Source: From M. Fernando et al., Plant Physiol. 100:1269-1276, 1992.

organic and inorganic osmotica in plants. There is a question, however, as to whether these can be provided quickly to fast-growing tissues, and in most cases it is the K+ that is delivered at sufficient rates. In natrophilic species, Na+ may substitute for K+ in this osmotic function. The high vacuolar turgor in expanding cells produces the pressure potential required for growth. This pressure may be insufficient (p< 0.6MPa) in plants suffering from K+ deficiency (31). In Figure 4.4, pressure potentials and the related cell size in leaves of common bean (Phaseolus vulgaris L.) are shown. Pressure potentials (turgor) were significantly higher in the treatment with sufficient K+ compared with insufficient K+ supply. This higher turgor (yp) promoted cell expansion, as shown in the lower part of Figure 4.4. From numerous observations, it is well known that plants insufficiently supplied with K+ soon lose their turgor when exposed to water stress. In recent experiments it was found that K+ increased the turgor and promoted growth in cambial tissue (32). The number of expanding cells derived from cambium was reduced with insufficient K+ nutrition. Photosynthesis and Respiration

Potassium ion transport across chloroplast and mitochondrial membranes is related closely to the energy status of plants. In earlier work, it was shown that K+ had a favorable influence on photore-duction and photophosphorylation (33). More recently, it was found that an ATPase located in the

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