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 greater than or equal to 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 mu mol CO2.m(-2).s(-1); inflorescence in bud, 3.7 mu mol CO2.m(-2).s(-1); inflorescence in flower, 2.5 mu mol CO2.m(-2).s(-1) and infructescence, 0.2 mu mol CO2.m(-2).s(-1). 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.