Colonisation dynamics of arbuscular mycorrhizal fungi and dark septate endophytes in the sugarcane crop cycle

The root systems of sugarcane (Saccharum L. spp. hybrids) form multifunctional associations with a variety of fungal symbionts including arbuscular mycorrhizal (AM) fungi and dark septate endophytes (DSE). A comprehensive understanding of the temporal changes in symbiotic fungal colonisation of sugarcane roots following ratooning is currently lacking, therefore limiting our ability to accurately assess the role of root-associated fungi on soil nutrition and crop productivity. This field study investigated the colonisation dynamics of native populations of AM/DSE in the root systems of 1st and 2nd ratoon phase sugarcane at two proximal sites where crops had previously been grown for either 1 or 2 years. Total percent colonisation of AM/DSE in two sugarcane root types (sett roots/shoot roots) was quantified over 28 weeks of crop development. Relevant AM functional structures (arbuscules, hyphae, spores and vesicles) were also delineated and quantified at each observation for both root types. Significant differences (P < 0.05) in total root colonisation over time for both fungal types and between sett roots and shoot roots were observed. At each site, a variety of AM morphologies were observed in both root types. Maximum sett root colonisation for both fungal types was observed at 5 weeks in the 1st ratoon and at 7 weeks in the 2nd ratoon crop, which may be due to difference in the ratoon year, but differences in field history between the sites cannot be ruled out as a contributing factor. Significantly lower levels of DSE sett root colonisation were observed in the 2nd ratoon compared to the 1st ratoon crop. Maximum AM shoot root colonisation occurred at 7 weeks in the 1st ratoon and 11 weeks in the 2nd ratoon crop. In shoot roots, significantly lower levels of DSE compared to AM colonisation were observed at both sites. For both fungal types, sett root colonisation was significantly higher than for shoot roots at both sites. There were no differences in total AM sett root colonisation between sites. For both fungal types, there were no differences in total shoot root colonisation between sites. Maximum levels of AM/DSE sett root colonisation occurred between 5-7 weeks, prior to shoot root establishment and rapid growth period of 1st and 2nd ratoon crops. While AM colonisation of sett roots was maintained across ratoon phases, there is a need to account for apparent differences in DSE sett root colonisation between 1st and 2nd ratoon crops. Our data provide the basis for the development of sampling strategies in future studies investigating impacts of crop management on AM/DSE symbioses in sugarcane systems.