Development of the Root System

It has been shown that in the primary root the elongation zone was modified in mierogravity. For instance, in lettuce seedling roots the length of the cell elongation zone was shorter in mierogravity than in the onboard 1-g centrifuge (Merkys and Laurinavicius 1990). The presence of cells elongating closer to the quiescent center9 indicated that there was an acceleration of the cell differentiation process. For example, the formation of root hairs occurred at a shorter distance from the root tip. The meristematic activity slowed down earlier in mierogravity than in the ground controls, and root apices of the same age in orbital flight contained only differentiated cells (review by Kordyum 1997). An early decrease or a removal of the apical dominance (i.e., the fact that the primary root tip inhibits the formation of secondary root primordia) led to the abundant formation of lateral roots described in many investigated species (Halstead and Dutcher 1987, Kordyum 1997).

Interestingly, Driss-Ecole et al. (1994) have shown that there was an increased biomass of roots in Veronica arvensis seedlings grown for 45 days on a slowly rotating clinostat (Figure 6-20). Dry weight of the hypocotyl and roots was 40% greater on the clinostat than in the controls due to a higher production of secondary roots.

1G Control

Figure 6-20. Veronica arvensis plants grown on a slowly rotating clinostat (on the left) or in the vertical position (on the right). The seedlings were cultivated on agar containing Heller medium (half-strength) for 45 days, ep: epicotyl part; hp: hypocotyl part; c: cotyledons; r: roots; numbers represent the order of the successive pairs of leaves. Note the greater development of the root system on the clinostat and the presence of adventitious roots growing at the level of the hypocotyl in simulated micro-gravity. Adapted from Driss-Ecole etal. (1994).

Clinostat

9 The quiescent center is a region of apical meristems in roots in which there is relatively little or no mitotic activity.

Aarrouf et al. (1999) have also observed similar results on rapeseed seedlings grown on the clinostat (see Figure 6-18). They have demonstrated that during the first 10 days of culture there was a faster development of the root system (primary root and secondary roots). However, after 15 days the apical dominance of the primary root was perturbed on the clinostat and the formation of secondary roots could occur very close to the tip of the primary root. Later, the primary root stopped growing on the clinostat whereas the secondary roots continued to elongate. The loss of apical dominance that was observed in microgravity on several species was also seen on the clinostat. It could be linked to a perturbation of the cell cycle in the primary root meristem since Aarrouf et al (1999) have shown that there were more cells in the G2 phase in the control than on the clinostat after five days of culture. But, after 25 days, there were more cells in the Gi phase on the clinostat. According to Aarrouf et al. (1999) the slowing down of the cell cycle could be due to a slight but continuous stimulation, which could occur on the clinostat and to the modification of the hormonal balance (IAA and ABA, at least).

If the results on the root system grown in microgravity are compared to those obtained on the clinostat, it can be concluded that at least during the first steps of development clinorotation has not the same effect as microgravity. In this way, clinostat could not be a good tool for simulating weightlessness. However, there is some similarity of the development of the root system in both conditions for longer period of growth. It is well known that the sensitivity of the primary root to gravistimulus is much greater during the first steps of its development. It is therefore possible that during this particular period (two days) there could be slight gravitropic stimulation on the clinostat that increases the growth of the primary root. Then after this period, the gravitropic sensitivity being lower, clinorotation could have the same effects as microgravity and could be a better simulation of microgravity.

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