Influence of potassium on drought resistance, root physiology, and root longevity of Hibiscus rosa sinensis L. cv. Leprecaun

Abstract

The effect of potassium (K) and drought stress on leaf elemental concentration, water relations, gas exchange, instantaneous water-use efficiency [(WUEj), the ratio of photosynthesis to transpiration], long-term water-use efficiency [estimated from carbon isotope discrimination (A)] and root longevity of Hibiscus rosa sinensis L. cv. Leprecaun were studied in a series of greenhouse experiments. Plants were fertilized with fullstrength Hoagland's nutrient solution containing 0,2.5, and 10 mM K (K1? K2 , and K3 , respectively). Root longevity was determined by the percentage of post-drought stress live-roots (PLR), using 8 6 Rb+ labeling. Total, shoot, root and leaf dry weight increased; while root:shoot ratio, and leaf area ratio of both drought-stressed and non-drought stressed plants decreased from Kj to K2 , respectively. Regardless of drought treatment, leaf area was always greatest in K3 plants, and drought stress reduced SLA of K2 and K3 plants. Thus at higher K, the transpiring surface area of the leaves was reduced relative to the photosynthetic mesophyll volume. High K (2.5 to 3.0% leaf K) increased the PLR under drought conditions (-1.5 MPa VPL). At K2 and K3, plants had higher relative water content (RWC), compared to Kj plants. Leaf Ca and Mg concentration decreased at K levels above 2.5 to 3.0%. At peak drought, photosynthesis (A) was highest in K2 plants, whereas transpiration (E) was lowest in K3 plants. WUE,- decreased as 'P l was reduced by drought stress from -0.5 MPa to -1.5 Mpa. Leaf osmotic potential ('FJC) was highest (less negative) and turgor potential ('Pp) was lowest in Kj plants. Drought-stressed plants had lower A compared to non-drought stressed plants at all K levels except for K2 in experiment 2. At peak drought, A was lower at K3, despite higher A in K2 plants. This indicates increased drought tolerance in H. rosa sinensis, at leaf K concentration in the range 2.5 to 3.0%.