Annals of Botany

Orchestrating organs? Roots and stems not always in harmony

Roots are often described as the forgotten parts of a plant, recalling the old saying ‘out of sight, out of mind’. And yet roots play vital roles in plant life, ranging from anchorage to nutrient absorption. Despite this, we know little about the timing and co-ordination of annual growth and development of roots in perennial plants. Thus, Thibeault-Martel et al. (Québec, pp. 667–674) have studied the annual pattern of root and shoot cambial activity and xylem formation in two gymnosperms, Abies balsamea and Picea mariana. At weekly intervals in the period May to November in three successive years, wood microcores were collected from roots and stems and were studied by standard histological techniques. In any one year, cambial activity showed the same pattern in roots and stems: cambial cell numbers in the cross-sections increased from May to June/July and then gradually decreased until late August/early September. However, initiation of xylem differentiation was not always strictly co-ordinated between root and stem. It occurred at the same time in both organs in 2004 and 2005 (although the actual timing differed between the two years), but in 2006 xylem differentiation in the root started a week later than in the stem. Wall thickening and the subsequent formation of tracheary elements was initiated in June with the stem always preceding the root. The end of xylem formation, indicated by lignification, occurred at the same time (late September) in both shoots and roots in 2004 and 2005, but in 2006 was 22 days later in roots than in stems. There is not a strict linkage between the roots and shoots in respect of renewal of cambial growth and differentiation. The authors suggest that this indicates a lack of dependence on auxin transported basipetally from young shoots: perhaps auxin is already available in the dormant cells.