Photosynthetic response of carrots to varying irradiances

Response to irradiance of leaf net photosynthetic rates (P-N) of four carrot cultivars: Cascade, Caro Choice (CC), Oranza, and Red Core Chantenay (RCC) were examined in a controlled environment. Gas exchange measurements were conducted at photosynthetic active radiation (PAR) from 100 to 1000 mumol m(-2) s(-1) at 20degreesC and 350 mumol(CO2) mol(-1)(air). The values of P-N were fitted to a rectangular hyperbolic nonlinear regression model. P-N for all cultivars increased similarly with increasing PAR but Cascade and Oranza generally had higher P-N than CC. None of the cultivars reached saturation at 1 000 mumol m(-2) s(-1). The predicted P-N at saturation (P-Nmax) for Cascade, CC, Oranza, and RCC were 19.78, 16.40, 19.79, and 18.11 mumol(CO2) m(-2) s(-1), respectively. The compensation irradiance (I-c) occurred at 54 mumol m(-2) s(-1) for Cascade, 36 mumol m(-2) s(-1) for CC, 45 mumol m(-2) s(-1) for Oranza, and 25 mumol m(-2) s(-1) for RCC. The quantum yield among the cultivars ranged between 0.057-0.033 mol(CO2) mol(-1)(PAR) and did not differ. Dark respiration varied from 2.66 mumol m(-2) s(-1) for Cascade to 0.85 mumol m(-2) s(-1) for RCC. As P-N increased with PAR, intercellular CO2 decreased in a non-linear manner. Increasing PAR increased stomatal conductance and transpiration rate to a peak between 600 and 800 mumol m(-2) s(-1) followed by a steep decline resulting in sharp increases in water use efficiency.