Introduction

Plants on Earth are subjected to a constant gravitational field, which has played a major role in their evolution. The actions of gravity on plants have been studied for more than a century (reviews by Larsen 1962, Sack 1991) and it is now well known that this physical factor has a great impact on

Figure 6-01. A dose-up view of a bloom on the Rasteniya-2/Lada-2 (Plants-2) plant growth experiment photographed by the astronauts on board the International Space Station. Photo courtesy of NASA.

the orientation of plant organs (gravitropism) and on the development of plants (gravimorphism).

Gravitropism is a response of bending due to a change in the orientation of plant organs or to an inadequate orientation of their extremity with respect to gravity. For instance, when a seedling root germinates on the ground, its extremity can be oriented in any direction but must penetrate into the soil quickly to survive. The final orientation of the root tip is the direction of gravity (even if it is reached only after one day of stimulation). The primary root therefore has a positive gravitropism. Shoots, on the contrary, have a negative gravitropism since their extremity curves in the opposite direction. The optimal orientation of growth of an organ can be parallel to gravity (orthogravitropism) or oblique (plagiotropism) with respect to the g vector. Thus, most of the plant organs have an optimal angle of orientation with respect to gravity that is called the Gravitropic Set-Point Angle (GSPA) (Firn etal. 1999).

Figure 6-02. Gravitropic bending of lentil roots. Left: This lentil seedling was grown in the vertical position for 27 h. Then, it was placed in the horizontal position for 3 h and photographed every hour. The counter-reaction (CR), which occurred after 3 h, led to a reduction of the curvature, c, cotyledon; r, root; g, direction of gravity. Right: Kinetics of the gravitropic response of 60 lentil roots grown in the vertical position and stimulated in the horizontal position as in A. The angle (a) of curvature (see insert) is reported as a function of time of stimulation in the horizontal position. After a latent time (LT) of about 20 min, there is a phase (LP) during which the response is linear as a function of time. After a strong slowing down of the rate of curvature a counter-reaction (CR) occurs which reduces the angle of curvature. The vertical bars represent the interval of confidence at the 5% level.

Figure 6-02. Gravitropic bending of lentil roots. Left: This lentil seedling was grown in the vertical position for 27 h. Then, it was placed in the horizontal position for 3 h and photographed every hour. The counter-reaction (CR), which occurred after 3 h, led to a reduction of the curvature, c, cotyledon; r, root; g, direction of gravity. Right: Kinetics of the gravitropic response of 60 lentil roots grown in the vertical position and stimulated in the horizontal position as in A. The angle (a) of curvature (see insert) is reported as a function of time of stimulation in the horizontal position. After a latent time (LT) of about 20 min, there is a phase (LP) during which the response is linear as a function of time. After a strong slowing down of the rate of curvature a counter-reaction (CR) occurs which reduces the angle of curvature. The vertical bars represent the interval of confidence at the 5% level.

Gravimorphism is the result of the effects of gravity on plant development. The actions of gravity on plant growth can be either quantitative (growth rate of plant organs) or qualitative (formation of plant organs). Some of these actions have been discovered by forcing a stem for instance to remain in an abnormal horizontal position by attaching its extremity. In this case the axillary buds (which are inhibited when the stem is in the upright position) begin to grow since the apical dominance of the apex over these buds is cancelled. This type of experiment shows that an inadequate orientation with respect to gravity provokes some changes in plant morphogenesis and not only on the orientation of the organs. However, the effect of this factor on a plant growing in the vertical position is not known, since the study of gravimorphism should include a comparison of plant growth on the ground and in microgravity. In that way, gravimorphism is more difficult to analyze than gravitropism. This is the reason why plant physiologists have used for more than a century special devices called clinostats in order to simulate microgravity.

The principle of these devices is simple: the clinostat prevents the unilateral effect of gravity by rotating the plant about a horizontal axis or about a point at 1 to 2 rpm (see Figure 1-19). These clinostats could actually simulate microgravity if the perception of gravity were too slow to induce a gravitropic signal. But, they can also induce slight omnilateral gravitropic stimulation if the perception time is short, i.e., in the order of 1 sec.

Plant physiologists have also carried out experiments with centrifuges assuming that there could a kind of continuum of the effect of gravity from 0 g to thousands g and by extrapolating the results obtained in the range of 1 g to thousands g. It is clear that works with clinostats or centrifuges can only give some information about what could be the action of microgravity on plant growth, and that space will remain a unique tool to study the effect of gravity on plant development (Figure 6-01).

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