Physiological and Ecological Analyses of Shade Tolerance of Plants 3. Effect of Shading on Growth Attributes of Helianthus annuus

As for shade tolerance the growth of Helianthus annuus was ananlysed under various light conditions, in comparison with that of Phaseolus aureus1, 2).1. The growth of H. annuus in total plant dry weight, leaf weight and leaf area was depressed markedly by shading. Particularly, the depression was striking in deeper shade (5, 10 and 22% light). In H. annuus the highest leaf area occurred in the full (temporarily 75%) light, while in Ph. aureus in light shade.2. The relative growth rate of H. annuus was evidently depressed by shading and turned to negative in later stage in deeper shade. The net assimilation rate (NAR) decreased with shading, though its variation with plant age was small.3. The leaf area ratio (LAR) tended to be large with decreasing the light intensity. However, in heavy shade (5 and 10% light) the LAR became smaller than in the 22% light, and this character is fatal to the shade tolerance.4. The pattern of response of the ratio of leaf area to leaf dry weight (F/F) to light intensity was similar to that of LAR. The ratio of leaf dry weight to total plant dry weight (F/W) decreased with shading. The range of variation with light intensity was larger in F/F than in F/W.5. The ratio of dry weight of non-photosynthetic organ to leaf dry weight (C/F) became large with decreasing illumination and this tendency was amplified with plant age.6. In order to confirm the effect of magnitude of C/F and of F/F on dry-matter production, the NAR was calculated on the basis of the photosynthetic and respiratory rate of leaf and the respiratory rate of non-photosynthetic organ using the results of growth analysis.7. The low shade tolerance of H. annuus may be attributed to the low F/F, which brings about the low LAR, and to the the high C/F under shade.