Project reference: 2007-02
Towards an understanding of the causes of physiological needle blight in Pinus radiata
Project description:
Under normal conditions water flow from roots to shoots is generally controlled by stomatal conductance to ensure a viable needle water potential is maintained and the soil–root xylem hydraulic pathway is not cavitated by air, blocking flow. Thus, when the coupling of water flow and stomatal conductance breaks down, control of needle water potential is lost. The uncoupling of the control mechanism may occur through a reduction in soil to needle hydraulic conductance with a concomitant increase in needle water loss. Reduced hydraulic conductance from roots to shoots late winter/early spring can be caused by long periods of root water logging (due to clay soils and high rainfall), low soil temperature, and lack of new root growth over winter.
At a low root–shoot hydraulic conductance, stomatal conductance must remain low to restrict needle water losses and prevent damaging root–shoot water potential gradients. But during late winter/early spring stomatal conductance can actually increase due to high irradiance, low relative humidity, high air temperature and increased wind flow. Once the water potential gradient gets beyond a certain limit (>2–3MPa), the xylem becomes vulnerable to cavitation (air bubbles) thus blocking the flow of water through the conduit. This phenomenon is more likely to occur in thin conduits with high resistances such as fine branches and needles in tall trees (whole plant hydraulic conductance being inversely related to the pathway length of water flow from bulk soil to needle).
Given the observations that PNB occurs in trees over 15 years old in early spring following high winter rainfalls, the Project aims to test the hypothesis that environmental conditions over winter and early spring can result in the breakdown in the regulation of water uptake to pine needles which manifests itself as xylem conduit cavitation in fine branches and needles.
The hypothesis will be tested by comparing hydraulic conductances through roots, branches, needles and stomata in different aged trees under a variety of environmental conditions experienced by pine late winter/early spring. This project aims to run over two years due to the seasonal nature of the occurrence of PNB.
Research Provider:
Forest Research
HortResearch
