Floral orientation in Eulychnia acida , an arborescent cactus of the Atacama Desert, and implications for cacti globally

Abstract The hyperarid Atacama Desert of northern Chile may be the driest place on Earth. Plants surviving there have adapted a number of unique strategies to cope with the harsh conditions. Many cacti in arid areas tend to produce reproductive organs in positions that maximize incidence of solar radiation. We sought to determine whether Eulychnia acida , an endemic cactus with an arborescent growth form, follows the same pattern. We conducted a study in two populations of the cactus. One population occurs where average annual precipitation ( AAP ) is 113 mm; AAP at the other site is 50 mm. We randomly sampled 10 plants at each location, and 300 reproductive structures on those plants. On each plant, we recorded the position of all reproductive organs relative to the center of the plant, and relative to the center of the stems on which they were located. The results of the two populations did not differ significantly, so results of the two populations were combined. Both populations lie south of the Tropic of Capricorn, beyond the point where the sun reaches its southernmost zenith. Hence, the sun is located at to the north all year long. We discovered that the reproductive tissue emerges predominantly on stems found the equatorial (north) side of the plant and on the equatorial (north) side of those stems. As reproduction is energetically expensive, the generation of reproductive organs from vegetative tissue that receives high levels of direct solar radiation minimizes the cost of translocating photosynthates from photosynthetic tissue to non‐photosynthetic reproductive tissue, thus begetting energetic efficiency. The strategy appears to be common among many cacti and in some other plants of arid regions worldwide and is especially prevalent at extra‐tropical latitudes. In addition to nectar rewards, the strategy may also benefit pollinators by offering a thermal reward, especially on overcast or otherwise cool days, thus improving the odds of successful pollination.