Contrasting patterns of leaf solute accumulation and salt adaptation in four phreatophytic desert plants in a hyperarid desert with saline groundwater

The seasonal variation of leaf solutes was investigated in four perennial phreatophytes in the natural vegetation surrounding a river oasis in the Chinese Taklamakan desert in order to elucidate their adaptation to saline groundwater. Leaves of the herbaceous perennial legume Alhagi sparsifolia, the poplar tree Populus euphratica, the salt cedar Tamarix ramosissima, and the C4 shrub Calligonum caput-medusae were collected at the end of each month during the growing season 1999 and analysed for cation, anion, organic acid, carbohydrate, glycinebetaine, and proline concentrations. The species revealed considerable differences in the foliar solute composition and their seasonal variation. Tamarix had high foliar mineral salt concentrations throughout the season but excreted the accumulated salts via salt glands. The three other species showed different degrees of ion regulation and ion selectivity. Calligonum had generally very low mineral ion concentrations, tolerated moderate Cl− concentrations and excluded Na+. Populus effectively excluded Cl− from its leaves but Na+ and total solute concentrations increased towards the end of the season which lead to K:Na ratio smaller than one in October. Alhagi showed the highest degree of ion selectivity by excluding Na+ but accumulating Ca2+ in its leaves. Mineral salt composition of xylemsap in Alhagi and Calligonum indicated that both species showed a similar degree of ion selectivity in the roots and that ion retransloction is probably another important mechanism to maintain low salt concentrations in the leaf. The cyclitols chiro-inositol and pinitol were the major compatible solutes in the plants and N-containing solutes like proline or glycinebetaine occurred only in minor concentrations or were absent, respectively. All plant species were well adapted to the moderate salinity of the NaCl dominated groundwater and no signs of salt related drought stress, ion toxicity or nutrient imbalance were observed.