Photosynthetic Profiles of Green, Purple, and Spotted-Leaf Morphotypes of Tipularia discolor (Orchidaceae)

Three morphotypes of Tipularia discolor (Cranefly Orchid) occur sympatrically in the piedmont of North Carolina, producing leaves with upper (adaxial) surfaces that are either green, purple, or green with purple spots; all leaves exhibit bright purple lower (abaxial) surfaces. The basis for differences in coloration between individuals in this species is unknown. Because leaf purpling is often a sign of physiological stress, we hypothesized that adaxially purple and/or spotted individuals would exhibit reduced photosynthetic capacity compared to adaxially green individuals, and that adaxially purple tissues would exhibit symptoms of shade acclimation relative to adaxially green tissues (consistent with a photoprotective function of anthocyanin pigments). We made the following measurements on samples of the 3 morphotypes: photosynthetic response to light and intercellular CO2, monthly photosynthetic gas exchange and chlorophyll fluorescence, and quantification of foliar pigments (chlorophylls, xanthophylls, and anthocyanins). Inconsistent with both hypotheses, the 3 morphotypes did not differ in any of the parameters measured, except anthocyanin content. Furthermore, corms continued to produce leaves with the same pigment patterns in subsequent years, even when sunlight, fertilizer, and water were controlled for in a greenhouse setting. We conclude that leaf purpling and spotting in Cranefly Orchid are most likely not related to physiological stress, and are more likely genetic in nature, perhaps representing alternative phenotypic adaptations related to herbivory defense.