TISSUE PRESSURE AND CELL-WALL METABOLISM IN AUXINMEDIATED GROWTH OF SUNFLOWER HYPOCOTYLS

The effect of auxin (indole-3-acetic acid, IAA) on growth, tissue pres sure and cell turgor of hypocotyl segments from etiolated sunflower se edlings (Helianthus annuus L.) was investigated. The pressure exerted by the inner tissues (cortex, vascular tissues and pith) on the growth -limiting peripheral cell walls (longitudinal tissue pressure) was alm ost identical with the average turgor pressure of the cortical cells, as measured with the pressure probe. The change in length during 6 h o f growth in water or IAA was inversely correlated with a corresponding decrease in tissue pressure and cell turgor. These results indicate t hat the turgor (tissue) pressure of the internal cells maintains the p eripheral cell walls under tension. This tensile stress in the walls o f the outer cell layers (tissue tension) decreases during auxin-mediat ed long-term growth of the organ segment. In a second set of experimen ts we determined the effect of IAA on biosynthesis and breakdown of ce ll wall material in the outer and inner tissues of abraded segments. D uring the first h of IAA-mediated growth no promotive effect on incorp oration of H-3-glucose into the cellulose and matrix fractions of the cell wall poysaccharides was observed. Likewise, IAA had no effect on incorporation of H-3-leucine into the protein fraction of the cell wal ls. To determine whether IAA induces a breakdown of cell wall polysacc harides abraded segments were given a pulse of H-3-glucose. Thereafter the labelled segments were incubated in water (without tracer or suga r) in the presence or absence of IAA. The amount of H-3-glucose retain ed in the cell walls during the chase period was unaffected by IAA, i. e. auxin does not induce a breakdown of cell-wall material. Finally, w e determined the pattern of polypeptides synthesized during the first 2 h of IAA-mediated growth. The sections were labelled with H-3-leucin e, and protein patterns were analyzed by one-dimensional polyacrylamid gel electrophoresis. Auxin treatment did not change the pattern of pr otein synthesis. However, we detected one major protein that was more abundant in the peripheral cell layers than in the inner tissues of th e hypocotyl.