Elongation of Wheat Leaves Exposed to Several Levels of Matric Potential and NaC1‐Induced Osmotic Potential of Soil Water1

Abstract Leaf elongation is a major indicator of plant growth. It is partly controlled by turgor potential which, in turn, is affected by soil water potential and/or by the osmotic potential of the soil water. This study was conducted to determine the interaction of osmotic potential of Soil water induced by NaCl (0, −4, −8, and −12 bars) and soil matric potential induced by carbowax 4000 (−0.3, −2.5, −5.0, and −7.5 bars) on the turgor potential and elongation rate of leaves of 2‐week‐old wheat seedlings (Triticum aestivum L.). Relationships between rate of leaf elongation and leaf turgor potential and rate of leaf elongation and leaf water potential at all combinations of the experimental variables were obtained. Seedlings of spring wheat (Triticum aestivum L. ‘Fielder’) were grown in a Xerollic Durargids loam soil separated from the surrounding nutrient solution by a semipermeable cellulose‐acetate membrane. The rate of leaf elongation of the 2‐week‐old plants, growing in a walk‐in growth room, was measured at 24‐hour intervals with a ruler. Turgor potential of leaves was obtained as the difference between leaf water potential measured with a pressure chamber apparatus and osmotic potential of leaf cells measured with a thermocouple psychrometer. At the matric potential of −0.3 bars, where cell dehydration does not occur, accumulation of salt in the leaves at osmotic potentials of −4, −8, and −12 bars appeared to increase the turgor potential of leaf cells. Under moderately low to very low soil water potentials of −2.5 to −7.5 bars combined with low osmotic potentials of the soil water, leaf water potential decreased more strongly than could be counteracted by osmoregulation through a decrease in the osmotic potential of the cell; consequently, the turgor potential decreased as the osmotic potential of soil water decreased. We found significantly different linear relationships between rate of leaf elongation and turgor potential and between rate of leaf elongation and leaf water potential at the experimental osmotic potentials of the soil water. The “gross extensibility” and threshold turgor potential of the leaves were different at the various osmotic potentials of the soil water, ranging from 5.29 to 2.01 μm/min‐bar and from +0.33 to +1.33 bars, respectively. The rates of leaf elongation decreased at an accelerated rate as the osmotic potential of the soil water decreased. Leaf elongation ceased at leaf water potentials of −18.2, −20.7, −24.4, and −29.7 bars at osmotic potentials of the soil water of 0, −4, −8, and −12, respectively. The toxic effects of NaCl on metabolic processes of cell wall extension have been discussed.