Use of Concentrated Macronutrient Solutions to Separate Osmotic from NaCl-specific Effects on Plant Growth

Abstract

The aim was to distinguish between osmotic and ion-specific effects of NaCl on plant growth and ion uptake by comparing plants grown in isosmotic solutions with and without NaCI. Preliminary experiments showed that polyethylene glycol 4000 and mannitol were unsuitable for even very-short-term studies because they caused immediate reductions in leaf elongation rate when plants were transferred from NaCl to isosmotic solutions of these compounds. However, concentrated solutions of macronutrients (modified Hoagland's nutrients) did not change the elongation rate.

Barley, wheat, Egyptian clover and white clover were grown in NaCl and concentrated macronutrient solutions of matching osmotic pressures. After 14 days, plants grown in concentrated macronutrients were smaller than controls (plants in normal strength nutrient solution) but had similar root : shoot ratios. NaCl-grown plants were less than half the size of plants in concentrated macronutrients, and had higher root : shoot ratios. NaCl-induced phosphate uptake did not cause this additional reduction in shoot growth. For barley, net transport of K⁺, Mg²⁺, Ca²⁺ and total nitrogen from the roots (per g root dry wt) was lower in NaCl-grown plants than in controls, but uptake by the shoot (per g shoot dry wt) of these minerals was similar. By contrast, both transport and uptake of these minerals in concentrated macronutrient-grown plants resembled control plants. NaCl-grown barley and wheat plants had higher osmotic pressures in both growing and mature tissue than did controls and macronutrient-grown plants, but a lower rate of uptake of solutes generating this osmotic pressure. We raise the possibility that growth in NaCl may be limited by a reduced rate of transport of an essential nutrient to the shoot.