GAS-EXCHANGE OF CITRUS SEEDLINGS AT DIFFERENT TEMPERATURES, VAPOR-PRESSURE DEFICITS, AND SOIL-WATER CONTENTS

Midday reductions of stomatal conductance and carbon dioxide assimilation rates (A(CO2)) in Citrus are typically attributed to large leaf-to-air vapor-pressure differences or high atmospheric vapor-pressure deficits (VPD). This study investigated air temperature (T-a) and available soil water (ASW) level as corollary factors of atmospheric VPD that influence midday reduction of net gas exchange in citrus leaves. The influence of elevated atmospheric CO2 under conditions that inhibit net canopy A(CO2) was also investigated. Net canopy A(CO2) and evapotranspiration rates of Carrizo citrange [Poncirus trifoliata Raf x Citrus sinensis (L.) Osbeck] and Swingle citrumelo (P. trifoliata Raf x C. paradisii Macf.) seedlings grown in outdoor controlled-environment growth chambers were measured under two levels of T-a with concomitant changes in VPD and two levels of atmospheric CO2 concentration, which were changed in steps over time. Cyclical depletion of ASW was allowed to occur at each set of T-a/VPD and CO2 combinations. Highest net canopy A(CO2) rates at ambient CO2 concentration (330 mu mol . mol(-1)) were obtained at the low T-a/VPD level (29C/2.4 kPa) and ASW >50%. Diurnal canopy CO2 uptake rates decreased at the high T-a/VPD level (37C/3.6 kPa), and midday depression of canopy A(CO2) was observed at ASW levels <50%. Net canopy A(CO2) decreased at higher levels of ASW under the high T-a/VPD treatment than at the low T-a/VPD treatment. At the elevated CO2 concentration (840 mu mol . mol(-1)) net canopy CO2 uptake rates were double those that occurred at ambient CO2 levels and they did not exhibit midday reduction. Our data indicate that, when soil water is not readily available, citrus seedlings are more sensitive to high levels of T-a and VPD which results in reduction of CO2 uptake. The inhibitory effects of elevated VPD and reduced ASW on citrus net A(CO2) were lessened at the elevated atmospheric CO2 level.