Growth–defence trade-off defines a diffusion limit. A commentary on ‘Boundary-line trade-off in bryophytes between UV photoprotection and photosynthetic capacity, but not desiccation tolerance’

Bryophytes ‘buy’ biochemical protection from UV radiation by investment in cell-wall compounds such as biflavonoids, and the ‘bill’ comes due in a reduced CO2 supply (Perera Castro et al., 2025; this issue). For CO2 to enter the chloroplast, it must first pass through the cell wall. It is known from work with tracheophytes that thicker or phenolic-rich walls lower the conductance to CO2. Bryophytes, being already constrained by relatively thick walls, appear to reach the same diffusional ceiling (Roig-Oliver et al., 2020; Flexas et al., 2021). This is because thicker walls increase diffusion path length with higher phenolic loading, lowering effective porosity; together these factors reduce internal CO2 conductance (Fig. 1A). The absence of leaves with stomata in bryophytes amplifies the role of the cell wall as the primary interface controlling gas exchange. However, the broad relationships between UV-absorbing compounds and photosynthesis need clarification. To complicate matters, photochemistry, measured by chlorophyll fluorescence, can be active while assimilation is limited by CO2 supply, making the detection of CO2 diffusion limitations difficult. Conceptual framework linking cell wall structure, CO2 supply and hydration cycles in bryophytes. (A) Effective internal CO2 conductance declines as diffusion path length increases and wall-bound phenolics reduce effective porosity. Thicker, phenolic-rich walls therefore restrict CO2 movement to the chloroplasts and limit photosynthetic capacity. (B) Idealized carbon balance of a poikilohydric plant during a single dry–wet–dry cycle. Vertical dotted lines and background colour intensity indicate rehydration and drying events. Net CO2 exchange shows three phases: an initial loss when respiration and repair exceed photosynthesis (a), a period of net carbon gain while tissues remain hydrated (b), and a final decline as photosynthesis ceases during drying (c). The total balance depends on the internal conductance set by wall properties. Curves represent low (lightest purple), typical (mid-purple) and high (darkest purple) conductance. To address this problem, Perera Castro and colleagues used a broad, multi-locality, cross-phylum dataset to show a boundary-line constraint: species that load their walls with UV-absorbing phenolics do not attain high photosynthetic capacity. Desiccation tolerance seems to sit largely outside that bargain, correlating instead with chloroplast-based regulatory photoprotective processes (in this case non-photochemical quenching, NPQ). Together, these results separate a possible ceiling on assimilation from a threshold for survival (Perera Castro et al., 2025). The authors sampled 47 bryophyte species (mosses, liverworts and hornworts) from Colombia, Spain, Australia and Antarctica. They quantified UV-absorbing compounds (UVACs) into two pools (cell-wall bound and soluble; likely vacuolar), measured apparent photosynthesis (PA; an estimate of gross photosynthesis) from gas exchange analyses, and assayed photochemistry by chlorophyll fluorescence and a desiccation tolerance index (DTI). The pattern between UVACs and PA formed an upper boundary rather than a linear trade-off: no species achieved both high wall UVACs and high PA, which is consistent with a diffusion ceiling, where wall-bound UVACs limit CO2 supply even when photochemistry remains fully competent. This diffusion ceiling has consequences beyond instantaneous photosynthesis as carbon balance after rewetting depends on both hydration duration and CO2 supply (Fig. 1B). Coe et al. (2012) showed that small rainfall events can yield negative net carbon balance in desert mosses because short wet periods and long preceding dry intervals combine high respiratory and repair costs with limited photosynthetic gain. Perera Castro et al. reveal a mechanistic basis for this limitation: internal CO2 diffusion through phenolic-rich walls constrains PA. Together, the two studies describe the same bottleneck, a structural limit on conductance within hydrated tissue paired with carbon costs of recovery after desiccation. Hence, plants with a large amount of cell-wall-bound UVACs are less likely to achieve a positive carbon balance during brief wet periods, even if they tolerate desiccation well (Proctor et al., 2007; Coe et al., 2012). Perera Castro et al. found that DTI aligns with chloroplast-located photoprotection rather than cell-wall UVACs. In this dataset, DTI increases steeply with NPQ beyond a threshold but shows no association with either wall-bound or soluble UVACs. This suggests a minimum NPQ capacity that must be in place for successful recovery after desiccation. Once that threshold is reached, further increases in NPQ coincide with higher DTI, consistent with the idea that strong energy dissipation or antioxidant buffering supports recovery during rehydration rather than a graded, linear response. A methodological nuance might create bias towards low DTI in some taxa. Because DTI normalizes at a point 24 h after dark rehydration to the pre-dry value, it likely underestimates recovery in species retaining slowly relaxing, zeaxanthin-linked quenching (Malnoë, 2018). A follow-up extending the recovery window beyond 48 h and tracking chlorophyll fluorescence, NPQ relaxation and xanthophyll-cycle pigments would help separate slow NPQ relaxation from injury and test whether the NPQ–DTI relationship strengthens once recovery is complete. Together, these findings describe two complementary functional modules. The first is physical and largely constitutive: a cell-wall-based UV screening system that attenuates incident UV radiation but imposes a diffusion ceiling on carbon gain. The second is dynamic and chloroplast-based: NPQ-mediated dissipation that defines a survival threshold after desiccation. They operate