GROWTH AND SEASONAL UTILIZATION OF WATER AND NUTRIENTS BY BANKSIA PRIONOTES

Banksia prionotes Lindley is a fire-sensitive, fast-growing tree of nu trient-impoverished deep sands of south-western Australia. Its root sy stem is dimorphic, comprising proteoid root-bearing, lateral roots abs orbing superficially concentrated nutrients during the wet winter seas on and a single main sinker (tap) root extending down to the water tab le. Shoot extension commences in early summer coincident with drying o f topsoil and ceases at the end of summer upon initiation of infloresc ences and resting buds. Shoot growth utilises nutrients accumulated th e previous and earlier wet seasons and current photosynthate formed at the expense of ground water abstracted by the sinker root. Rooting mo rphologies of differently aged trees are described and yearly changes in dry matter distribution between leaves, trunk and parts of root sys tems are related to dry matter gain and foliage area. The seasonality of nutrient uptake by proteoid, lateral and sinker roots and nutrient translocation in shoots is assessed by xylem and phloem sap analyses. Specific hydraulic conductivities of xylem of sinker roots are conside rably higher than in lateral roots and higher again than in trunk xyle m. The differences involved relate to vessel lengths and diameters and proportional transectional areas devoted to conducting tissues. Seaso nal changes in dependence on ground water as opposed to recent rain ar e estimated using deuterium:hydrogen natural abundance ratios of water extracted from xylem of lateral roots, tap root and trunk. Relationsh ips between water stress and timing and progress of extension growth o f shoots are studied using carbon isotope natural abundance ratios of new leaf dry matter and abscisic acid levels in xylem and phloem sap.