EFFECTS OF VIRUS-INFECTION AND GROWTH IRRADIANCE ON FITNESS COMPONENTS AND PHOTOSYNTHETIC PROPERTIES OF EUPATORIUM-MAKINOI (COMPOSITAE)

We examined the effects of geminivirus infection on fitness components and on photosynthetic properties of the host plant, Eupatorium makino i, grown at two irradiance levels in a natural-light greenhouse. Under the low-light condition (13% full sunlight), more than a half of the infected plants died during the 9-mo experiment, while most of uninfec ted plants survived. Growth rate was also lowered by infection. At hig h light (50% full sunlight), by contrast, virus infection did not caus e mortality despite slight decrease in growth rate. Flowering occurred only at high light, and reproductive outputs of the plants were marke dly reduced by the infection. Infected leaves had distinct yellow vari egations and, when compared with uninfected leaves, they showed (1) co mparable light-saturated photosynthetic rate per unit area, but (2) lo wer initial slope of light-response curve of photosynthesis on an inci dent irradiance basis. The lower initial slope was mainly due to reduc tion of light-harvesting chlorophyll-protein complexes in the variegat ed parts. Since the differences in plant performance, depending both o n infection and on growth irradiance, were largely explained by the di fferences in growth rate and/or plant size, the reduced photosynthetic production in the infected plants would be a major factor explaining the inferior performance of the host plants.