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

Investigations were designed to gain fundamental information on the mi crobial ecology of endophytic bacteria in model dicotyledonous and mon ocotyledonous hosts. Population dynamics of indigenous endophytic bact eria in cotton (Gossypium hirsutum L. 'DES119') and sweet corn (Zea ma ys L. 'Silver Queen') stems and roots were studied in a 2-year field t rial by quantifying culturable bacteria at intervals during the season on three media: R2A, medium SC, and tryptic soy agar. Population dyna mics 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 seas ons, and were present throughout most of the growing season at populat ions 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 cotto n roots and stems in 1991 but were not detected until 2 days after eme rgence in 1990. Endophytic populations in cotton roots ranged from 4 t o 6 log(cfu/g-fw) for most of the growing season in 1990 and 1991, whi le 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 re covered from bells (minimum detectable limit = 1.30 log(cfu/g-fw)). Th e relative contribution of seeds and soil as sources of endophytic bac teria recovered from inside plants was assessed using surface-disinfes ted seed in a potting mix or on water-agar. With sweet corn, the mean endophytic bacterial population in seedlings grown on water agar was b elow 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-d isinfested 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 cott on. These results indicate that endophytic bacteria are natural inhabi tants of internal regions of roots and stems and that the endophytes m ay arise from both seeds and soils.