Читать книгу Ecology - Michael Begon - Страница 69

The responses of plants to changes in the quantity of light include the induction at high light intensities of photoprotective mechanisms that prevent the photosynthetic machinery from getting ‘overexcited’ and risking the generation of damaging oxidising radicals, instead dissipating excess light as heat. However, when intensities return to harmless levels, there is typically a delay before these protective mechanisms are fully switched off, such that rates of photosynthesis at these times are lower than they might otherwise be. Some calculations suggest that this could cost field crops as much as 20% of their potential yield (Kromdijk et al., 2016). It would therefore clearly be valuable if that switching off of the protective mechanisms could be speeded up. Bioengineering (the insertion of new or altered genes into a plant) offers the opportunity of applying our understanding of the physiology of photoprotection to effect such an accelerated response. Results are shown in Figure 3.5 for a study in which variants of three different genes known to be instrumental in the operation of the mechanism were selected for increased expression levels, following a screen of seedlings of the model plant Arabidopsis thaliana. These variants were then inserted into tobacco plants, Nicotiana tabacum, itself used as a model for crop plants in general, since the photoprotective mechanism being altered is common to all plants.

Figure 3.5 Bioengineering of photoprotection can improve crop plant performance. (a) To the left, a comparison for two measures of photosynthetic efficiency (of CO₂ uptake and of electron transport) and of the rate of harmlessly dissipating excess light as heat – the rate of ‘quenching’ of chlorophyll fluorescence (NPQ) – at steady levels of light, between wild type (WT) Arabidopsis plants and three strains bioengineered to switch off photoprotection more rapidly. There were no differences. To the right, a similar comparison but with fluctuating light levels. The bioengineered strains were all significantly more efficient in photosynthesis than the wild type because fluorescence was dampened down more rapidly. (b) The consequences for the bioengineered plants in terms of weight, leaf area and plant height, following 22 days of growth in the field. All strains grew better. In both (a) and (b), bars are SEs and * indicates a significant difference between bioengineered lines and the wild type (P < 0.05)

Source: After Kromdijk et al. (2016).

When the supply of light was constant, all three types of bioengineered plant behaved similarly to wild type plants in terms of photosynthetic efficiency and the harmless dissipation (‘quenching’) of excess light as heat (Figure 3.5a, left). But in the field, most leaves experience continually fluctuating light due to clouds and intermittent shading from the leaves above. It is notable, therefore, that in the fluctuating regime, photosynthetic efficiency was higher in the bioengineered plants than the wild types, and their overall level of quenching was lower, because it was compressed into a shorter period (Figure 3.5a, right). As a result, the bioengineered plants grew much better than the wild types (Figure 3.5b). Bioengineering of any sort must always be applied with caution, but these results do hold out the prospect of significant increases in yield for a wide variety of crops, since this process is common to all land plants.

pigment variation in aquatic species

In aquatic habitats, much of the variation between species is accounted for by differences in photosynthetic pigments, which contribute significantly to the precise wavelengths of radiation that can be utilised. Of the three types of pigment – chlorophylls, carotenoids and biliproteins – all photosynthetic plants contain the first two, but many algae also contain biliproteins; and within the chlorophylls, all higher plants have chlorophyll a and b, but many algae have only chlorophyll a and some have chlorophyll a and c. These different forms of chlorophyll all have slightly different absorption spectra, so that in combination, the plant or alga can trap more light. We see an example of variation in the nature of light with the concentration of dissolved organic matter in lake water, and the consequences of this for the photosynthetic microphytoplankton living there, in Figure 3.6. Of two lakes in north‐western Patagonia, Argentina, one, Lake Morenito, had lower concentrations of dissolved organic matter, leading to ‘greener’ light (Figure 3.6a) and hence to higher densities of cryptophyte algae (Figure 3.6b). Cryptophytes have a unique combination of pigments – chlorophylls a and c, but also the carotenoid alloxanthin and one of two biliproteins – allowing them to function effectively in that range. The other, Lake Escondido, with yellower light, had a microphytoplankton community dominated by chrysophytes (‘golden algae’), which lack these biliproteins.

Figure 3.6 Variation in light quality in lakes can give rise to different communities of photosynthesisers. (a) The mean spectrum (wavelength distribution) of downward irradiance in the water columns of two lakes in Argentina: Lake Escondido and Lake Morenito. (b) As a result of the greener light in Lake Morenito, it supported a phytoplankton community with a higher proportion of cryptophytes that have photosynthetic pigments enabling them to function effectively at such wavelengths.

Source: After Gerea et al. (2017).