Résumé — Ce travail a pour but de caractériser les paramètres descriptifs de la structure et de la cinétique de figure 1) on a branching zone surrounded by 2 bare parts: "a" located just above the apex and "b", at the top of the root. The length of "a", given in figure 2 according to the method used, fluctuates between 10 and 19 cm and seems to be independent of the insertion number of the primary root (called E i ). The length of "b" increases with E i and varies from 0.4 to 5.3 cm as shown by the relation:Numbers of secondary roots are given per unit length, ie cm of primary mother root, in figure 3 for the needle boards and in 4 for the rhizotrons. They increase with E i , vary from 6.5 ramification/cm for E 2 to 16.0 RII/cm for E 7 and have a tendency to decrease with depth. They also vary significantly between the 2 years (see variance figure 6a and 6b versus number of days after sowing and number of visible leaves. However, the emission duration of the first 90 cm of a primary root (length which can be followed through the rhizotron glass wall) varies widely (table III) with its internode rank.Zea mays L = maize / root structure / secondary root / root development / root number
fig 2) for selecting the roots in contact with the transparent wall. Table / gives the number of evaluation performed for each method versus year and depth. The number of secondary roots per cm of primary roots versus internode and depth is given in figures 3 and 4 for the differents methods. The variability inherent in the ramification process is important and can attain 30%. The coefficients of variation are greater for the results obtained with rhizotrons. They include the consequences of the specific perturbations due to the method. The observations made through the acrylic wall are representative of the ramification kinetics inside the box. Thus, figure 5 allows comparison of the different counts performed in the rhizotrons on the basis of the slope variation between 2 consecutive internodes. This figure shows that the acrylic wall did not disturb the secondary ramification, and that the results of the observations performed through the transparent face are proportional to those of the total number of ramifications on the observed primary roots. But these data can be extended to the field conditions for the first internodes (E 2 to E 5 ) only. This is certainly caused by the soil temperature differences between the root boxes and the field which is especially important at the end of the vegetative growing season (fig 6) when the laterals on internode 6 and 7 are initiated.Zea mays L = maize / method / rhizotron / needle board / root development / number of secondary roots
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