In small seeds, light often promotes germination and longer-term exposure to darkness reduces light sensitivity. In cacti inhabiting harsh environments, a rapid response to light exposure is potentially advantageous for seedling establishment. We exposed dark-imbibed seeds of the cactus Cereus repandus to doses of red (RED) light and far-red (FR) light. The seeds exhibited positive photoblastism to RED light. Although the initial levels of germination varied between seed lots, the sensitivity to increasing the RED dose did not. As little as 5 min per day for 4 d was sufficient to saturate the light requirement for germination. The effects of RED light were reversed by FR exposure as long as the interval between RED and FR did not extend to 2 d, by which time the seeds had ‘committed’ to germinate. Dark incubation for 1–2 weeks prior to RED exposure reduced light sensitivity in two seed lots, such that RED only promoted around 20% germination. Phytochrome is assumed to mediate the reversibility of the RED:FR response. High sensitivity to light spectral quality suggests that seeds of C. repandus are able to germinate quickly in high-quality microsites, but seed burial or shading may commit the seeds to form a soil seed bank. The light characteristics of the germination trait in this species are typical of many small seeded species of the drylands.

在小种子中，光通常会促进发芽，而长期暴露在黑暗中会降低光敏感性。在生活在恶劣环境中的仙人掌，对光照的快速反应可能有利于幼苗的建立。我们暴露了仙人掌Cereus repandus的黑暗吸收种子红光 (RED) 光和远红光 (FR) 光的剂量。种子对红光表现出阳性成光细胞。尽管发芽的初始水平因种子批次而异，但对增加 RED 剂量的敏感性没有变化。每天只需 5 分钟，持续 4 天就足以满足发芽所需的光照。只要 RED 和 FR 之间的间隔没有延长到 2 天（此时种子已经“承诺”发芽），FR 曝光就会逆转 RED 光的影响。在暴露于 RED 之前 1-2 周的黑暗孵化降低了两个种子批次的光敏感性，因此 RED 仅促进了大约 20% 的发芽。假定光敏色素介导 RED:FR 响应的可逆性。对光谱质量的高度敏感性表明C. repandus的种子能够在高质量的微型站点中快速发芽，但种子掩埋或遮荫可能会使种子形成土壤种子库。该物种发芽性状的光照特征是旱地许多小种子物种的典型特征。