POPULATION-DYNAMICS OF ENDOPHYTIC BACTERIA IN FIELD-GROWN SWEET CORN AND COTTON

Investigations were designed to gain fundamental information on the microbial ecology of endophytic bacteria in model dicotyledonous and monocotyledonous hosts. Population dynamics of indigenous endophytic bacteria in cotton (Gossypium hirsutum L. 'DES119') and sweet corn (Zea mays L. 'Silver Queen') stems and roots were studied in a 2-year field trial by quantifying culturable bacteria at intervals during the season on three media: R2A, medium SC, and tryptic soy agar. Population dynamics of endophytic bacteria inside cotton petioles and boils were also determined in 1 year. Endophytes were recovered from sweet corn roots and stems at seedling emergence at mean population densities of 4 log (colony-forming units per gram fresh weight (cfu/g-fw)) for both seasons, and were present throughout most of the growing season at populations ranging from 4 to 6 log(cfu/g-fw) in 1990 and 4 to 7 log(cfu/g-fw) in 1991. Endophytic bacteria were also present at emergence in cotton roots and stems in 1991 but were not detected until 2 days after emergence in 1990. Endophytic populations in cotton roots ranged from 4 to 6 log(cfu/g-fw) for most of the growing season in 1990 and 1991, while populations in cotton stems fluctuated between 3 and 7 log(cfu/g-fw) during both seasons. In cotton petioles, mean populations generally ranged from 1 to 4 log(cfu/g-fw), while no endophytic bacteria were recovered from bells (minimum detectable limit = 1.30 log(cfu/g-fw)). The relative contribution of seeds and soil as sources of endophytic bacteria recovered from inside plants was assessed using surface-disinfested seed in a potting mix or on water-agar. With sweet corn, the mean endophytic bacterial population in seedlings grown on water agar was below 2 log(cfu/g-fw), while with cotton the mean was 5 log(cfu/g-fw) 6 days after germination. Internal populations resulting from surface-disinfested seed planted in nonsterile potting mix were 6 log(cfu/g-fw) at 6 days after planting with corn but only 2 log(cfu/g-fw) with cotton. These results indicate that endophytic bacteria are natural inhabitants of internal regions of roots and stems and that the endophytes may arise from both seeds and soils.