ADJUSTMENT MECHANISMS OF PLANTS SUBJECTED TO VARIED OSMOTIC PRESSURES OF NUTRIENT SOLUTION

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Abstract
Bean and pepper plants grown in nutrient solution in a controlled environment were subjected to water stress by gradual addition of NaCl or polyethylene glycol (PEG) to increase the osmotic pressure of the nutrient solution. The osmotic pressure, percent soluble solids, and Ca++, Na+, Cl- content of expressed juice of roots, leaves and stems and the relative turgidity and percent dry weight of leaves were determined. With each increase in osmotic pressure of the nutrient solution, there was a proportional but smaller increase in the osmotic pressure of the expressed juice of the plants. This response was the same whether plants were growing in PEG or NaCl solution. There was an abrupt decrease in relative turgidity of the leaves and an increase in concentration of K+ and Ca++ in juice as the osmotic pressure of the nutrient solution was changed from 0.5 bar to approximately 2 bar. Additional increases in the osmotic pressure of the solution resulted in similar but smaller changes in the relative turgidity and concentration of K+ and Ca++. The concentration of Na+ and Cl- in the juice of the plants increased as the concentration of NaCl in the solution increased. Addition of PEG to the solution had little or no effect on the concentration of Na+ and Cl- in the juice. Associated with the increase in the osmotic pressure of the solution was a gradual increase in soluble solids of the juice. The increase was greatest in plants grown in PEG solutions. Calculations were made of the effect that changes in concentration of each measured component had on the increase in the osmotic pressure of the juice. The initial response was dehydration, followed by accumulation of soluble compounds, NaCl when it was used as solute, and soluble solids when PEG was used as a solute.