Aplicación del supuesto normativo racional.

Phase change from the juvenile to the adult state is often accompanied by a range of morphological and physiological changes in leaf characteristics, as reviewed by Hackett ( 1 985), Poethig ( 1 990), Haffner et al. ( 1 99 1 ) and Greenwood ( 1 995) (see Section 1 .2.2). Consequently, carbon isotope discrimination, which is affected by leaf morphology and physiology, might provide a useful tool for the investigation of phenomena associated with phase change. However, reports of research that

explicitly set out to examine discrimination in relation to phase change are not common in the literature. The information concerning differences in carbon isotope discrimination between the juvenile or adult states and its possible relevance to phase change in M. excelsa is reviewed below.

Changes in leaf characteristics, such as the number and/or position of stomata (Bauer and Bauer, 1 980), the thickness of epidennal and mesophyll tissue (Brand and Lineberger, 1 992b:. Bostrack, 1 993; Clearwater and Gould, 1 993; Hansen, 1 996; Fleck et al., 1 996; Wang et al., 1 997; Day et al., 1 997), and the development of tomentum have been reported to be associated with phase change in a number of woody perennial species, including M. excelsa (Cockayne, 1 928� Dawson, 1 968a; Clemens et al., 1 9(9).

Diffusion of CO2 into the leaf through surface tomentum and stomata, as well as diffusion within the leaf tissue, are also likely to be altered by phase change transformations of leaf thickness or stomatal distribution (Bauer and Bauer, 1 980; Day et al., 1 997). Longer-tenn changes in Pi and thus CO2 availability at the site of carboxylation should then be reflected in values of carbon isotope discrimination (0 ' Leary, 1 98 1 ; Farquhar et al., 1 982). Bauer and Bauer ( 1 980) compared

photosynthetic and anatomical attributes of juvenile and adult leaves of Hedera helix and found that light-saturated net photosynthesis was 1 .5 times higher in adult leaves than juvenile, owing partly to the lower stomatal and residual conductances to the CO2 in juvenile leaves.

M ore directly, phase change and carbon isotope discrimination in leaves were examined in association with ecophysiological differences between j uvenile and adult plants in an open field trial of several woody species (Acer negundo,

A rtemisia tridentata, Chrysothamnus nauseosus. and Salix exigua) by Donovan and Ehleringer ( 1 99 1 ; 1 994). These researchers concluded that the difference in discrimination between plants of the two ontogenetic states was probably related to plant size, and consequently to the soil moisture availability. They also found that a lower WUE in juveniles was accompanied by higher rates of photosynthesis and stomatal conductance than in adult plants. As expected, WUE was negatively correlated with carbon isotope discrimination. However, the effects of water availability and ontogenetic state were not separated in this experiment.

Rundel et al. ( 1 999) also found that (presumed j uvenile) seedlings of Zygophyllum prismatocarpum showed consistently higher discrimination than adult plants. In

this study, however, no correlation between leaf morphology and carbon isotope discrimination was found.

Carbon isotope discrimination in j uvenile and adult leaves was compared in Acacia by Hansen ( 1 996) and in Quercus by Fleck et al. ( 1 996). Hansen ( 1 996) hypothesised that juvenile leaves of A . koa would possess suites of adaptations that would be consistent with rapid establishment in seedlings, such as high growth rate, while phyllodes borne by adult plants would exhibit attributes of drought resistance in response to unpredictable stress situations. The author used carbon isotope discrimination to quantify long-term physiological differences between these two types of photosynthetic organs. He concluded that leaf morphology, including the mass per unit area, accounted for differences in leaf physiological performance in juvenile leaves, which had higher discrimination. The author quoted Hansen and

Steig ( 1 993), who had earlier concluded that xeromorphic leaves have a lower Pi than mesomorphic leaves, which in turn lead to a lower discrimination against !3C02 (Farquhar et aI., 1 982).

Fleck et al. ( 1 996) attributed an unexpectedly lower discrimination in resprouts (re­ growth after tree felling and fire damage) compared to 40-year-old undisturbed trees

of Quercus ilex to the importance of underground organs, and a higher physiological adaptive capacity of resprouts in response to environment.

