The growth physics and water relations of red-light-induced germination in lettuce seeds

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Irradiation of Grand Rapids lettuce (Lactuca sativa L.) seeds with red light (R) results in an increase in the growth response in the embryonic axes which enables them to overcome the mechanical restriction of the endosperm or the osmotic restriction of an external solution. When the growth rates of axes from R-and far-red(FR)-treated seeds are equalized with polyethylene-glycol-4000 solutions, a water potential difference of 3.4 bars between the axes is obtained. Measurements of the osmotic potential indicate a 1.0-2.0-bar difference between the treatments, depending upon incubation time. Although the difference in water potential remained constant during the growth of the axes, the osmotic constituents of the axes of R-treated seeds were diluted to a greater extent than those of the axes of FR-treated seeds. The length-to-volume ratio of the growing axes of R-treated seeds increased at a greater rate than that of the axes of FR-treated seeds. These data indicate that changes in the cell-wall properties may account for the remainder of the observed water potential difference. We propose that the phytochrome-mediated growth increase in these embryonic axes is an integrated function of the cells: increased wall loosening may be inextricably linked to simultaneous increases in the osmotic constitutents. The driving force for growth may not involve solely a change in the osmotic potential or the pressure potential, with passive readjustment of the other parameter, but the activation of a cellular component that links changes in the wall properties with the accumulation of osmotic constituents.