Relationship between carbon partitioning and Na+, Cl- and ABA allocation in fruits of salt-stressed bean

Ion toxicity is considered one of the most important limiting factors affecting plants growing under long-term saline conditions. Abscisic acid participates in the plantʹs adaptation to different environmental stresses. However, most of the information available on this subject is for plants exposed to water deficit during the vegetative growth phase. In this study, we examine the role of ABA and ion allocation in the control of fruit growth in salt-stressed bean. Plants of Phaseolus vulgaris were grown in solution culture with 1, 25, 50, or 75 mmol/L NaCl. After 40 days, growth, photosynthetic parameters and ion, protein and ABA concentrations were recorded in the different plant organs. Leaf area and instantaneous C02 assimilation rates were inhibited by salt. Salinity induced a decrease in harvest index (fruit dry weight/total plant dry weight) simultaneously with an increase in root index (root dry weight/total plant dry weight). The changes observed in carbon allocation were greatly related to sodium index (fruit Na+/total plant Na+). No differences among treatments were found either in chloride index (fruit Cl-/total plant Cl-) or ABA index (fruit ABA/total plant ABA). Since bean is a Na+ excluder species, an adaptation mechanism that promotes root over fruit growth would enhace Na+ exclusion and favor plant survival. The signalling mechanism may be triggered by a possible effect of apoplastic Na+ on the plasma membrane, but seems not to be mediated by an increase in ABA.