Little is known about the effect of varying levels of potassium (K) on the mineral element concentration, growth, and gas exchange, characteristics of woody ornamental plants. The commercially important woody ornamental species Hibiscus rosa-sinensis L. cv. Leprechaun was evaluated for K response in a series of three experiments with full strength Hoagland's nutrient solution, which supplied 0 to 10 mM K. Plants grown with 4 mM K in nutrient solution (2.4% leaf tissue K) had the greatest shoot growth and root extension. Gas exchange rates (net photosynthesis, transpiration, and stomatal conductance) were also highest at 4 mM K compared to the control (0 mM K /0.6% leaf tissue K), 0.2, 2.0 and 10 mM K treatments. The application of 4 mM K increased net photosynthesis and tranpiration by 2.1 fold and stomatal conductance by 4.5 fold over 0 mM K controls. Increasing K in nutrient solution correlated positively with tissue K, manganese (Mn), and zinc (Zn), but negatively with nitrogen (N), phosphorus (P), calcium (Ca), and magnesium (Mg). There was a stronger sink for K in yonger leaves (the first to fourth fully expanded leaf from the shoot apex) which had higher K concentration than older leaves (the eighth to twelfth fully expanded leaf from the shoot apex). However, with increasing K in nutrient solution, K concentration in leaf tissue increased regardless of leaf age, and the difference between the younger and older leaf was constant. Daily application of 10 mM K resulted in 6.9% leaf tissue K and caused a decrease in plant total dry matter, net photosynthesis, compared to 4 mM K treated plants. However, these parameters remained higher in 10 mM K plants, which retained high ornamental quality than in 0 mM controls. Plants fertilized with 10 mM K had the highest leaf tissue K and Zn, but lowest P, Ca, Mg, iron (Fe), copper (Cu) and boron (B). Nevertheless, the 10 mM K treated plants exhibited no morphological differences or deficiency symptoms; rather those plants had similar vegetative vigor and newer bud formation rate as those at 4 mM K.