In the above studies, carbon isotope discrimination in true juvenile (Hansen, 1 996) or rejuvenated resprouts (Fleck et aI., 1 996) was examined in comparison to that in adult leaves. However, differences were attributed to the role of underground organs (Fleck et a1. 1 996), the smaller size of juvenile leaves resulting in lower WUE, and to higher rates of photosynthesis and stomatal conductance (Donovan and Ehleringer,

1 99 1 , 1 994; Hansen, 1 996), rather than to the ontogenetic state of the plant itself The somewhat scattered studies of carbon isotope discrimination yield contradictory results in tenns of an overall pattem of response in the juvenile versus the adult state. For example, rejuvenated sprouts had lower discrimination than adult material of the same tree (Fleck et al., 1 996). Conversely, Donovan and Ehleringer, ( 1 99 1 ; 1 994), Hansen ( 1 996) and Rundel et a1. ( 1 999) found the opposite, i.e. leaves of seedlings or rej uvenated plants had higher discrimination against 13C02 than those of adult plants.

Donovan and Ehleringer ( 1 99 1 ) and Rundel et al. ( 1 999) found that differences in carbon isotope discrimination due to ontogenetic state were more pronounced on wet sites, presumably as a result of juveniles being as water stressed as adults on drier sites. However, in the study of Rundel et a1. ( 1 999) a wet site was considered relative to other observed sites in an arid zone with ca. 1 00 mm of annual precipitation, while Donovan and Eh1eringer ( 1 99 1 ) considered a wet site to have 580 mm of aIIDual precipitation. Consequently, what was considered in the report of Rundel et a1. ( 1 999) to be a relatively wet site, was a dry site in the study of Donovan and Ehleringer ( 1 99 1 ).

Differences in discrimination between juvenile and adult leaves should arise from differences in _P/Pa for CO2 and/or the CO2 concentration at the site of carboxylation. These arise from numerous changes in the morphology and anatomy of leaves that are associated with phase change in general (e.g. Bauer and Bauer, 1 980; Hackett,

1 985 ; Clearwater and Gould, 1 993), and in leaf morphology of M. excelsa in

leaf thickness associated with phase change reported by Komer and Diemer ( 1987), Hansen ( 1 996), and Day et al. ( 1 997) is likely to alter C02 diffusion through the leaf, chlorophyll content and its distribution within leaf cells leading to differences in CO2 assimilation (Bauer and Bauer, 1 980; Woo et aI., 1 994; Fleck et aI., 1 996),

anatomical changes, such as in thickness and composition of the mesophyll (Komer and Diemer, 1 987; Clearwater and Gould, 1 993; Day et aI., 1 997;Wang et aI., 1 997).

A number of papers reported on differences in discrimination between pubescent and glabrous leaves of Metrosideros polymorpha (e.g. Vitousek et aL, 1 990; Cordell et aI., 1 998). In these reports, the differences in discrimination were explained as an effect of the growth environment, but were also strongly

correlated with the increasing leaf mass/area ratio of the pubescent leaves across all examined environments. Morphological differences between the leaves of j uvenile and adult plants of M. excelsa (Cockayne, 1 928; Dawson 1 968, 1 972;

Clemens et aI., 1 999) provide a visual indicator of the progress of phase change. Moreover, the morphological differences of the leaves indicate that differences in leaf carbon isotope composition could be expected to exist between ontogenetic states oiM. excelsa. There are no reports of a link between leaf anatomy and/or carbon isotope discrimination and ontogenetic changes in M. excelsa.

Summary

Based on the literature reviewed above, it was hypothesised that carbon isotope discrimination in leaves of M. excelsa would be affected by ontogenetic state. Thus, changes in discrimination would mark the progress of phase change. Moreover, if the hypothesis was verified, carbon isotope discrimination could be indicative of underlying physiological processes associated with phase change. Measuring carbon isotope discrimination would provide an integrated measure of the physiological processes associated with phase change over a relatively long time scale. However, Borchert ( 1 976) considered that a more dynamic means of observation in ontogenetic studies was also necessary.