BackgroundMany 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.MethodsIndividuals 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.ResultsA 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.ConclusionOur 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.

中文翻译：

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超干旱阿塔卡马沙漠中短花桉（仙人掌科）生殖组织的定向

背景许多桶状或柱状仙人掌，包括一些在阿塔卡马沙漠中的仙人掌，在向赤道倾斜的单生或最少分枝的茎的末端或附近产生它们的生殖组织，据报道，最大程度地暴露于光合有效辐射 (PAR)。那些具有侧向生殖组织的，通常在茎的赤道侧产生组织。对多茎丛枝仙人掌 Eulychnia breviflora 进行了检查，以确定它是否遵循相同的总体策略。方法沿阿塔卡马沙漠 100 公里的横断面对该物种的个体进行了评估。记录所有花蕾和开放花相对于植物中心和相对于它们所在的单个茎的中心的位置。结果绝大多数生殖组织位于植物的赤道（北）侧和发现它的茎的赤道（北）侧。结论我们对这一现象的解释与其他研究人员不同。由于生殖组织含有很少或不含叶绿素，我们建议花朵从接受大量 PAR 的茎区域出现，而不是因为生殖组织本身需要暴露于 PAR。由于仙人掌中光合产物的易位很困难且能量消耗很大，因此将生殖组织定位在具有高光合能力的茎区的能量效率更高。此外，阿塔卡马沙漠并不是特别温暖。将花朵暴露在太阳辐射下可能会对传粉者产生热奖励，