An analysis of growth rates and directions of growth of primary roots of field-grown banana trees in an andisol at three levels of soil compaction

In monocotyledons, soil exploration by the root system is mainly due to primary roots. Classical root observations, from soil cores or minirhizotrons, do not allow local root growth rates to be related directly to soil physical conditions around the roots. The pattern of growth of banana primary roots in an andisol under field conditions was determined by destructive, architecturally-based samplings of the root system over a four-month period at three levels of soil compaction. Primary root growth rates were estimated from morphological measurements, while soil porosity around growing roots was assessed by soil coring. In all treatments, root growth rates appeared to be variable with time, and a linear regression between the root growth rate and root apical diameter, soil porosity and the degree-day sum (base 14 degreesC) accounted for 92% of the observed variance. Primary root growth was reduced by nearly 40% in the most compacted soil. Above-ground plant growth was also affected after a few weeks. Primary root growth directions were not affected by soil compaction and roots appeared to curve toward the horizontal plane when ageing. These results highlight the variability of root growth in field conditions, even in uniform soil conditions, and suggest possibilities for the development of root growth models and models of root system architecture.