Solar furnaces or swamp coolers:: costs and benefits of water use by solar-tracking flowers of the alpine snow buttercup, Ranunculus adoneus

Solar tracking or heliotropism simultaneously raises organ temperature and light interception. For leaves and flowers carbon gain is maximized at the expense of water loss. In this study I explore how costs and benefits associated with water use by solar-tracking flowers of the alpine snow buttercup, Ranunculus adoneus change with ambient temperature. First, I test whether heliotropism increases the water cost of reproduction in the snow buttercup under extant alpine conditions. I then explore whether water use for evaporative cooling in solar-tracking flowers reduces the risk of over-heating as temperatures increase. Solar tracking, by elevating floral temperature and irradiance causes a 29% increase in water uptake by flowers. Gas exchange measurements suggest that the extra water taken up by solar-tracking flowers is released through transpiration. Transpirational cooling in turn allows solar-tracking flowers to gain advantages of enhanced light interception and warmth while reducing the risk of over-heating. Transpiration reduces excess temperature in solar-tracking flowers, but at a water cost. Results show that even in cool alpine habitats, flower heliotropism has water costs to balance its reproductive advantages. Plants with solar-tracking flowers may tolerate hotter conditions if soil moisture is plentiful, but not under drought.