Acid Phosphatase Activity in Karri (Eucalyptus diversicolorF. Muell.) in Relation to Soil Phosphate and Nitrogen Supply

Abstract

O'Connell, A. M. and Grove, T. S. 1985. Acid phosphatase activity in karri (Eucalyptus diversicolor F. Muell.) in relation to soil phosphate and nitrogen supply.—J. exp. Bot. 36: 1359-1372. Soluble acid phosphatase activity was measured in tissues of karri (Eucalyptus diversicolor F. Muell.) seedlings and five year old karri trees to which P and N fertilizer had been applied. Addition of P from 0 to 1250 mg P kg1 soil with a basal treatment of other nutrients produced significant increases in growth, P content and P concentration of karri seedlings. In each of five plant components (shoot tips, partly expanded leaves, mature leaves, young stems and old stems) soluble acid phosphatase activity was greatest at low levels of added P and decreased with increasing soil P supply. The range of acid phosphatase activity (0-5-6-5 pmol NPP g 1 f.wt. min ') was similar to that reported for a number of agricultural and horticultural plants. Enzyme activity was highest for shoot tips and lowest for old stems. Flowever, the relative change in activity with decreasing soil P supply was greatest for stems (4-3 fold) and least for shoot tips (2-7 fold). Mature leaves of seedlings grown in 'high P' and 'low P' soil at four levels of added N showed, in addition to the effect of P, a significant N-P interaction on soluble acid phosphatase activity. In leaf samples from five year old karri trees there was a significant decrease in soluble acid phosphatase with increasing P fertilization. Addition of N fertilizer had no significant effect on enzyme activity, probably because added N had little effect on foliar N concentrations. Exponential models relating (1) plant growth to enzyme activity and (2) plant growth to P concentration in stems and mature leaves of plants grown in soil with a range of added P accounted for 78-92% and 63-87%, respectively, of the variation in top dry weight. The results suggest that for the diagnosis of plant P status, (1) stem components may be the most appropriate tissue to sample, and (2) nutrient and enzyme assays may complement each other, P concentration being most useful where P supply is adequate and phosphatase activity where P supply limits growth.