Growth patterns of bacterial communities in the rhizoplane and rhizosphere of white clover ( Trifolium repens L.) and perennial ryegrass ( Lolium perenne L.) in long-term pasture

Abstract

Rhizoplane bacteria in white clover roots (0–5 cm), perennial ryegrass roots (0–5 and 5–10 cm) and surrounding rhizosphere soil from a long-term pasture were studied for the rate of colony development over a 10 day period. Serial dilutions were inoculated onto 0.1 strength tryptose soy agar (TSA) and 0.1 strength modified King's medium B (KB). Bacterial counts were made eight times over 10 days. Bacterial colony development ( CD) and eco-physiological ( EP) indices were calculated from these data. There were no significant differences in CD and EP indices of fluorescent pseudomonads growing on KB medium. After 5 days, the number of colonies developing on KB as a proportion of total colonies appearing over 10 days was lower for ryegrass roots than for soil ( P < 0.05). The community structure derived from the colonies growing on 0.1 strength TSA showed that the rhizoplanes of white clover and ryegrass had a greater proportion of K-strategists and a smaller proportion of r-strategists than did the rhizosphere soil ( P < 0.001). CD values for roots were significantly lower than those for soil ( P < 0.001), while EP values were not significantly different. Randomly selected bacterial isolates were identified using the Microlog ™ system. The fastest growing (24 h) strains belonged to the genera Pseudomonas and Bacillus spp. Other genera identified included Acinetobacter, Sphingomonas, Streptococcus, Cellulomonas, Comamonas, Xanthomonas and Erwinia. The slowest growing bacteria belonged to the genera Cellulomonas, Rhizobium, Xanthomonas and Erwinia.