JXB Advance Access published online on September 26, 2007
Journal of Experimental Botany, doi:10.1093/jxb/erm183
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RESEARCH PAPER |
Cell wall water content has a direct effect on extensibility in growing hypocotyls of sunflower (Helianthus annuus L.)
School of Biosciences, University of Westminster, 115 New Cavendish Street, London W1W 6UW, UK
* To whom correspondence should be addressed. E-mail: thompss{at}wmin.ac.uk
It has been proposed that spacing between cellulose microfibrils within plant cell walls may be an important determinant of their mechanical properties. A consequence of this hypothesis is that the water content of cell walls may alter their extensibility and that low water potentials may directly reduce growth rates by reducing cell wall spacing. This paper describes a number of experiments in which the water potential of frozen and thawed growing hypocotyls of sunflower (Helianthus annuus L.) were altered using solutions of high molecular weight polyethylene glycol (PEG) or Dextran while their extension under constant stress was monitored using a creep extensiometer (frozen and thawed tissue was used to avoid confounding effects of turgor or active responses to the treatments). Clear reductions in extensibility were observed using both PEG and Dextran, with effects observed in hypocotyl segments treated with PEG 35 000 solutions with osmotic pressures of 
0.21 MPa suggesting that the relatively mild stresses required to reduce water potentials of plants in vivo by 0.21 MPa may be sufficient to reduce growth rates via a direct effect on wall extensibility. It is noted, therefore, that the water binding capacity of plant cell walls may be of ecophysiological importance. Measurements of cell walls of sunflower hypocotyls using scanning electron microscopy confirmed that treatment of hypocotyls with PEG solutions reduced wall thickness, supporting the hypothesis that the spatial constraint of movement of cellulose microfibrils affects the mechanical properties of the cell wall.
Key words: Cell wall extensibility, extensiometry, PEG, plant growth, sunflower, water relations
Received 1 June 2007; Revised 12 July 2007 Accepted 13 July 2007