Gas Exchange of Carrot Leaves in Response to Elevated CO2 Concentration

Short-term responses of four carrot (Daucus carota) cultivars: Cascade, Caro Choice (CC), Oranza, and Red Core Chantenay (RCC) to CO₂ concentrations (C_a) were studied in a controlled environment. Leaf net photosynthetic rate (P_N), intercellular CO₂ (C_i), stomatal conductance (g_s), and transpiration rate (E) were measured at C_a from 50 to 1 050 mol mol^–1. The cultivars responded similarly to C_a and did not differ in all the variables measured. The P_N increased with C_a until saturation at 650 mol mol^–1 (C_i= 350–400 mol mol^–1), thereafter P_N increased slightly. On average, increasing C_a from 350 to 650 and from 350 to 1 050 mol mol^–1 increased P_N by 43 and 52 %, respectively. The P_Nvs.C_i curves were fitted to a non-rectangular hyperbola model. The cultivars did not differ in the parameters estimated from the model. Carboxylation efficiencies ranged from 68 to 91 mol m^–2 s^–1 and maximum P_N were 15.50, 13.52, 13.31, and 14.96 mol m^–2 s^–1 for Cascade, CC, Oranza, and RCC, respectively. Dark respiration rate varied from 2.80 mol m^–2 s^–1 for Oranza to 3.96 mol m^–2 s^–1 for Cascade and the CO₂ compensation concentration was between 42 and 46 mol mol^–1. The g_s and E increased to a peak at C_a= 350 mol mol^–1 and then decreased by 17 and 15 %, respectively when C_a was increased to 650 mol mol^–1. An increase from 350 to 1 050 mol mol^–1 reduced g_s and E by 53 and 47 %, respectively. Changes in g_s and P_N maintained the C_i:C_a ratio. The water use efficiency increased linearly with C_a due to increases in P_N in addition to the decline in E at high C_a. Hence CO₂ enrichment increases P_N and decreases g_s, and can improve carrot productivity and water conservation.