Growth and Seasonal Utilisation of Water and Nutrients by Banksia prionotes

Abstract

Banksia prionotes Lindley is a fire-sensitive, fast-growing tree of nutrient-impoverished deep sands of south-western Australia. Its root system is dimorphic, comprising proteoid root-bearing, lateral roots absorbing superficially concentrated nutrients during the wet winter season and a single main sinker (tap) root extending down to the water table. Shoot extension commences in early summer coincident with drying of topsoil and ceases at the end of summer upon initiation of inflorescences and resting buds. Shoot growth utilises nutrients accumulated the previous and earlier wet seasons and current photosynthate formed at the expense of ground water abstracted by the sinker root. Rooting morphologies of differently aged trees are described and yearly changes in dry matter distribution between leaves, trunk and parts of root systems 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 considerably higher than in lateral roots and higher again than in trunk xylem. The differences involved relate to vessel lengths and diameters and proportional transectional areas devoted to conducting tissues. Seasonal changes in dependence on ground water as opposed to recent rain are estimated using deuterium : hydrogen natural abundance ratios of water extracted from xylem of lateral roots, tap root and trunk. Relationships between water stress and timing and progress of extension growth of shoots are studied using carbon isotope natural abundance ratios of new leaf dry matter and abscisic acid levels in xylem and phloem sap.