How and why do species break a developmental trade‐off? Elucidating the association of trichomes and stomata across species

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