THE ROLE OF SETT ROOTS AND SHOOT ROOTS IN THE ESTABLISHMENT OF SUGARCANE PLANTED INTO YIELD DECLINE SOILS

TO EXAMINE the role of sett roots in the early growth of sugarcane, the production of sett roots was impaired by damaging root primordia and the number and biomass of primary and secondary shoots was measured 70–100 days after planting. Interactions between sett roots and the soil-borne pathogens typically found in association with sugarcane were examined by damaging root primordia and then planting the setts into soil that was fumigated with methyl bromide, treated with a nematicide or left untreated. The results of two of three experiments showed that the size of the sett root system did not affect the number of primary shoots established, presumably because the parent stem cutting was capable of supporting the developing bud until shoot roots became functional. However, the results of a third experiment suggested that the effect of sett roots on primary shoots depended on the quality of the stem cutting and the inherent dormancy of buds. Mortality of primary shoots was relatively high when root primordia were removed from setts taken from the upper part of the stem, while shoot establishment from relatively dormant buds on cuttings taken from the lower stem was reduced when root primordia were removed. Loss of sett roots had a greater impact on the weight of secondary shoots than primary shoots, as primary shoot weight in untreated soil was significantly reduced in only one experiment whereas 38–51% reductions in secondary shoot weight were observed in all experiments. Soil fumigation and nematicide treatments markedly improved plant growth but regardless of soil treatment, the removal of root primordia generally reduced the number and weight of secondary shoots. This result demonstrates that sett roots are important in sustaining the initial growth of sugarcane. The fact that removing root primordia sometimes had a greater impact in untreated soil than in fumigated soil indicates that damage to sett roots from soil-borne pathogens may affect crop establishment.