Leaf K+ Interaction with Water Stress Inhibition of Nonstomatal-Controlled Photosynthesis

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Abstract
The relationship between leaf K+ concentration, in vitro dehydration, and nonstomatal-controlled photosynthesis was investigated using spinach [Spinacia oleracea] leaf slices that were vacuum infiltrated with media containing varying sorbitol concentrations. The leaf slices were from plants either supplied with complete or K+-deficient medium throughout a 35-day growth period. During this time, leaf K+ concentration, water potential, osmotic potential, and turgor pressure were monitored. Leaf K+ concentration averaged 239 micomoles per gram (fresh weight) in control plants, and dropped to 74.3 micromoles per gram (fresh weight) in K+-deficient plants. Less negative osmotic potentials and resultant turgor loss in K+-deficient plants indicated that the osmotically active pool of cellular K+ was lower in those plants. The decrease in leaf K+ concentration enhanced the dehydration inhibition of photosynthesis. For example, increasing sorbitol from 0.33 to 0.5 molar during incubation inhibited photosynthesis in the controls by 14% or less. This same protocol resulted in an inhibition of photosynthesis by as much as 41% in K+-deficient tissue. In contrast to the data obtained with leaf slices, dehydration inhibition of isolated chloroplast photosynthesis was not affected by K+ status of parent plant material. These data are consistent with the hypothesis that one effect of leaf K+ deficiencies on photosynthetic response to dehydration may be mediated by extra-choloroplastic factors. Ammonium ions, which facilitate stromal alkalinization, reversed the increased sensitivity of K+-deficient leaf slice photosynthesis to cell dehydration. However, NH4+ had no effect on photosynthesis of K+-deficient leaf slices under nonhypertonic conditions. These data suggest that endogenous extra-chloroplastic K+ may modulate dehydration inhibition of photosynthesis, possibly by facilitating stromal alkalinization.