K+ starvation increases water uptake in whole sunflower plants

Eleven- to twenty-day-old intact sunflower plants (Helianthus annuus L. cv. Sun-Gro 380) grown in nutrient solutions with different K+ levels were used to study the effect of K+ status and abscisic acid (ABA) on water flow. Changes in nutrient potassium levels prompted no significant changes in growth, although there were evident differences in internal K+ content and water uptake capacity. Water uptake was greater, and water use less efficient, in moderately K+ deficient plants than in controls. Addition of 4 μM ABA to the external medium induced a differential effect between the two types of plant, inhibiting water uptake more effectively in plants with normal K+ status. The effect of internal K+ content on xylem sap pH was also studied, pH was more acidic in moderately K+ deficient plants than in controls. These results suggest that water flux regulation could be impaired in K+-starved plants.