Evaluation of Chloride Tolerance IN Selected Vegetable Crops.

Chloride ion (Cl^-) is regarded as an essential micronutrient for optimal plant growth at levels 0.3-1.0 mg g^-1 dry matter in most plants. But such influence is plant dependent. Its benefits include in several physiological metabolism processes in plants, such as osmoticum and stomatal regulation (together with K), photosynthesis, and disease resistance or tolerance. Adequate supply of Cl^- can improve the yields and quality of some crop species. However, excessive Cl^-, acting as a salinity stress, can be toxic to plants. The objective of this study was to investigate Cl^- tolerance of four vegetables under hydroponic conditions with or without Ca²⁺. In this study, four vegetable plants (bean, pepper, tomato, and lettuce) were selected. Chloride was supplied as NaCl only or NaCl+CaCl₂ (1:1 as of Cl^-) at seven levels (mM): CK, 0.05, 1.0, 5.0, 10.0, 20.0, 40.0. After growth for 1 week in nutrient solution, plants were treated with Cl^- at different levels every week, and harvested after 4 week’s treatment of Cl-. Chlorophyll a (Chla) in plant leaves was measured before and 1 week after Cl^- treatment and before harvest. Plant yields and both macro- and micro-elements in plants were measured after harvest.   Chla concentration in bean leaves was not different before Cl^- treatment, but apparently lower (P< 0.05) after Cl^- treatment as NaCl only at 40mM level of Cl- after one week and before harvest. Such difference was not evident when Cl- was supplied as NaCl+CaCl₂. In tomato leaves, lower Chla concentration (P< 0.05) was observed after Cl- was supplied as NaCl at 10 mM or above for more than one week, but as NaCl+CaCl₂ at 20mM or above. In pepper leaves, no differences in Chla concentration were observed among different supply levels of Cl^- and between the two sources. However, in lettuce leaves, Chla concentration was not different among Cl^- levels when it was supplied as NaCl only, while 1mM treatment of Cl^- as NaCl+CaCl₂ resulted in lower Chla as compared to other treatments.   Chloride treatments resulted in lower shoot dry matter yields of bean as NaCl only at >10 mM, but no differences as NaCl+CaCl₂. A significant decrease in shoot yields of tomato was observed with Cl^- as NaCl₂ only at ≥10 mM, but no differences when Cl^- was supplied as NaCl+CaCl₂. Effect of Cl on shoot yields of pepper were complicated, when Cl^- was supplied at 1.0 mM, it resulted in the lowest yield, but peaked at 40 mM Cl as NaCl only and at 5 mM Cl^- as NaCl+CaCl₂. Lettuce had lower yields of shoots when treated with Cl- as NaCl only at ≥20 mM, but increasing shoot yields as NaCl+CaCl₂ from 0.1 mM to 20 mM, then dropped at 40 mM. Root dry matter yields of bean plant were the lowest at 40 mM Cl as NaCl only, however, increased at Cl- supply of ≥5mM as NaCl+CaCl₂. No apparent decreases in tomato yields of roots were measured at different Cl^- levels as NaCl only, however, when Cl^- supply exceeded 10 mM as NaCl+CaCl₂, tomato yields of roots significantly increased (P <0.05). Root yields of pepper were comparable to its shoot yields, lowest at 1.0mM Cl^-, but peaked at 40mM Cl as NaCl only and at 5mM Cl as NaCl+CaCl₂. No evident differences of lettuce yields of roots were observed at different Cl- supply levels, however, ≥10mM Cl as NaCl+CaCl₂ apparently increased lettuce root yields (P <0.05). Root tips, surface area, and total length at different Cl levels varied similarly as root yields.   The results from this study indicated that Cl^- tolerance in plant is specie-dependent and Ca may in some degree alleviate the negative effect of overdose Cl.

Key words: High plants; Chlorophyll a; Yields; Root morphology; Calcium.