Sunflower plants (Helianthus annuus L. cv. Zwerg Sonnengold) were cultivated in pots containing 1 kg of soil fertilized with 0.6 g K (K1) and 5.0 g K/pot (K2). At the age of 5 and 11 weeks, respectively the plants were transferred to a growth chamber. 50% of the plants per K treatment received sufficient watering (controls), the remaining 50% were subjected to water stress until visible signs of severe wilting were observed. At that stage the plants were rewatered.
In both growth stages and even under water stress conditions the up to 5 times higher K concentrations in the tissue of K2 plants caused an increased dry matter production during the experimental periods and a larger total leaf area which was less reduced under water stress than the leaf area of K1 plants.
During continuous watering the water content per leaf area of K2 plants was on the average 1.5g H2O dm−2 (K1: 1.35g), the water content of older K2 plants being on the average 1.33g H2O dm−2 (K1: 1.25g). During water stress and subsequent recovery this relation observed between K2 and K1 remained constant. In young K1 plants, however, no intensified loss of succulence was found during severe drought compared with 11‐week‐old K1 plants where the water content decreased from 1.2g to 0.6g H2O per dm2 of leaf area.
At almost equally high soil water availability stomatal diffusive resistance especially of the older unstressed K2 plants was higher than in K‐deficient plants. Under water stress, the degree of stomatal opening of K2 plants at first decreased more rapidly, but at equally low soil water potentials diffusive resistance in the leaves of K2 plants remained lower than in K1 plants.
A relation of the leaf water characteristics, total water potential (41), osmotic potential (42), and turgor potential (Φp) to the corresponding soil water potential showed that under stress Φ1 and Φs in plants with sufficient K nutrition always remained on a higher level than in K1 plants. Consequently, they were able to maintain a higher turgor pressure even under conditions of restricted water availability.
