Na+ accumulation in shoot is related to water transport in K+-starved sunflower plants but not in plants with a normal K+ status.

Sunflower plants (Helianthus annuus L. cv Sun-Gro 380) grown in nutrient solutions with different K+ levels were used to study the effect of potassium status on water uptake, Na+ uptake and Na+ accumulation in the shoot. Changes in nutrient potassium levels induced evident differences in internal potassium content. When both low and normal-K+ plants were exposed to 22 degrees C and salinity conditions (25 or 50 mM NaCl) during a short time period (9h), water uptake in low-K+ plants was greater than in normal-K+ plants. In addition, K+ starvation favoured the Na+ uptake and the Na+ accumulation both in the root and in the shoot. When the plants were exposed to heat stress by a sharp increase of the temperature to 32 degrees C during the same period of time, the stimulating effect of K+ starvation on the water uptake was even greater. The high temperature increased Na+ uptake in both types of plants, but the Na+ accumulation in the shoot was only favoured in low-K+ plants. The results suggest that Na+ accumulation in the shoot is more dependent on the water uptake in low-K+ plants than in normal-K+ plants, and this effect could explain the greatest susceptibility to the salinity in K+-starved plants under high transpiration conditions, which are typical in dry climates.