Go to OSU College of Forestry LWGate Home Page.From: Geoff.Downes@csiro.au
Subject: Re: Radial transport
Date: Tue, 8 Oct 2002 16:08:56 +1100
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Interesting discussion. I am not sure how relevant my comments below are and how far we have drifted from Barb's question but.... In 1988, working as a post doc on turnover of mycorrhizal fine roots in sitka spruce, we, and other studies, calculated a fine root halflife in the order of 4-9 months. These are the roots doing a lot of the water uptake. It occurred to me that this characteristic would also tend to keep the newest formed roots in contact with the newest formed xylem and consequently the newest form leaves i.e. water uptake would tend to flow into the newest wood just beneath the cambium, as the vascular connections would be made there. This would possibily tend to minimise the need for large radial transport. In recent years, working with dendrometers logging data sub-hourly in eucalypts (that could be pulling down to -3MPa), the diurnal cycles / longer-term drought induced shrinkage suggested a sequence of water movement into / out of the cambium / phloem that could have a role in generally mixing the water solution surrounding the developing cells. Simple simulation models of water uptake limited by root surface area and driven by stomatal conductance / VPD showed very similar patterns of diurnal movement to those observed in the real dendrometer data. Even showing that the real differences observed in the magnitude of diurnal variation (between Euc. nitens and globulus) could be modelled by limiting the rate of root water uptake, for a given transpirational demand. Thus as Rod pointed out gradients across the cambium are very dynamic. To what extent transport is passive or active...? Geoff Downes CSIRO Forestry and Forest Products GPO Box 252-12, Hobart 7001, Tasmania, Australia College Rd Sandy Bay 7005, Tasmania, Australia Ph +61 3 6226 7962 Fax +61 3 6226 7901 Mobile 0438 289 703
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