The study was carried out in the “Genetic Improvement Diversity” Unit of the Research Center on Industrial Plantations, in the main coastal forest plantations, at Pointe-Noire, Republic of Congo.

The study involved four interspecific Eucalyptus urophylla × Eucalyptus grandis clones referred to as: 18 - 50, 18 - 147, 18 - 170, and 18 - 228 produced by rooted cuttings according to two different procedures. For each clone, two types of cuttings, F and C were distinguished: c plants were produced from 2 yr-old field coppicing stumps as initially described by [1] and then by [4] whereas plants derived from intensively managed container-grown stock plants from 1 yr-old. These two propagation systems are detailed in [4] and illustrated in Figure 1.

The F and C rooted cuttings were cultivated for 3 months in containers filled with substrate (1V of genulite and 3v of vermiculite) before they reach the suitable average size of 30 cm to be field planted after the addition of 200 g of NPK 13-13-21 in the planting hole 1-41.

In this study, the description of the plant made by [4] was supplemented by the characterization of the orthotropic of the newly formed axis through the measurement of the angular value of the deviation of this axis compared to the vertical.

The different clones by type of plant were installed in a device in complete randomized blocks with three replicates and an elementary experimental unit made up of 7 × 7 plants, installed according to the spacings of 4.7 m × 2.5 m, i.e. at a density of 851 plants per hectare. A total of 49 plants were installed per type of plant and per clone. All the elementary experimental units were all framed by two border lines of 1-41, a natural hybrid clone planted at the same spacing as the artificial hybrid clones used. The experimental setup covered a total area of 1.64 ha. A starter fertilization consisted of burying, in four pockets, 200 g of NPK 13-13-21 fertilizer, around the plant, at planting.

In the field, the diameters of the seedlings at the collar were measured at 3 and 6 months. From 12 months the circumference was measured at 1.30 from the ground. The height of the trees was measured with the pole (from 6 to 12 months) then the suunto inclinometer (from 18 to 36 months). The diameter at the collar (C) was measured with a precision 10⁻² electronic caliper then with a tape measure. The measurements were taken every three months from 0 to 12 months and then every six months from 18 to 36 months.

To test the differences in height and circumference among types of plants or among clones, one-way analysis of variance (ANOVA) was used for each age after plantation. A multilevel model was first fitted according to:

Y i = μ + δ i + ε i (1)

with

Y i is alternatively height or circumference;

μ is the overall mean value of height or circumference;

δ i represents the factor of the data, i.e., that each individual, i belongs to a type of plant or a clone;

and ε i is the residual term which includes any measurement error.

The conditions of normality and homoscedasticity of residuals were checked graphically and with Shapiro-Wilk and Breusch-Pagan tests, respectively. When these conditions were invalidated, the ANOVA with a nonparametric test (Kruskal-Wallis rank sum test) was used to test the differences in height and circumference. For the nonparametric Kruskal-Wallis test [9], the null hypothesis was “no difference between medians for each variable”. When the null hypothesis was rejected, we conducted post hoc Kruskal-Wallis multiple comparisons between medians [10]. All statistical analyses were computed using the open source R environment [11], using packages PGIRMESS package for post-hoc test [12].

One of the most remarkable morphological differences noted between the two types of cuttings at the nursery stage is the orientation of the newly formed axis resulting from the development of the axillary bud for traditional cuttings and that formed by the development of the terminal bud for cuttings intensively managed container-grown stock plants. Observations show that the newly formed main axes of plants from rooted cuttings produced from coppicing stumps have an inclination with respect to the vertical. While those from the stock plants above ground are relatively orthotropic.

The newly formed axis of conventional plants on average is deviated from the vertical by an average angular value of 40.28˚. Analysis of variance showed a very significant clone effect (P < 0.001) on the inclination of the newly formed axis relative to the vertical. For the clones studied, 18 - 50 is more orthotropic than 18 - 52 (Figure 2). The latter is in turn more vertical than the 18 - 209. The most plagiotropic of the clones studied is 18 - 72 with an average spacing of the newly formed axis from the vertical of 49.75. The test of means comparison indicates that the angular values obtained are about 49.95 a for the clone 18 - 72, 47.00 ab for the clone 18 - 209, 36.56 b for the clone 18 - 52 and 29.16 c for the clone 18 - 50.

The height growth curves of two types of plants are confused from planting to eighteen months of age (Figure 3). Twenty-four months after planting, growth curves show a difference in development up to the age of thirty-six months in favor of plants resulting from the shoots of open stumps. The analysis of variance carried out at the 5% level confirms the observations made by showing

significant differences (P < 0001) in the growth in height between the two types of plants of the plantation from the age of eighteen months.

