Photosynthetic Responses of the Citrus Varieties Rangpur Lime and Etrog Citron to Salt Treatment

Abstract

Seedlings of Rangpur lime (Citrus reticulata var. austera hyb.?) and Etrog citron (C. medica L.), which differ markedly in ability for salt exclusion (i.e. the ability to restrict the uptake and/or transport of salt between roots and shoots), were grown under glasshouse conditions and supplied with dilute nutrient solution containing either 0 or 50 mM NaCl. Photosynthetic response to salt treatment and subsequent recovery were followed for 105 days for Etrog citron and for 119 days for Rangpur lime. Photosynthesis in mature leaves of both varieties was progressively reduced by salt treatment irrespective of ability for salt exclusion. The photosynthetic decline in each case was related to increases in stomatal and internal resistances. The reduction in photosynthetic capacity in Etrog citron leaves was associated with high leaf chloride concentrations while in Rangpur lime, a salt excluder, it could be related to a loss of leaf turgor. Leaf sodium concentrations were not markedly increased by salt treatment in either variety.

Cessation of salt treatment led to a progressive recovery of photosynthesis for both varieties accompanied by a reduction in both stomatal and internal resistances. Recovery in Etrog citron leaves occurred despite little change in leaf chloride concentrations. These leaves tolerated up to 350 mM chloride (leaf water basis) under glasshouse conditions without sustaining a permanent reduction in photosynthetic capacity.