Leaf development coloured young leaves

A common feature of tropical rain forest trees is the production of new leaves of distinctive colours other than green. Young leaves are often a shade of red, varying from pale pink to bright scarlet, but white or even purple or blue can be observed. In a survey of 250 species across the tropics (Coley & Kursar 1996), 33% of species were found to have non-green young leaves. These included representatives of 61% of the families in the sample. Among tropical trees, coloured young leaves are particularly common in the Sapindaceae and the caesalpinoid legumes. The proximate reason for the young leaves not being green is that they lack chlorophyll (Fig. 2.23). Greening is delayed in these species in comparison with those that do have young leaves containing chlorophyll. Anthocyanins, generally located in the cell vacuoles, are the pigments responsible for the red, purple or blue hues of the flush leaves. Physiological reasons for delayed greening have been put forward. Possibly young leaves are susceptible to damage from high light intensities and chlorophyll would be destroyed if it were put in sooner, and maybe the anthocyanin pigments are protectants against bright light. However, the observation that coloured young leaves are commoner in understorey species than elsewhere (Kursar & Coley 1992a) seems to discount this argument.

Other theories to explain delayed greening involve selection due to herbivore pressure. Chlorophyll is a nutritious molecule and will add to the attractiveness of leaf tissue to herbivores. The cost of not synthesising chlorophyll and beginning photosynthesis in young leaves is a loss of photosynthate. But young leaves with chlorophyll would be more likely to be eaten by herbivores. It is already well established that young leaves are particularly susceptible to herbivory (Coley & Barone 1996). This is probably because of their low fibre concentrations, and despite the fact that young leaves are often relatively well protected chemically (Fig. 2.24), often with twice the

Figure 2.23 Chlorophyll per area as a function of leaf age in Ouratea lucens, Xylopia micrantha, Desmopsispanamensis and Annona spraguei. The error bars indicate plus or minus one standard error. In some cases, the error bars are smaller than the point and therefore are not shown. Exp is the average time taken for leaves from 10% of full leaf expansion until 4d prior to full leaf expansion. The data at 0 d are the averages for leaves three days prior to full leaf expansion until +4d. The data at 10 d are the averages for leaves at +5 d to +15 d, etc. After Kursar & Coley (1992b).

Figure 2.23 Chlorophyll per area as a function of leaf age in Ouratea lucens, Xylopia micrantha, Desmopsispanamensis and Annona spraguei. The error bars indicate plus or minus one standard error. In some cases, the error bars are smaller than the point and therefore are not shown. Exp is the average time taken for leaves from 10% of full leaf expansion until 4d prior to full leaf expansion. The data at 0 d are the averages for leaves three days prior to full leaf expansion until +4d. The data at 10 d are the averages for leaves at +5 d to +15 d, etc. After Kursar & Coley (1992b).

Figure 2.24 Schematic representation of changes occurring during leaf development for species with delayed greening. Age 0 indicates full leaf expansion. Negative ages are for expanding leaves, and positive ages are for days since full expansion. Low molecular mass secondary compounds decrease during expansion, but no data exist to test this. After Coley & Kursar (1996).

Figure 2.24 Schematic representation of changes occurring during leaf development for species with delayed greening. Age 0 indicates full leaf expansion. Negative ages are for expanding leaves, and positive ages are for days since full expansion. Low molecular mass secondary compounds decrease during expansion, but no data exist to test this. After Coley & Kursar (1996).

concentrations of tannins and other polyphenols of mature leaves (Coley 1983; Turner 1995a). The reduction in nitrogen and defence chemical concentrations as leaves mature may be due more to dilution by cell wall material than an actual reduction of the total amount of the substances in the leaf. Anthocyanins are possibly anti-fungal compounds (Coley & Aide 1989). Pathogens, as well as herbivores, find young leaves more tempting. Therefore, delayed greening may be a plant strategy to reduce the attractiveness of its already vulnerable young leaves to herbivores.

Cost-benefit analysis shows that delayed greening should be more favoured where the foregone benefits of greening are low. This will be in sites where potential photosynthetic rates are only moderate, and these include the forest understorey where, as already mentioned, delayed greening is commoner. Delayed greening may also be a corollary of very rapid leaf expansion (Coley & Kursar 1996). If a tree develops its leaves quickly it will reduce the period when it is particularly at risk from herbivory. It may be that the degree of physiological activity required both to expand rapidly and import and synthesise the pigments and other photosynthetic apparatus is not feasible, so the latter is delayed to allow greater speed in leaf expansion (Coley & Kursar 1996). It is possible that coloured young leaves are rarer in the temperate region than the tropics because lower temperatures slow down leaf expansion rates and allow the importation and synthesis of chlorophyll to keep pace with expansion. Coley & Kursar (1996) found that the amount of herbivory on expanding leaves was positively correlated with expansion rate in a wide-ranging survey. Aide (1993) also failed to find the expected negative correlation between herbivory and expansion rate. Clearly the hypothesised advantage of rapid expansion reducing the likelihood of herbivore attack is not evident in cross-species comparisons. Coley & Kursar (1996) suggest that slow-expanders are better protected and protection slows expansion. This may represent another 'grow or defend' trade-off. The evolutionary advantages of rapid leaf expansion, presumably in terms of reducing the period of susceptibility to herbivores and the earlier implementation of full photosyn-thetic potential, may outweigh the risk of losing leaf material to herbivores during expansion.

Juniper (1993) has taken a somewhat different line in the debate about coloured young leaves, concentrating more on why young leaves have the non-green and limp form. He hypothesised that the main advantage of flush leaves is that they do not appear to be leaves at all. Thus they do not provide a strong signal to herbivores, particularly insects, that they are potential food. He noted the apparently large intrapopulation, and even intragenotype, variation in flush-leaf form and proposed that this may be a way to avoid reinforcing the cue to insect attackers that new leaves are available.

Lucas et al. (1998) proposed that leaf colour may be used as a signal of food quality by some herbivores. In tropical rain-forest leaves, low toughness and probably peak protein concentrations correspond to a phase where leaves are light and predominantly yellow in hue, sometimes dappled red. Increasing greenness and darkness reflect increasing toughness and fibre concentration as leaves mature, which would reduce food quality. The authors proposed that trichromacy (possession of three colour receptors on the retina) in primates may have evolved to assist in foraging for leaves, as colour vision greatly facilitates distinction of the best quality food.

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