The circumference at the collar and at 1.30 m, on the other hand, evolves differently. It differs significantly from the nursery stage and varies during growth in favor of plants resulting from the cuttings collected from field coppicing stump stock plants at 12 and 18 months when the values of the two types of cuttings merge (Figure 4).

Analysis of variance revealed a very significant clone effect (P < 0.001) on the growth parameters studied (Table 1). The four clones studied do not grow in the same way (Figures 5-8). Also, all clone propagation technical interactions are always significant (Table 1), indicating that some clones are better with plants resulting from cuttings from the shoots of open-ground stump shoots, others with plants from cuttings from stock plants above ground. The differences remain however limited. The growth of the clone 18-147 is better with plants from conventional cuttings, on the other hand, the clone 18-228 is better with plants from cuttings from stock plants above ground.

Above-ground stock plants benefit from intensive cultivation conditions - severe and repeated pruning, adapted irrigation and fertilization [4], reputed to have a

positive effect on rejuvenation and maintenance of the juvenile state [13] and [14]. In addition, the plants resulting from the stock plants above the ground are ramets of the stock plants in the ground and can therefore be considered as being more juvenile than the cuttings resulting from the shoots of open stumps, by reference to the principle of cuttings “cascade” successfully used in some cases to physiologically rejuvenate selected aged genotypes [15] and [16]. The juvenility of cuttings from above-ground stock plants for clones of the interspecific hybrid Eucalyptus urophylla × Eucalyptus grandis, has been proven by the work of [5] at the microscopic morphological scale by studying the conformation of the domes of the stem apical meristems, [7], at the molecular level through a study of the dosage of 5-methyl-Cytosine in the stem apices and the immunolocalization of 5-methyl-Cytosine in the apical stem meristems.

At the macroscopic morphological level, the rooting dynamics of cuttings from stock plants outside and that of cuttings from stock plants in open ground differs significantly [7]. However, the plants resulting from the technique of cuttings from above-ground stock plants lose the advantage acquired in terms of duration of precocity to adventitious rooting. Indeed, the two types of plants grow in the same way in the first months of installation in the field, from 3 to 18 months, and express lower growth parameters than those of plants resulting from the classic cutting technique. Although the differences are relatively insignificant between the two types of cuttings.

Cuttings from container-grown stock plants, C, above ground have an early rooting. After a 25 long rooting period, the C cuttings are ready for acclimatization [4] and to be field-planted. The plants resulting from this production technique are obtained 2.5 months after the date of cuttings. These plants should go into planting at this age. The size of training received by these plants, which is the removal of roots and secondary axes of the main stem would be detrimental to their growth in the field and reduce the performance of this type of cuttings.

It should be noted that collar circumferences were significantly superior form cuttings F than for C ones larger neck diameters were noted in favor of plants from full-ground stock plants. This observation must be put into perspective, however, because cuttings from stock plants in the ground have a larger diameter than cuttings from stock plants above ground even before the initiation of the roots.

The difference in morphological shape observed between the two types of plants [4] must take into account the fact that the plant resulting from the stock plant in the ground results from the development of a newly formed axillary bud while the plants above ground come from the development of a cutting with apex. The first level of influence then emerges with the above ground or full ground origin of the cuttings on the morphological shape of the cuttings. This study has just highlighted another level of possible influence on tropism. This is the clone effect on the angular value of the inclination of the plant's main axis from the vertical, which would assume a genetic effect on the tropism. The orientation of the main axis of the plant, and even of the young shoots [17], during its growth is symptomatic. Numerous studies, including that [14] [18] and [5] confirm that orthotropic growth is relative to young material, while plagiotropic growth is associated with a mature material type.

The plants studied come from cuttings from the shoots of open stumps. These plants are relatively mature because they come from cuttings from stump shoots. On the other hand, the plants resulting from cuttings outside the ground are ramets of the stock plants of full ground [5] in the process of rejuvenation of the clones by cuttings. They are therefore more juvenile than plants obtained from the shoots of open-ground stumps. This study supports to some extent the assertion of [18].

The major consequence of tropism is the presence of basal curvatures in the first months of growth on plants from cuttings from full-ground stock plants, curvatures due to a straightening of the plagiotropic axis. 80% of plants resulting from cuttings of open-ground stump shoots show a bend at 6 months of growth in the field.