Growth and reproduction in relation to light availability
5.4 Discussion
results indicate that light intensity has an effect on average plant size. In terms of the magnitude of absolute weights, this pattern of response to available light (45 % > 71 % > 17 %) was observed for the absolute dry weights ofleaves, sterns and roots (Fig. 5.4. b-d).
The greatestpercentage allocation of dry weight by plants in all treatments was to the sterns (Fig.
5.5). Furthermore, this proportion differed significantly between treatments and increased with decreasing light levels(F=15.7; p<0.05). The proportion of biomass allocated to secondary stolon production also differed and was significantly larger for those plants maintained at 45 % PAR (F=5.95, p<0.05) (Fig. 5.6). Proportional biomass allocated to the leaves was not significantly different at the range oflight intensities(F=I.26, p>0.05) despite the fact that there was a magnitudeofdifference in both the number and absolute dry weights ofthe leaves produced by the plants in each treatment (Fig.5.2b&5Ab). Allocation ofbiomass to roots for plants in all treatments was very low «20 %) and increased with increasing light levels (Fig. 5.5).
Flowers
The flowering season for D. odorata is during the months ofMay-June. For this experiment very few individuals flowered and the experiment was terminated in late June when existing capitula were in full bloom. Although flowering was low for plants in all treatments, dry weight of flowers produced was greatest for plants grown at 45 % PAR (Fig.5.7); 40 % (10 individuals) ofthe plants grown at 45 % PAR flowered while only 8 % (2 individuals) flowered at 71 % and 17% PAR. For this experiment flowering was not dependent on plant size as there was no relationship between plant size and production of flowers (Fig. 5.8).
1.6 --- ---.---------.-- ----._..~------ --- -- .- -.--- ---- --_....---...--~-.----.--.
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Fig. 5.7. Total dry weight (g) of inflorescences produced by D. odorata transplants following eight months
treatment at one ofthree light levels.
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plant weight (g)
Fig. 5.8. Dry weight (g) of inflorescences produced as a function of whole plant dry weight (g). T-test showed no significantdifference between weight of non-flowering plants vs. flowering plants (p>O.OS).
leaves produced at 17 % and 71 % PAR. This suggests that the photosynthetic assimilation rate was highest at an intermediate light level; however, detailed physiological studies need to be conducted to confirm this observation. Zimdahl (1993) noted that weeds with great competitive ability are often shade tolerant in that their highest carbon dioxide assimilation does not occur in full sunlight. Mack (1996) suggested a viney growth habit and a low light compensation point (the point at which the amount of CO2given out by the respiratory process is equal to that taken up by photosynthesis) to be traits indicative ofinvasive potential in natural or semi-natural forests.
The most consistent plastic responses to availability oflight are manifested in internode elongation (etiolation) and branching (the proportion of axillary meristems which grow out) (Bazzaz and Harper, 1977; Sultan, 1987;Ellison and Niklas, 1988; Hutchings, 1988; Sutherland and Stillman, 1988; Hutchings and De Kroon, 1994; De Kroon and Hutchings, 1995). Most plants show reduced branching and internode elongation at lower flux densities (De Kroon and Hutchings, 1994), however, this is not always the case (Lovett Doust, 1987; see review by Hutchings and De Kroon, 1994). In an experiment to determine the effect oflight on growth of the clonal herb Glechoma hederacea L., Slade and Hutchings (1987b) found that clones grown under shading had significantly longer internodes than stolons produced by unshaded clones but total stolon length ofunshaded clones was over three times greater than for shaded clones, as a consequence ofprofuse branching. In this experiment, total stolon length, branching frequency and secondary stolon production was greatest for plants growing at an intermediate light level (45 % PAR) as opposed to those at 71 % available sunlight. Thus althoughD.odorata shows plasticity in growth response to light availability, it does not respond to changes in the light environment as predicted for plants withforaging behaviour. Instead resource acquisition appears to be consistently highest at 45 % PAR, a result consistentwith observations on the success and proliferation ofD. odorata in moist, shadyenvironments (Fagg, 1989; Chipping, 1993; Alvarez, 1997). Ifinternode lengths were shortest for plants exposed to 45% PAR then D. odorata would be foraging for intermediate light levels. However, internode length was consistently shorter at the highest light level (71 %).
The maximum weight ofinflorescences was produced by plants grown at 45 % PAR. However, as only a few plants flowered the results of this experiment should be interpreted with reservation. After eight months growth, there was no relationship between plant size and sexual reproduction, a relationship which has been reported in other studies (Whigham, 1974; Werner,
1975; Pitkellaet al., 1980). Pitkella et al. (1980) found sexual reproduction in Aster acuminatus Michx., a forest understorey herb, to be indirectly affected by light through its effect on plant size. However, flowering is not always dependent on plant size (Salonen, 1994) and the results ofthis investigation indicate that floweringinD. odorata is independent ofplant size and directly affected by light availability. Furtherfield tests need to be conducted to confirm this observation.
Finally, following data collection, it was realized that this experiment could be improved by subjecting the stolons of each replicate clone to different light regimes. As stolons grow predominantly horizontally, sets of ramets along a stolon could be subjected, in experiments, to different light levels. Furthermore, to test for foraging behaviour, each set of ramets could be provided witha supply oflight, nutrients and water different from that supplied to other sets of ramets. Such an experiment would test implicitly the plasticity of a clone, as all variation in morphology could be ascribedto differences in growing conditions experienced by ramets of an individual. Unfortunately there was insufficient time to conduct this experiment.