EFFECT OF POTASSIUM AND DROUGHT STRESS ON GAS EXCHANGE, WATER RELATIONS AND WATER USE EFFICIENCY OF HIBISCUS ROSA SINENSIS L. cv. LEPRECHAUN

Abstract

Rooted cuttings of Hibiscus rosa sinensis L. cv. Leprechaun were irrigated with full strength Hoaglands solution containing 0, 2.5, or 10 mM K+. Half of the plants at each K+ level were subjected to a 21-day slowly developing drought stress cycle (DS) followed by a recovery period (day 22), while the other half were non-drought stressed (NDS). Midday leaf water potential (Ψleaf) at day 21 were -1.5 to -1.6 MPa for DS and -0.5 MPa for NDS plants. Photosynthesis (A) was lowest during early stress and recovery of 0 mM K+ plants. Transpiration (E), stomatal conductance (g), and instantaneous water use efficiency (A/E), were generally lowest in 0 mM K+ plants. During peak stress, A was highest in the 2.5 mM K+ plants, whereas E was lowest and A/E highest in 10 mM K+ plants. Ψleaf did not differ among K+ treatments during peak stress and recovery, but osmotic potential was highest (least negative) and turgor potential lowest in 0 mM K+ plants. DS plants had lower carbon isotope discrimination (Δ) compared to NDS plants at all K+ levels, suggesting higher A/E for DS plants. Although there was no significant K+ effect, there was a trend at peak drought stress of Δ lower A and higher A at the 2.5 mM K+ level.