Males differ more between developmental stages than females, and plasticity to light is stage-dependent in the tropical plant Marchantia inflexa.

Plants have evolved strategies to maintain photosynthesis and mitigate tissue-damaging high light. In some dioecious seed plants, these strategies are sexually dimorphic and are linked to spatial segregation of the sexes (SSS) along light gradients. In vascular tissue-free plants (bryophytes) with separate sexes, SSS is common, but how light gradients, sexual dimorphisms, and SSS correlate is not well understood. To test if sexual dimorphisms in vegetative or sexual stages lead to light-associated SSS in bryophytes, we used Marchantia inflexa whose males occupy a wider range of light conditions, including higher light conditions, than females. We also tested if changes in development differed between sexes. We grew 25 males and 25 females in a glasshouse with clones in low and high light and assessed pigment and biomass allocation traits in vegetative and sexual thalli (analogous to leaves), representing non-sexual and sexually reproductive stages. We expected males to exhibit traits consistent with high light acclimation more than females and greater sex differences in sexual thalli due to specialization. Further, we reasoned that males would change more between stages than females. For sexual thalli, males had higher carotenoid/chlorophyll ratios (consistent with expectation), while females had higher chlorophyll a/b ratios and dry matter content (opposite from expectations). Vegetative thalli were not sexually dimorphic but were more plastic to light than sexual thalli. Overall, the stages differed more for males than females, but without regard for light. However, female stages differed more for dry matter content. Males generally need greater change in pigmentation and biomass allocation than females between stages, and we posit links for individual traits to sex function. Specialization in sexual thalli constrains their plasticity to light compared to vegetative thalli. Yet, neither sexual dimorphism in sexual thalli nor greater change between stages for males than females clearly leads to light-associated SSS.