Responses to flooding and drought stress by two citrus rootstock seedlings with different water-use efficiency

Leaf water relations, net gas exchange and leaf and root constituent responses to 9 days of drought stress (DS) or soil flooding were studied in 6-month-old seedlings of Carrizo citrange [Citrus sinensis (L.) Osb. x Poncirus trifoliata L.; Carr] and Cleopatra mandarin (Citrus resnhi Hort. ex Tanaka; Cleo) growing in containers of native sand in the greenhouse. At the end of the drought period, both species had similar minimum stem water potentials but Cleo had higher leaf relative water content (RWC) and higher leaf osmotic potential at full turgor (Psi(100)(pi)) than Carr. Flooding had no effect on RWC but osmotic adjustment (OA) and Psi(100)(pi) were higher in Cleo than in Carr. Net CO2 assimilation rate (A(CO2)) in leaves was decreased more by drought than by flooding in both species but especially in Carr. Leaf water-use efficiency (A(CO2)/transpiration) was lower in Carr and was decreased more by DS and flooding stress than in Cleo. Higher values of intercellular CO2 concentration (C-i) in stressed plants than in control plants indicated that non-stomatal factors including chlorophyll degradation and chlorophyll fluorescence [maximum quantum efficiency of PSII (Fv/Fm, where Fm is the maximum fluorescence and F-0, minimum fluorescence in dark-adapted leaves)] were more important limitations on A(CO2) than stomatal conductance. In both genotypes, leaf proline was increased by drought but not by flooding, whereas both stresses increased proline in roots. Soluble sugars in leaves were increased by DS, and flooding decreased leaf sugars in Cleo. In general, DS tended to increase the concentrations of Ca, K, Mg, Na and Cl in both leaves and roots, whereas flooding tended to decrease these ions with the exception of leaf Ca in Cleo. Based on water relations and net gas exchange, Cleo was more tolerant to short-term DS and flooding stress than Carr.