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Te red light inhibition of growth of the intact pea (Pisum sdtvuwm L. cv. Alaska) third interode was correlated with an increase in the content of cell wail-bound hydroxyprolie. These changes were detected 3 hours after irradiation, and possibly at 1 hour. Far red light reversed the effects of red Nght. The iron chelator a,-dipyridyl reversed the red lht effects on both growth and hydroxyprolie content. Using sets incubated hi vnto, no phytochrome-medyated che in hydroxyproile content could be observed, perhaps because of an overwhelming wounding response. If plants were irradiated X situ and grown for 8 hours before excision and incubation of segments, some ement of hydroxylation by red ight was detectabl both corimetrkalBy and radolsotopicaly. The red fight inhibltio of segment growth was reversed by adipyridyL These results are examine in reference to the role of extensin in norm and induced growth cessatiozThe growth of a plant cell is significantly influenced by the properties of the cell wall. For elongation to proceed, the wall must be in a loose condition. Changes in growth rate are often caused by changes in certain crucial acid-labile, alkaline-stable bands (3). The initial stage of growth promotion by auxin may be the result of H+ secretion into the wall and consequent wall loosening (14). One type of acid-labile bond implicated in growth control is the hydroxyproline-O-arabinose bond, linking extensin, a hydroxyproline-rich glycoprotein, to the wall polysaccharides (8). There is considerable evidence that cells that have ceased elongating contain more wall-bound hydroxyproline (and thus have less plastic walls) than do elongating cells (4, 10, 17). Nonelongating pea epicotyl segments incorporated and hydroxylated more I'4Clproline than did elongating segments (17). The inhibition of growth by agents such as ethylene and benzimidazole was also accompanied by an increase in wall-bound hydroxyproline (15, 16) and in [14CJproline hydroxylation (16). The increase in hydroxyproline content was suggested to precede and to cause the growth inhibition, but the first assays were made 6 h (16) or 1 day (15) after hormone application.R5 strongly inhibits the growth of intact pea epicotyls (5) and epicotyl segments incubated in a buffered sucrose-cobalt medium (2), but little is known of the mechanism of action. R might accelerate the transition of cells from the elongation stage to the maturation stage (19). There is some evidence concerning a relationship of phytochrome control of growth and hydroxyproline. Pea seedlings grown in the light were much shorter and contained much more wall-bound hydroxyproline than etiolated seedlings (17). Although white light inhibited the growth of radish hypocotyls, light had no effect on wall hydroxyproline content (9). This study was undertaken to determine the influence of phytochrome on wall hydroxyproline level.The iron chelator a,a'-dipyridyl is widely used as an inhibitor of the hydroxylation of peptidyl proline (1,9,16). The reversal by this compound of the horm...

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