Leaf photosynthesis is enhanced, not impaired, under severe potassium limitation in spinach despite arrested growth

Potassium (K) deficiency has conventionally been reported to suppress leaf photosynthesis and growth. We show that in hydroponically grown spinach (Spinacia oleracea L.), severe K limitation maintained or increased the net photosynthetic rate per unit leaf area (PN) at both ambient and saturating CO2 concentrations with increased stomatal conductance, while whole-plant growth was severely restricted. Compared with K-sufficient controls, K-limited leaves had higher leaf mass per area, nitrogen and phosphorus concentrations, and SPAD values, whereas CO2-saturated photosynthetic rates per unit dry mass remained near control levels. This indicates that the increase in area-based PN reflects structural leaf densification, with the photosynthetic machinery remaining functional per unit mass. Sucrose concentrations tended to increase without hexose accumulation, and the carbohydrate-induced photosynthetic downregulation typical of nitrogen or phosphorus deficiency was absent. Shoot dry mass nonetheless fell by about 38%, revealing a marked decoupling between leaf photosynthesis and whole-plant growth under severe K limitation.