UNCORRELATED CHANGES IN THE DISTRIBUTION OF STEM ELONGATION, TISSUE EXTENSIBILITY AND CELL-WALL PEROXIDASE-ACTIVITY ALONG HYPOCOTYL AXES OF CUCUMIS SEEDLINGS EXHIBITING DIFFERENT PATTERNS OF GROWTH

Elongation rates of hypocotyl regions of cucumber (Cucumis sativum L.) grown in dim-red-light are similar along much of the stem axis, while dark-grown seedlings show a marked decline in elongation rate from the apex to the base. In vivo tissue extensibility measurements show that plastic extensibility was highest in the apical region of dark-grown seedlings, lowest in the basal region of dark grown plants, an exhibited similar, intermediate, values in both apical and basal tissue from dim-red-light grown seedlings. Cell wall peroxidase (EC 1.11.1.7) levels were determined in three stem regions for both dark-grown and dim-red-light grown seedlings. Peroxidases were extracted both by low-speed centrifugation of stems vacuum infiltrated with 50 mM CaCl2, and by high-salt extraction of cell walls separated from homogenized stem tissue. Cell wall free-space peroxidase in both dark-grown and dim-red-light-grown cucumbers exhibited an apical to basal increase in specific activity. Salt-extracted peroxidase and cytoplasmic peroxidase showed qualitatively similar patterns of activity, with apical and middle regions exhibiting low specific activities, and basal regions possessing relatively higher specific activities. The hypothesis that cell wall peroxidases control cell wall extensibility and cell expansion cannot be supported by these results.