Tobacco plants (Nicotiana tabacum L.) were transformed with a mannitol-1-phosphate dehydrogenase gene resulting in mannitol accumulation. Experiments were conducted to determine whether mannitol provides salt and/or drought stress protection through osmotic adjustment. Non-stressed transgenic plants were 20-25% smaller than non-stressed, non-transformed (wild-type) plants in both salinity and drought experiments. However, salt stress reduced dry weight in wild-type plants by 44%, but did not reduce the dry weight of transgenic plants. Transgenic plants adjusted osmotically by 0*57 MPa, whereas wild-type plants did not adjust osmotically in response to salt stress. Calculations of solute contribution to osmotic adjustment showed that mannitol contributed only 0-003-0-004 MPa to the 0 2 MPa difference in full turgor osmotic potential {KJ between salt-stressed transgenic and wild-type plants. Assuming a cytoplasmic location for mannitol and that the cytoplasm constituted 5% of the total water volume, mannitol accounted for only 30-40% of the change in K^ of the cytoplasm. Inositol, a naturally occurring polyol in tobacco, accumulated in response to salt stress in both transgenic and wild-type plants, and was 3-fold more abundant than mannitol in transgenic plants.Drought stress reduced the leaf relative water content, leaf expansion, and dry weight of transgenic and wild-type plants. However, Ji^was not significantly reduced by drought stress in transgenic or wild-type plants, despite an increase in non-structural carbohydrates and mannitol in droughted plants. We conclude that (1) mannitol was a relatively minor osmolyte in transgenic tobacco, but may have indirectly enhanced osmotic adjustment and salt tolerance; (2) inositol cannot substitute for mannitol in this role; (3) slower growth of the transgenic plants, and not the presence of mannitol per sc, may have been the cause of greater salt tolerance, and (4) mannitol accumulation was enhanced by drought stress but did not affect 71^ or drought tolerance.

The relative contributions of xylem, phloem, and transpiration to fruit growth and the daily patterns of their flows have been determined in peach, during the two stages of rapid diameter increase, by precise and continuous monitoring of fruit diameter variations. Xylem, phloem, and transpiration contributions to growth were quantified by comparing the diurnal patterns of diameter change of fruits, which were then girdled and subsequently detached. Xylem supports peach growth by 70%, and phloem 30%, while transpiration accounts for ;60% of daily total inflows. These figures and their diurnal patterns were comparable among years, stages, and cultivars. Xylem was functional at both stage I and III, while fruit transpiration was high and strictly dependent on environmental conditions, causing periods of fruit shrinkage. Phloem imports were correlated to fruit shrinkage and appear to facilitate subsequent fruit enlargement. Peach displays a growth mechanism which can be explained on the basis of passive unloading of photoassimilates from the phloem. A pivotal role is played by the large amount of water flowing from the tree to the fruit and from the fruit to the atmosphere.

In peach (Prunus persica [L.] Batsch.), sorbitol and sucrose plants in proportion to water supply. Osmotic adjustment was are the two main forms of photosynthetic and translocated detected by the second week of treatment in mature leaves and by the third week in shoot tips. Both SDH and S6PDH carbon and may have different functions depending on the activities were reduced by drought stress within 4 days of organ of utilization and its developmental stage. The role and treatment and positively correlated with overall c w levels. interaction of sorbitol and sucrose metabolism was studied in However, only SDH activity was correlated with c s . Among mature leaves (source) and shoot tips (sinks) of 'Nemaguard' the sucrose enzymes, only SS was affected by drought, being peach under drought stress. Plants were irrigated daily at reduced after 3 weeks. Sorbitol accumulation in both mature rates of 100, 67, and 33% of evapotranspiration (ET). The relative elongation rate (RER) of growing shoots was mea-leaves and shoot tips of stressed plants was observed starting sured daily. In mature leaves, water potential (c w ), osmotic from the second week of treatment and reached up to 80% of total solutes involved in osmotic adjustment. Sucrose content potential (c s ), sorbitol-6-phosphate dehydrogenase (S6PDH, was up to 8-fold lower than sorbitol content and accumulated EC 1. 1.1.200), and sucrose-phosphate synthase (SPS, EC only occasionally. We conclude that a loss of SDH activity in 2.4.

cause the variability in root hydraulic properties that exists among plant species. Previous research suggested that the hydraulic properties of root systems of intact plants could be

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