PPFD and temperature conditions to assess their photosynthetic response.

Rate of photosynthesis, WUE and Ci decreased by 50 %, 53 % and 10 % respectively, and Ci/Ca, E and g_s increased by 25 %, 7 % and 3 % respectively when measurements were made at 250 μmol mol-1 as compared to ambient CO₂(350 μmol mol⁻¹) level. Elevated CO₂ concentration (750 μmol mol⁻¹) suppressed E and g_s ∼ 29% and 42% respectively, and stimulated P_N, WUE and Ci by 50 %, 111 % and 115 % respectively as compared to ambient CO₂ concentration. The study reveals that this species can be efficiently cultivated in the range of 25 to 30 °C and ∼1500 μmol m⁻²s⁻¹ PPFD. Furthermore, higher PN, WUE and nearly constant Ci/Ca ratio under elevated CO₂ concentrations in C. sativa, reflects its potential for better survival, growth and productivity in drier and CO₂ rich.