In situ and Ex situ Conservation of Commercial Tropical Trees - ITTO

486during the selection of the isolates. It was cautious that the host plant height,despite of its simplicity for undisturbed measurement, was not appropriateparameter for this selection. As also reported by Sprent (1994), the activity ofnitrogen fixation (as measured as acetylene reduction activity) did not indicatestrong correlation to their biomass, possibly this physiological trait was onlymeasured its potential rather than its actual value. It also suggested thatheterogeneity of the host genotypes may contribute to an inconsistentmeasurement of this trait. The importance of biomass and nodule during selectionwere also similar to those noted in other tropical tree species, such as Acaciamangium and with Leucaena leucocephala (Karyanto-unpublished). Werecommended a direct inoculation into an ultisol as a straight-forward procedureafter the in vivo screening, rather than by using a sand culture supplied withcontrolled nutrient solution. It was noted that the growth response obtainedfrom an artificial media (sand culture) did not represent the isolates performanceon ultisols (Karyanto-unpublished). It was suggested that a multi-constraintnature of ultisol could not be simply imitated by articifially stress conditions.A sub-optimal dosage of the basal fertilizer was required during thesein vitro selection. Without these applied nutrient, nodulation were very poor. Incontrast, an excessive nutrient application suppressed the potency of the tolerantisolates, therefore could not distinguish the Al tolerant from the sensitive ones.Phosphorous was shown to be major component in this basal fertiliser, in addition,nitrogen should be completely omitted since it may inhibit early step of thenodulation.Genotypic interaction between host plant and rhizobial isolates wereexamined by inoculating the nine families derived from three differentprovenances/landraces of P.f. with the three selected isolates of rhizobia asshown in Figure 5 and 6. An interaction was shown by statistical analysis of thefollowing parameters: (i) host plant biomass, (ii) number of root nodule, and (iii)biomass of root nodule. Host plant diversity at both family and provenance/land-race levels as well as the isolate of rhizobia contributed to this interaction.The best response was achieved by a compatible interaction, on the other hand,reduction on plant growth caused by incompatibility during P.f. - rhizobiaassociation was apparent. It was interesting to note that the growth responseof a P.f. provenance collected from Morotai island was negative. Theassociations was characterized by small and under-developed nodules formationaccompanying with negligible or even undetected of nitrogenase activity.Genotypic interaction between host and rhizobia are frequently reported(Pvovorov et al. 1994, Josephson et al. 1991), but limited informations havebeen available in tree species. Galiana et al. (1996) reported this evidence inrhizobia – Acacia mangium symbiosis. We, therefore, strongly recommendedthat genotype of host plant is considered during the selection of rhizobial isolates.