Some Effects of Root Anatomy on K, Na and Cl Loading of Citrus Roots and Leaves

Abstract

Loading of K, Na and Cl into fibrous roots of salt-treated citrus genotypes, Rangpur lime (Citrus reticulata var. austera hybrid?) and Etrog citron (C. medica L.) was investigated in relation to root anatomy, in particular, the differentiation of the epidermal-hypodermal layers with distance from the root tip. The influence of duration of salinity treatment on the characteristics of K, Na and Cl accumulation in leaves of the two genotypes was explored in two experiments respectively, covering the short term (14 d) and long term (12 weeks). This study focused on two regions of the fibrous root, a segment 2–12 mm from the root tip, immediately basipetal to the zone of elongation and a differentiated region of the mature suberized, fibrous root, 40–50 mm from the root tip. In the distal root segment (2–12 mm) the epidermis and hypodermis of both genotypes was observed as two closely packed, uniseriate layers of living cells. In the proximal root segment (40–50 mm) the differentiated hypodermis was evident as a uniseriate layer of thick-walled lumina interspersed with ‘passage cells’ which were frequently associated with clusters of viable epidermal cells. The characteristics of Na and Cl loading in the two root zones differed profoundly during the short term loading (acclimation) phase. Attainment of quasi-steady-states for Na and Cl in the distal region (with the exception of Na in Etrog citron) was rapid as was Na equilibration of the proximal root segments in both genotypes. In contrast, Cl loading in the proximal region took c. 14 d to reach a quasi-steady-state by which time Cl levels were 2 to 3 times higher in the proximal than in the distal root segments. The superior tolerance of Rangpur lime to long term salinity was highly correlated to Cl exclusion from the leaves. However, during the first 14 d of acclimation to 50 mol m⁻³ NaCl there was no segregation of the two genotypes based upon leaf Cl levels. Expression of differential accumulation of Cl in leaves appeared to be a time dependent process and was manifest only after Cl saturation of the proximal root which represents the bulk of the fibrous root system. The salt tolerance of Rangpur was also associated with high selectivity of fibrous roots for K. over Na. A pronounced loss of K from cortical cells in the proximal root segment of salt-stressed Etrog citron was also evident by X-ray microanalysis.