Comparative growth and symbiotic performance of four Acacia mangium provenances from Papua New Guinea in response to the supply of phosphorus at various concentrations

Plant growth performance, the P content in root and nodule tissues, and nodulation and N₂-fixing ability were studied in four provenances of Acacia mangium from Papua New Guinea following different levels of P fertilizer application. A. mangium did not seem to need high levels of P for growth and N₂ fixation. The response by this leguminous tree to the P supply varied significantly according to provenance and to P concentrations in the culture solution. The provenances of A. mangium were classified into three types according to their P response: (1) Growth performance, nodulation, and N₂ fixation of plants were stimulated as concentrations of P increased (provenance PH 482); (2) the maximal effect of P on plant growth was found only at P concentrations higher than 500 M (provenance PH 484); and (3) the plant response to P fertilization was low, even with nutrient solutions containing P concentratins higher than 500 M (provenances PH 483 and PH 485). Provenance PH 483 was distinguished by its low nodulating ability. However, this provenance grew well, probably because of its high N₂ fixation efficiency as expressed by specific acetylene reduction activity and its high P content in nodule tissues. Therefore, in certain cases, these two parameters may be useful criteria in selecting leguminous plants for field use. Statistical analyses of the study results showed that the effect of the factor P supply on N₂ fixation efficiency and nodule development was only significant at P concentrations lower than 250 M whereas the effect of the factor plant provenance was significant regardless of the P concentration used. This observation emphasizes the value of provenance screening in the identification of plants for use in a wide range of soil types.