Elucidating the association of trichome and stomatal densities across species

<p>Previous studies suggested a trade-off between trichome density (<em>D</em>_t) and stomatal density (<em>D</em>_s) due to shared cell precursors. We clarified how, when, and why this developmental trade-off may be overcome across species. We derived equations to determine the developmental basis for <em>D</em>_t and <em>D</em>_s in trichome and stomatal indices (<em>i</em>_t and <em>i</em>_s), and the sizes of epidermal pavement cells (<em>e</em>), trichome bases (<em>t</em>) and stomata (<em>s</em>), and quantified the importance of these determinants of <em>D</em>_t and <em>D</em>_s for 78 California species. We compiled 17 previous studies of <em>D</em>_t-<em>D</em>_s relationships to determine the commonness of <em>D</em>_t-<em>D</em>_s associations. We modelled the consequences of different <em>D</em>_t-<em>D</em>_s associations for plant carbon balance. Our analyses showed that higher <em>D</em>_t was determined by higher <em>i</em>_t and lower <em>e</em>, and higher <em>D</em>_s by higher <em>i</em>_s and lower <em>e</em>. Across California species, positive <em>D</em>_t-<em>D</em>_s coordination arose due to <em>i</em>_t-<em>i</em>_s coordination and impacts of the variation in <em>e</em>. A <em>D</em>_t-<em>D</em>_s trade-off was found in only 30% of studies. Heuristic modeling showed that species sets would have the highest carbon balance with a positive or negative relationship or decoupling of <em>D</em>_t and <em>D</em>_s, depending on environmental conditions. Shared precursor cells of trichomes and stomata do not limit higher numbers of both cell types, or drive a general <em>D</em>_t-<em>D</em>_s trade-off across species. This developmental flexibility across diverse species enables different <em>D</em>_t-<em>D</em>_s associations according to environmental pressures.Developmental trait analysis can clarify how contrasting trait associations would arise within and across species.</p>