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The Effects of Water Stress on the Development of the Photosynthetic Apparatus in Greening Leaves ¹

^a Department of Botany, Duke University, Durham, North Carolina 27706

The effects of low and high relative humidity and of polyethylene glycol-induced root water stress on chlorophyll accumulation, on formation of the lamellar chlorophyll-protein complexes, and on the development of photosynthetic activity during chloroplast differentiation were examined. Low relative humidity or polyethylene glycol-induced root water stress (stress conditions) resulted in a 3 to 4 hour lag in chlorophyll accumulation, retarded the rate of chlorophyll b accumulation, and reduced the rate of formation of the light-harvesting chlorophyll a/b protein. All of these effects could be overcome by high relative humidity (nonstress) conditions. Concomitant measurement of leaf water potential showed that under stress conditions greening leaves were subjected to initial water deficits of –8 bars which decreased to –5 bars after 3 to 4 hours of illumination corresponding to the end of the lag phase. Leaves greening under nonstress conditions did not experience leaf water deficits greater than about –5 bars. It seems that the attainment of a minimum leaf water potential of –5 bars may be critical in the control of early chloroplast development. These results demonstrate that the lag phase is not indicative of a programmed event in chloroplast development, but rather is attributable to environmental conditions prevailing during leaf development and greening.

¹ Research was supported in part by the Duke University Research Council, by National Science Foundation Grants GB 31207, awarded to J. P. Thornber, University of California, Los Angeles, and GB 30367X, awarded to P. J. Kramer, Duke University. Support for Phytotron facilities was made available through National Science Foundation Grants GB 19634 and GB 28950, awarded to H. H. Hellmers, Duke University.