Germination and Root Orientation

Many species have been grown in mierogravity or on clinostats and it appeared that the absence of gravity or simulation of weightlessness had no real effect on germination (Halstead and Dutcher 1987, Kordyum 1997). However, the orientation of growth of the radicle, which is strongly dependent upon gravity on Earth, is related to the position of the embryo in mierogravity or in simulated mierogravity (Volkmann et al. 1986). Thus, as it develops on a clinostat the primary root shows spontaneous curvatures, which have been studied extensively on maize by Hoson (1994). The maize root on the 3D clinostat did not grow straight (Figure 6-17A). Control roots grown in 1 g showed some degrees of curvature in three regions. The curvature around the basis (angle K) of the root was always prominent. The distribution of angle A and M of control roots was more concentrated around 0 deg than angle K. Clinorotation greatly enhanced the curvature and caused an increase in the dispersion of the angle of bending. These curvatures in the segments of the root under simulated microgravity conditions may be derived from inherent properties of plants, which could be modified by the gravity vector on the ground. Such an automorphogenesis appears to play a major role in the regulation of plant development under a microgravity environment. The spontaneous curvatures were not due to osmotic concentrations within the cell or to mechanical properties of the cell wall between the two sides of the bending roots. According to Hoson (1994), the bending could be linked to circumnutation (regular oscillations of the tip), which have been observed by Volkmann et al. (1986).

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Figure 6-17. A. Root development in a Zea mays seedling on a 3D clinostat (simulation of microgravity). The angles K, A, and M indicate the angles of bending at the level of three points. Adapted from Hoson (1994). B. Root orientation in lentil seedlings grown in space after 25 h and 29 h spent inside an onboard 1-g centrifure or in microgravity. Asterisks indicate seedlings whose extremity was subjected to a strong change in orientation between 25 and 29 h. r: root; s: sponge. Adapted from Legué et al. (1996).

Figure 6-17. A. Root development in a Zea mays seedling on a 3D clinostat (simulation of microgravity). The angles K, A, and M indicate the angles of bending at the level of three points. Adapted from Hoson (1994). B. Root orientation in lentil seedlings grown in space after 25 h and 29 h spent inside an onboard 1-g centrifure or in microgravity. Asterisks indicate seedlings whose extremity was subjected to a strong change in orientation between 25 and 29 h. r: root; s: sponge. Adapted from Legué et al. (1996).

Such movements have also been observed in lentil roots by Perbal et al. (1987) and Legué et al. (1996). After a growth period of 25 h in microgravity, the emerging root was bent and its tip was most often pointing away from the cotyledons (Figure 6-17B). Although the mean angle of curvature was about the same after 25 and 29 h, some roots were subjected to a strong change in orientation during 4 h. This meant that the movement was not synchronous from one root to another.

Root growth in Lepidium sativum has been studied by Johnsson et al. (1996) in order to determine whether the root tip was subjected to random walk (growth in a random direction). The seedlings were grown between two agar slices and the deviation angle of the root tip (at) at time t was measured with respect to a fixed reference direction (a0). Theoretically, random walk is characterized by a mean deviation equal to 0, a, - a0 = 0 and the variance of the deviation should be proportional to time: (a, - a,,)2 = k x t (k constant).

These authors showed that the displacement of the root tip could be considered to be random walk since it fulfilled the two criteria cited above, at least at the beginning of the root growth. These results could appear contradictory to those obtained by Volkmann et al. (1986) since these authors observed that the orientation of the root tip depended upon the position of the embryo and that it was subjected to nutations (movement of oscillation). This controversy can be eventually explained by the fact that these nutations are asynchronous even at the beginning of the root development (Figure 6-17B). It could be also due to different culture conditions since in the Johnsson's experiment the displacement of the root tip could occur only in a plane between two agar slices whereas in the Volkmann's experiment the root tip had the possibility of 3D movement. In any case, the experiments with clinostat or in mierogravity have shown that without the unilateral effect of gravity, the root tip is subjected to various movements, its orientation during germination being strongly dependent upon the position of the embryo in the seed.

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