Maize nodal root ramification: Absence of dormant primordia, root classification using histological parameters and consequences on sap conduction
Résumé
Maize plants grown in field conditions were used to describe the histological organisation of the nodal roots, those of their laterals, and also to test the presence of critical stages where the subsequent capability for growth and development of young laterals was determined irreversibly. The absence of undeveloped primordia, which stop their development before boring through the nodal mother-root epidermis, proved that the number of laterals could not be regulated between the differentiation and the emission stage. Cross sections performed on nodal roots beared by the internodes 2, 4 and 6 and their long (>3 cm) and short (<3 cm) laterals showed that:-The short laterals have almost the same root and stele diameter, cortex width and xylem size regardless of the insertion position of their mother root. Consequently they stopped their development at nearly the same stage, which seems to be concomitant with late metaxylem vessel differentiation. Thus, 92% of these short roots had only one of these vessels. On the contrary, the dimensions of the tested histological parameters, as well as the numbers of late and early metaxylem vessels and of xylem poles increased with internode rank for both long laterals and nodal roots. Moreover, for the nodal roots issued from internode 6, important decreases (up to 50%) in the sizes of most of the measured parameters were observed along the roots with the distance from the stem insertion point. Complementary experiments performed in mist boxes, under strict homogeneous conditions, showed that the observed variability is not generated by the culture medium.-Up to 750/xm, the root diameter is correlated to most of the histological characters tested.-Nodal roots have a higher proportion of xylem than lateral roots. Consequently their conduction abilities, evaluated using the C/E ratio of Hagen-Poiseuille's law, were particularly important.-No relationship was found between the diameter of a nodal root and those of its laterals.