CARPELS AS LEAVES - MEETING THE CARBON COST OF REPRODUCTION IN AN ALPINE BUTTERCUP

We investigated the role of photosynthesis by reproductive organs in m eeting the carbon costs of sexual reproduction in the snow-buttercup, Ranunculus adoneus. The exposed green carpels of snow-buttercup flower s have 1-2 stomata each. Net carbon assimilation rates of flowers are negative during bud expansion, but rise to zero at maturity, and becom e positive during early fruit growth. Experimental removal of separate whorls of flower parts demonstrated that the showy, nectary-housing p etals account for most of the respiration cost of flower presentation. Conversely, photosynthesis by female organs contributes to a flower's carbon balance. Dipteran pollinators of R. adoneus are most active in sunny mid-morning to mid-afternoon intervals. At this time of day, ra tes of carpel photosynthesis (A(max) meet respiratory costs of pollina tor attraction in fully expanded flowers. Achenes remain photosyntheti cally active until dispersal, and positive net carbon assimilation rat es characterize infructescences throughout fruit maturation. Photosynt hetic rates of achenes are positively correlated with infructescence g rowth rates. We tested the causal basis of this relationship by experi mentally shading developing infructescences. Mature achenes from shade d infructescences were 16-18% smaller than those from unshaded control s. Leaf photosynthetic rates did not differ between plants bearing sha ded and unshaded seed heads. Since female reproductive organs are only 8% more costly,in terms of caloric investment than male ones and cont ribute to their own carbon balance, it is plausible that the energy co st of male function equals or exceeds that of female function in this hermaphroditic species.