Time Trends of Solute Accumulation and the Influence of Potassium Fertilizer on Osmotic Adjustment of Water-Stressed Leaves of Three Tropical Grasses

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Abstract
Osmotic adjustment, calculated from the difference in concentrations of ions and sugars between stressed and unstressed leaves of green panic, buffel grass and speargrass given normal or high potassium supply in the field, was determined during a 37-day drying cycle and following rewatering. Pressure/volume (P/V) curves were used to characterize the water relations of buffel grass with normal K supply on eight occasions during the experiment, and of the remaining treatments only at the end of the drying cycle. The maximum osmotic adjustment for the three grasses was 0.84-1.0 MPa calculated from chemical analyses of solutes and 0.5-0.7 MPa derived from the P/V analyses. High K supply did not enhance osmotic adjustment of water-stressed leaves. Osmotic adjustment reached a plateau 22 days after watering ceased, and its relationship with leaf water potential (Ψ) was the same for all species. When midafternoon Ψ was c. -4.8 MPa and leaves were permanently wilted, osmotic adjustment was 90% of maximum, suggesting that osmotic adjustment ceased when growth and most physiological activity were curtailed by loss of turgor. Osmotic adjustment delayed the onset of permanent wilting in water-stressed leaves by c. 4 days. Therefore, its contribution to the maintenance of leaf growth during water stress in these three grasses was small.