SOLUTE ACCUMULATION AND DECREASED PHOTOSYNTHESIS IN LEAVES OF POTATO PLANTS EXPRESSING YEAST-DERIVED INVERTASE EITHER IN THE APOPLAST, VACUOLE OR CYTOSOL

Potato (Solanum tuberosum cv. Desiree) plants expressing yeast inverta se directed either to the apoplast, vacuole or cytosol were biochemica lly and physiologically characterised. All lines of transgenic plants showed similarities to plants growing under water stress. Transformant s were retarded in growth; and accumulated hexoses and amino acids, es pecially proline, to levels up to 40-fold higher than those of the wil d types. In all transformants rates of CO2 assimilation and leaf condu ctance were reduced. From the unchanged intercellular partial pressure of CO2 and apoplastic cis abscisic acid (ABA) content of transformed leaves ii. was concluded that the reduced rate of CO2 assimilation was not caused by a limitation in the availability of CO2 for the ribulos e-l,5-bisphosphate carboxylase-oxygenase (Rubisco). In the transforman ts the amount of Rubisco protein was not reduced, but both activation state and carboxylation efficiency of photosynthesis were lowered. In vacuolar and cytosolic transformants this inhibition of Rubisco might be caused by a changed ratio of organic bound and inorganic phosphate, as indicated by a doubling of phosphorylated intermediates. But in ap oplastic transformants the pattern of phosphorylated intermediates res embled that of leaves of water-stressed potato plants, although the ca use of inhibition of photosynthesis was not identical. Whereas in wate r-stressed plants increased contents of the phytohormone ABA are suppo sed to mediate the adaptation to water stress, no contribution of ABA to reduction of photosynthesis could be detected in invertase transfor mants.