This study investigated the physiological response to drought of a 41-year-old <I>Pinus</I> <I>sylvestris </I>stand in south central Scotland. The imposed drought (April-August) resulted in an approximate 15% reduction in the number of functional conduits but no adverse effects on the above ground hydraulic resistance were evident. This may be the result of 'spare capacity' in the amount of conducting tissue or may be indicative of refilling. Under water-stress a significant increase in xylem embolism was detected in 1-year-old shoots. Leaf-level conductance measurements established a strong reduction in conductance with increasing vapour pressure at the leaf surface and a reduction in transpiration and assimilation. Maximal levels of stomatal conductance, under ambient conditions, were 280 mmol m<SUP>-2</SUP> s<SUP>-1</SUP> in water-stressed trees and 315 mmol m<SUP>-2</SUP> s<SUP>-1</SUP> in control trees. Maximum net photosynthesis rates were approximately 4.5 μmol m<SUP>-2</SUP> s<SUP>-1</SUP> in both treatments. Stomatal control of transpiration prevented needle water potential from declining below -1.5 MPa. Strong restrictions on midday conductance at both the leaf and stand levels were apparent when soil volumetric water content declined below a threshold of 12%. The results show an immediate response of leaf conductance to the onset of drought and suggest a mechanism involving sensing of soil water status. There was an increase in ABA in the xylem sap to 600 μmol ml<SUP>-1</SUP> at the height of the drought, but the observed stomatal closure preceded any increase in ABA<SUB>xyl</SUB>. Calculations of ABA delivery times suggest it may take a chemical signal in excess of six weeks to travel from root to shoot, in a mature conifer, at the peak of the prolonged water-stress. It is therefore suggested that ABA signals from the roots are not the primary cause of stomatal closure in conifers.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:660539 |
| Date | January 1998 |
| Creators | Perks, Michael Philip |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/15631 |
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