Intrinsic changes in photosynthetic parameters of carrot leaves under increasing CO2 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 P Nmax by nearly 85 % compared to controls. Dark respiration rate (RD) showed no consistent response to moisture deficit. The CO 2 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/C a 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. © 2007 Institute of Experimental Botany, ASCR.