The number of bacteria in the rhizosphere during plant development: relating colony-forming units to different reference units

The number of bacteria was determined during the growth of chrysanthemum pl ants on young (tip) and old (base) root parts. We assessed if the same conc lusions could be drawn on the dynamics of bacterial populations during plan t development when different reference units were used to express the bacte rial counts. The results indicated that the total number of bacteria on the base decreased significantly during plant development, when expressed per root length, per root fresh weight or per root surface. The number of bacte ria on the tip only decreased significantly when expressed per root length. Using the unit of dry weight of adhering soil, contradictory results were obtained for both base and tip; in general, the number of bacteria increase d significantly during plant development. Thus, different reference units m ay lead to different conclusions. Root surface seemed to be the best unit t o use, but the use of this unit requires time-consuming measurements. Regre ssion analyses indicated that the reference unit "root surface" was highly correlated with root fresh weight (R-2=93%). Thus, once this relation is de termined, the less time-consuming unit can be measured in the experimental work. To analyse the data, the colony-forming units should be expressed per root surface. Besides bacterial numbers during plant development, we asses sed whether the bacterial populations collected showed different growth rat es on agar plates. The growth rates of bacteria from the tip and base and d ifferent development stages of the plants showed differences, indicating di fferences in the metabolic state of the collected populations.