Reproductive effort and floral photosynthesis in Spiranthes cernua (Orchidaceae).

This study examined the cost of reproduction and photosynthetic characteristics of the reproductive structures of Spiranthes cernua, an agamospermic, terrestrial orchid. Reproduction was frequent: two-thirds of the plants flowered at least 2 yr in a row and one-fourth of the consecutive-year runs were ~3 yr. Neither a significant decrease in leaf area nor a reduced likelihood of flowering was observed following 1 or 2 yr of inflorescence production. While there was a tendency for plants producing >16 flowers to have decreased size the next year, plants with the greatest number of flowers (31+) were the most likely to reproduce. Leaf and reproductive gas exchange were measured in the field. Low but positive rates of net photosynthesis were documented at all stages of inflorescence development. The average rates of photosynthesis for each stage were: leaves, 9.2 mmol CO2/m2s; inflorescence in bud, 3.7 mmol CO2/m2s; inflorescence in flower, 2.5 mmol CO2/m2s and infructescence, 0.2 mmol CO2/m2s. Based on diurnal gas exchange, the contribution of leaves and reproductive structures to seasonal carbon assimilation was 91.6 and 8.4%, respectively. The role of the inflorescence as a source and sink for carbon assimilation may lower the cost of reproduction and support frequent inflorescence production.