Intrinsic changes in photosynthetic parameters of carrot leaves under increasing CO<sub>2</sub> concentrations and soil moisture regimes

Abstract

A controlled growth chamber experiment was conducted to investigate the short-term water use and photosynthetic responses of 30-d-old carrot seedlings to the combined effects of CO2 concentration (50–1 050 µmol mol –1 ) and moisture deficits (–5, –30, –55, and –70 kPa). The photosynthetic response data was fitted to a non-rectangular hyperbola model. The estimated parameters were compared for effects of moisture deficit and elevated CO2 concentration (EC). The carboxylation efficiency (α) increased in response to mild moisture stress (–30 kPa) under EC when compared to the unstressed control. However, moderate (–55 kPa) and extreme (–70 kPa) moisture deficits reduced α under EC. Maximum net photosynthetic rate (PNmax) did not differ between mild water deficit and unstressed controls under EC. Moderate and extreme moisture deficits reduced PNmax by nearly 85 % compared to controls. Dark respiration rate (RD) showed no consistent response to moisture deficit. The CO2 compensation concentration (Γ) was 324 µmol mol –1 for –75 kPa and ranged 63–93 µmol mol –1 for other moisture regimes. Interaction between moisture deficit and EC was noticed for PN, ratio of intercellular and ambient CO2 concentration (Ci/Ca), stomatal conductance (gs), and transpiration rate (E). PN was maximum and Ci/Ca was minimum at –30 kPa moisture deficit and at Ca of 350 µmol mol –1 . The gs and E showed an inverse relationship at all moisture deficit regimes and EC. Water use efficiency (WUE) increased with moisture deficit up to –55 kPa and declined thereafter. EC showed a positive influence towards sustaining PN and increasing WUE only under mild moisture stress, and no beneficial effects of EC were noticed at moderate or extreme moisture deficits.