Abstract Of critical importance to plants is the ability to secure adequate photosynthetically active radiation (PAR) for functioning, growth and reproduction. The Opuntia genus represents plants that produce new cladodes (=pads, the primary photosynthetic surface) annually and whose orientation is a response to PAR. Because of the great variability in orientation, I assess pad orientation and tilt at highly local scales to quantify plant response to variations in PAR receipt, using three datasets collected over two field seasons. Several species and putative hybrids were sampled in the West Coyote Hills in Southern California. Data for orientation, shade direction, surface temperatures on differently oriented cladode faces, and cladode tilt were collected. Following normalization of variables, a variety of parametric and non-parametric tests show that the pattern of pad orientation is not unique to species, but is unique to the microsite, where even shade from another pad 5 cm away strongly affects PAR receipt and orientation response. Orientation is not constrained by position on plant. Pads tilt to increase S and E exposure (at the expense of N and W) in response to local shade. Pad face temperatures are higher on sun-facing sides, but the temperature differences are generally small in winter. I observe that pad orientation is best predicted by shade but at the scale of the areole, pad or plant. Results confirm that the high variability observed in orientation in many populations is due to the lack of consideration of shade at highly local scales (plant- or pad-scale), and that even in environments with seemingly ample PAR (sunny, open environments), plants benefit from orienting photosynthetic surfaces to maximize PAR receipt.

中文翻译：

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微环境的重要性：仙人掌植物的生长、形态和对阳光的反应

摘要 对于植物来说至关重要的是确保足够的光合有效辐射 (PAR) 用于功能、生长和繁殖的能力。仙人掌属代表每年产生新枝条（=垫，主要光合表面）的植物，其方向是对 PAR 的响应。由于方向的巨大可变性，我使用在两个田间季节收集的三个数据集，在高度局部尺度上评估垫的方向和倾斜，以量化植物对 PAR 接收变化的响应。在南加州的 West Coyote Hills 采样了几个物种和假定的杂交种。收集了方向、阴影方向、不同方向的枝条表面的表面温度和枝条倾斜的数据。在对变量进行归一化之后，各种参数和非参数测试表明，垫方向的模式不是物种所独有的，而是微型地点所独有的，即使是 5 厘米外另一个垫的阴影也会强烈影响 PAR 接收和方向响应。方向不受植物位置的限制。垫倾斜以增加 S 和 E 曝光（以 N 和 W 为代价）以响应局部阴影。朝阳一侧的垫面温度较高，但冬季温差通常较小。我观察到垫的方向最好通过阴影预测，但在乳晕、垫或植物的范围内。结果证实，在许多种群中观察到的高度可变性是由于在高度局部尺度（植物或垫尺度）上缺乏考虑阴影，即使在看似充足的 PAR（晴天，