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Background: Many barrel or columnar cacti, including some in the Atacama Desert, produce their reproductive tissue at or near the terminal apices of solitary or minimally branched stems that lean toward the equator, reportedly to maximize exposure to photosynthetically active radiation (PAR). Those with lateral reproductive tissue, often produce the tissue on the equatorial side of the stems. An examination of the multi-stemmed, arbuscular cactus, Eulychnia breviflora, was made to determine if it follows the same general strategy.

Methods: Individuals of the species were evaluated along a 100 km transect in the Atacama Desert. The position of all floral buds and open flowers was documented relative to the center of the plants and relative to the center of the individual stems on which they were located.

Results: A highly significant majority of the reproductive tissue was located on the equatorial (north) side of the plant and on the equatorial (north) side of the stems on which it was found.

Conclusion: Our explanation of the phenomenon differs from other researchers. Inasmuch as reproductive tissue contains little or no chlorophyll, we suggest that the flowers emerge from areas of the stems that receive abundant PAR, not because the reproductive tissue itself requires exposure to PAR. Because the translocation of photosynthates in cacti is difficult and energetically expensive, positioning of reproductive tissue in zones of the stems with high photosynthetic capacity is more energetically efficient. In addition, the Atacama Desert is not particularly warm. Exposure of flowers to solar radiation may produce a thermal reward for pollinators, in addition to any nectar rewards received.