The presence of stomata on both leaf surfaces (amphistomy) increases photosynthesis by reducing the distance for CO 2 diffusion between stomata and chloroplasts. Paradoxically, most leaves are hypostomatous (stomata on lower surface), despite the photosynthetic advantage of amphistomy. Across 29 diverse populations of "wild tomatoes", leaves developed under high light intensity benefit more from amphistomy in terms of CO 2 assimilated for a given stomatal conductance than plants developed under low light. Furthermore, populations native to open habitats benefit more from amphistomy than those from more closed habitats. Thus, plasticity and adaptation together may explain why amphistomatous leaves are prevalent in sunny, open habitats, including many crops. Contrary to common assumptions, amphistomy can save water because hypostomatous leaves evaporate 10-65% more to achieve the same photosynthetic rate.
Muir et al. (Sat,) studied this question.