The Mechanism Behind the Promotive Effect of Foliar Application of 5-Aminolevulinic Acid (ALA) in Tomato Plants Under Salt Stress

Abstract

The effect of foliar ALA application on the internal ALA concentration in tomato plants grown in soil containing high levels of NaCl  was investigated. Six week old plants were treated with 100, 50, and 25 mmol/L NaCl on a weekly basis and they were simultaneously treated with 5-ALA at a concentration of 5%  by foliar spray. The effect of foliar ALA application on plant growth, chlorophyll contents and internal ALA concentration was studied. The internal ALA shoot concentrations ranged between 27.50±2.12 and 34.35±1.48 µg g^-1 dry weight with no significant difference (p<0.05) recorded between plants treated with NaCl alone and those treated with both NaCl and ALA. The concentrations of chlorophyll a and b were elevated only in tomato plants treated with NaCl and ALA, whereas their levels decreased in plants treated with NaCl only. An adverse significant effect (p<0.05) of salinity stress was recorded on plants length, number of leaves, shoot and root fresh and dry weight. However, no significant difference  (p<0.05) was observed in plants treated with  NaCl alone with those treated with  NaCl plus ALA in comparison to the control. The results of the present study suggested that foliar ALA treatment had no effect on the Na and Cl uptake, the internal ALA concentration, and had no role in adverting the effects of salinity on plant growth. The present study has proven that foliar ALA is directly used by the plant for the synthesis of chlorophyll in order to increase the photosynthetic rate and thus to help tomato plants to survive the salinity stress.