8.2  Plant and organ orientation

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Vascular plants orient themselves in space to optimise shoot exposure to radiant energy and CO2 in the atmosphere, and to maximise root access to water and nutrients in the soil. To achieve this, there is a range of directional control systems, which change as a plant proceeds through its life cycle. Regardless of how a seed falls to the ground, on germination a seedling root grows downwards and the shoot grows upwards. What controls these opposite directions of growth?

First, seedling shoots are very sensitive to low-intensity light, curving strongly towards any directional light which may indicate a break in the leaf canopy that the shoot can utilise. In mature plants, leaf orientation can follow the sun during the day to maximise light capture, but if midday radiant energy becomes excessive the leaf blade may instead orient at right angles to the sun’s rays. Flower buds are usually bent downwards, but on opening the stem straightens and holds the flower upright to maximise exposure to insects and other pollinating agents.

Second, gravity is an all-pervasive and constant orienting signal. However, roots and shoots generally show opposite responses to gravity, reflecting the intrinsic polarity in all higher plants. One half, the root system, is adapted for life in dense dark soil, while the other half, the shoot system, has evolved to exist in the fragile atmosphere, and harvests sunlight for photosynthesis. Conforming with this dichotomy, main roots exhibit a positive directional response to gravity, whereas shoots generally show a negative reaction.


Table 8.6

Directional growth responses to directional stimuli are called tropisms. There are three main kinds:

1. Gravitropism — gravity sensing

2. Thigmotropism — touch sensing

3. Phototropism — light sensing

The characteristics of the major tropisms are shown in Table 8.6. All these responses are due to different growth rates on two sides of a responding organ, resulting in curvature either towards or away from the stimulus. The positioning, or orientation in space, of many plant organs can be due to several tropisms and nastic (non-directional) responses acting together.