Comparative Growth and Symbiotic Performance of Seedlings ofAcaciaspp. in Defined Pot Culture or as Natural Understorey Components of a Eucalypt Forest Ecosystem in S.W. Australia

Abstract

Hansen, A. P. and Pate, J. S. 1987. Comparative growth and symbiotic performance of seedlings of Acacia spp. in defined pot culture or as natural understorey components of a eucalypt forest ecosystem in S.W. Australia.—J. exp. Bot. 38: 13–25 Growth, nitrogen accumulation, nodulation and nitrogenase activity (C₂H₂ reduction activity) were monitored in a dense stand of seedlings of Acacia pulchella R.Br. and A. alata R.Br. during two growing seasons after fire at a jarrah (Eucalyptus marginata Donn ex Sm.) forest site at Illawarra, 35 km south-east of Perth. Nitrogen fixation, estimated by a previously calibrated C₂H₂ reduction assay, was essentially restricted to the winter and spring months (July to October) and was estimated to contribute 37% and 9% respectively of the total N accumulated by A. pulchella during the first and second seasons of growth. Comparable values for A. alata were 29% and 2%. Comparisons with fully symbiotic plants raised in a glasshouse in supposedly non-limiting growth conditions in minus nitrogen sand culture indicated that water stress rather than high temperatures was responsible for loss of nodules, cessation of symbiotic activity and attenuation of growth during summer in the field. By 19 months glasshouse plants had gained 130–230 times the dry weight and accumulated 110–160 times the total N of similarly aged, field grown plants. Growth, nodulation and N₂ fixation of symbiotically-dependent sand cultured plants of A. pulchella and A. alata responded markedly to phosphate, and additions of water and nutrients to 3-year plants of A. pulchella during a winter growing season in the field indicated that periods of reduced soil moisture and low availability of nutrients, particularly phosphorus, might limit symbiotic performance under natural conditions.