BACKGROUND AND AIMS The idea that genome (size) evolution in eukaryotes could be driven by environmental factors is still vigorously debated. In extant plants, genome size correlates positively with stomatal size, leading to the idea that conditions enabling the existence of large stomata in fossil plants also supported growth of their genome size. We test this inductive assumption in drought-adapted, prostrate-leaved Cape (South Africa) geophytes where, compared with their upright-leaved geophytic ancestors, stomata develop in a favourably humid microclimate formed underneath their leaves. METHODS Stomatal parameters (leaf cuticle imprints) and genome size (flow cytometry) were measured in 16 closely related geophytic species pairs from seven plant families. In each pair, representing a different genus, we contrasted a prostrate-leaved species with its upright-leaved phylogenetic relative, the latter whose stomata are exposed to the ambient arid climate. KEY RESULTS Except for one, all prostrate-leaves species had larger stomata, and in 13 of 16 pairs they also had larger genomes than their upright-leaved relatives. Stomatal density and theoretical maximum conductance were less in prostrate-leaved species with small guard cells (<1 pL) but showed no systematic difference in species pairs with larger guard cells (>1 pL). Giant stomata were observed in the prostrate-leaved Satyrium bicorne (89-137 µm long), despite its relatively small genome (2C = 9 Gbp). CONCLUSIONS Our results imply that climate, through selection on stomatal size, might be able to drive genome size evolution in plants. The data support the idea that plants from 'greenhouse' geological periods with large stomata might have generally had larger genome sizes when compared with extant plants, though this might not have been solely due to higher atmospheric CO2 in these periods but could also have been due to humid conditions prevailing at fossil deposit sites.

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气孔大小的环境压力可能会推动植物基因组大小的进化：来自开普地植物自然实验的证据

背景和目的 真核生物基因组（大小）进化可能受环境因素驱动的观点仍在激烈争论中。在现存植物中，基因组大小与气孔大小呈正相关，导致化石植物中能够存在大气孔的条件也支持其基因组大小的增长。我们在适应干旱的匍匐叶海角（南非）地球植物中测试了这一归纳假设，与它们的直立叶地球植物祖先相比，气孔在其叶子下方形成的有利潮湿的小气候中发育。方法在来自七个植物科的 16 个密切相关的地球植物物种对中测量气孔参数（叶角质层印记）和基因组大小（流式细胞术）。在每一对中，代表一个不同的属，我们将一个匍匐叶物种与其直立叶系统发育亲属进行了对比，后者的气孔暴露于周围的干旱气候中。主要结果 除了一个之外，所有平卧叶物种都具有更大的气孔，并且在 16 对中的 13 对中，它们的基因组也比直立叶近缘种更大。具有小保卫细胞 (<1 pL) 的匍匐叶物种的气孔密度和理论最大电导率较低，但在具有较大保卫细胞 (>1 pL) 的物种对中没有显示出系统差异。在匍匐叶 Satyrium bicorne（89-137 µm 长）中观察到巨大的气孔，尽管其基因组相对较小（2C = 9 Gbp）。结论 我们的结果表明，通过对气孔大小的选择，气候可能能够推动植物基因组大小的进化。数据支持植物来自'