Light Quality Signalling Functions of Light

As already mentioned above (Sect. 3.4.1), light quality changes in relation to horizontal and vertical structure of forests.

Ultraviolet light (UV) is interesting because blue light and UV may exert signalling functions. Shade adapted tropical tree seedlings can gradually adapt to UV (Krause et al. 2003b) and UV radiation may be attenuated by UV absorbing substances and plant cuticles (Krauss et al. 1997). Often, however, natural UV-A and

UV-B radiation may cause photoinhibition and photodamage particularly in shade leaves exposed to full sunlight (Krause et al. 1999, 2003a).

Most interesting in relation to signalling, however, is the other end of the spectrum because filtration by canopies eliminates the red light (R) from the solar spectrum much more effectively than the far-red light (FR). Sunlight has a mean R/FR ratio of 1.2 but under green canopies the ratio may be reduced to levels below 0.5 (Vazquez-Yanes and Orozco-Segovia 1993). This affects all processes regulated by the phytochrome system. Irradiation with red light generates the active PFR form of phytochrome, which elicits various photomorphogenetic responses. Far-red light inactivates the phytochrome, shifting the phytochrome equilibrium towards the inactive PR-form (Box 4.7). The light intensities required in phytochrome effects are often extremely low. It is the signalling function of light, which is sensed by the phytochrome system and not its function as an energy source.

Box 4.7 The reversible phytochrome system

Photo-morphoses

Photo-morphoses

Wave length (nm)

4.3 Seedlings: Germination, Establishment and Growth 4.3.1 Regulation of Seed Dormancy and Germination

Among the many processes governed by phytochrome (Sect. 4.2.2) one of the most well known is the germination of the seeds of "light-germinators", i.e. positive pho-toblastic seeds. In this way phytochrome also plays an important role in the regulation of succession and regeneration in tropical forests, because light dependence of seed germination is one of the most fundamental differences between pioneer species and late successional or climax species, where competition is not only explained by the substrate and energy aspects of irradiance but also its signalling functions (Aphaloet al. 1999).

Only seeds of late successional and climax species can germinate and establish seedlings under deep canopy shade. These seeds germinate very soon after dispersal and also remain alive in the soil only for a short time. The mean ecological longevity of seeds in the tropical rainforest may be one of the shortest of any plant community (Vâzquez-Yanes and Orozco-Segovia 1993). The advantage of this behaviour lies in the fact that seeds are more threatened by predators and parasites in the soil environment of the tropical rainforest with continuous moisture and high temperature, than are seedlings. Thus, the seed banks in rainforest soils are depleted of seeds of late succession and climax species. On the other hand, seedlings may grow extremely slowly and a persistent nursery of small plants is built up, i.e. a seedling bank instead of a seed bank. Flores (1992) has studied two species of the cloud forest of the northern coastal range of Venezuela and his observations give a good idea about the actual longevity of tree seedlings after germination:

• Aspidosperma fendleri (Apocynaceae), an emergent species, which grows its crowns above the canopy (see Fig. 3.25),

- germination time 5 days,

- longevity of cotyledons 2 months,

- longevity of 1st leaf pair 2 years.

• Richeria grandis (Euphorbiaceae), a canopy species,

- germination time 20 days,

- longevity of cotyledons 2.5 years,

- longevity of leaves 3 years.

The small plants remain in a state of slow growth until a canopy gap provides an opportunity for stimulation of growth (see Sect. 4.3.2). The survival of the seedlings is independent of photosynthetic parameters and largely determined by morphological characteristics which are likely to provide protection from and enhance defence against herbivores and pathogens, i.e. dense and tough leaves, a well established root system and a high wood density. Furthermore, reserves of non-structural carbohydrates in stems, roots and storage cotyledons support long-term survival of seedlings of shade tolerant species enabling them to cope with periods of biotic and abiotic stress (Myers and Kitajima 2007). Seedlings in gaps proved to be more resistant to herbivory than seedlings in the undergrowth of tropical forests (Blumwald and Peart 2001). Seedling survival of 13 tropical tree species was found to be negatively correlated to relative growth rate (RGR), i.e. both low RGR of plants raised in the shade and high RGR of plants in the sun, and to leaf area ratio and positively correlated to root/shoot ratios and wood density (Kitajima 1994; Fig. 4.15). Fast growing pioneer plants need lower stem support and afford a lower wood density, but this reduction of support costs is related to higher mortality rates (Fig. 4.15E; King et al. 2006).

In contrast to late successional species seeds of woody pioneer species are capable of dormancy. They are often the most abundant components of the soil seed bank in tropical forests (Vâzquez-Yanes and Orozco-Segovia 1993). Dormancy may be enforced by hard seed coats which are impermeable to water and oxygen and need many weeks for breakdown by weathering and microbial action. However, germination is mainly determined by light.

Light may act via temperature effects, especially via temperature alternations, which are required by some seeds for germination. Canopy gaps and clearings lead to greater fluctuations of soil surface temperatures due to direct insolation (see Fig. 3.29). Often, however, germination is regulated by light quality and the involvement of the phytochrome system rather than by light intensity. A higher proportion of red light activates and a higher proportion of far-red light inactivates phytochrome and the reversibility of phytochrome effects may be important in excluding reactions to short light flecks (Sect. 4.2.1) and sensing true light gaps. The photoreversibility of phytochrome mediated germination within certain time limits may be essential to prevent germination resulting from light flecks (Vâzquez-Yanes and Orozco-Segovia 1993).

The sophisticated regulation of dormancy and germination, respectively, is most frequent among pioneer species and gap colonizers, with germination inhibited under closed canopies and stimulated in clearings.

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Renewable Energy 101

Renewable Energy 101

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