in different places (wall versus chloroplast), on different timescales, and together maintain energy balance under fluctuating light and hydration (Proctor et al., 2007; Coe et al., 2012). This modular framework helps explain why some bryophytes persist under extreme desiccation yet fix carbon and grow very slowly; survival and productivity often diverge under episodic water and high radiation (Furness and Grime, 1982). A similar trade-off recurs across land plants, due to shared biochemistry. The phenylpropanoid pathway, which leads to the creation of UVACs that accumulate in bryophyte cell walls, is also used in tracheophytes to produce the lignin that strengthens vasculature, and in tracheophytes UV-B exposure induces this pathway as part of a general photoprotective response (Jansen et al., 1998). Thicker or more lignified cell walls, in turn, are associated with lower internal CO2 conductance (Roig-Oliver et al., 2020; Flexas et al., 2021). In bryophytes, UVAC responses to UV-B are well documented but variable across taxa and extraction pools, which are strong in some species and absent in others (Martínez-Abaigar and Núñez-Olivera, 2022). Although the specific compounds differ, the outcome is similar across lineages as greater phenylpropanoid investment improves photoprotection while constraining CO2 diffusion. The fact that this boundary-line pattern emerges across the three bryophyte phyla suggests that it reflects something fundamental about the lineage as a whole. Unlike tracheophytes, bryophytes have leaves that contain fewer specialized cell types, so individual cells often serve multiple functional roles. In bryophytes, the same cells that provide structural integrity also perform photosynthesis, whereas tracheophytes divide these roles between the photosynthetic mesophyll and an epidermis that provides structural integrity and is the primary host to cell wall UVACs (Jansen et al., 1998). This multi-functionality in bryophytes creates an intrinsic conflict: what is good for structural integrity or physical protection, such as phenolic-rich walls that screen UV radiation, can limit maximum PA by restricting CO2 diffusion. The absence of this boundary within individual phyla suggests that each group balances structure and function differently. Variation within lineages is substantial, and while 47 species make a rigorous test in this study, they ultimately represent only a small fraction of the diversity within the 20 000+ bryophyte species. Species with greater cellular differentiation also exhibit stronger specialization along these axes. In xerophytic Syntrichia, leaves curl when drying and the costa or midrib, with its band of thick-walled, often pigmented stereid cells on the abaxial side, becomes outward-facing (Mishler, 2007). When hydrated, these cells may be structural tissue; when dry, they are exposed, likely contributing both to the hygroscopic curling mechanism and to protection from solar radiation. This dual role illustrates specialization, with non-photosynthetic structural cells providing both mechanical support and photoprotection in the same organ. In liverworts, ventral scales on the thallus underside offer a parallel case: they are frequently darkly pigmented (purple to black), contain UVACs, and can also shield the thallus from solar radiation while dry (Jain et al., 2024). Liverworts overall provide a particularly promising system for further testing, as the group spans simple thalloid, leafy and complex thalloid forms, the latter with pronounced tissue differentiation and internal air chambers that decouple structure from photosynthetic function. This kind of structural and functional differentiation would have been averaged over in the present study, since UVACs were measured from whole shoots rather than from individual tissues, potentially obscuring finer relationships between wall chemistry and photosynthetic capacity. Such anatomical diversity offers a natural experiment for testing how the separation of structure and function shapes the diffusion–screening trade-off identified by Perera Castro and colleagues. A few limitations should be kept in mind. The causal path from wall phenolics to a conductance ceiling is inferred rather than measured; internal conductance was not quantified, and the dataset spans sites, seasons and instruments. The desiccation assay used dark desiccation and rehydration, so any passive benefit of wall screens in the dry state would not raise the DTI under those conditions. In nature, excess light, including UV radiation, must also be managed during the desiccated phase, though thought to be often through physical reflectance or canopy structure (Proctor et al., 2007). Because the boundary is strongest across the full dataset and weaker within individual phyla, the results should not be over-generalized to all bryophyte groups. These two limits – diffusion and dissipation – define physiological boundaries of bryophyte performance, but they also reflect broader principles of plant function. Across land plants, carbon balance depends on how quickly tissues can resume assimilation after stress and how effectively they can prevent photodamage during that transition. In poikilohydric plants, the dry–wet–dry cycle makes this especially visible: when tissues rehydrate, respiration and repair draw from stored carbon while assimilation ramps up along a trajectory constrained first by internal CO2 conductance and then by the duration of hydration (Fig. 1B). Similar interactions between wall structure, CO2 supply and energy dissipation shape recovery and performance in many plant lineages. Phenolic-rich walls restrict CO2 diffusion and slow recovery, whereas NPQ capacity governs protection during rehydration – complementary strategies that mirror the broader growth–defence balancing acts seen throughout land plant evolution. By making these constraints visible, Perera Castro et al. provide a framework that can be extended to link structural investments with carbon gain, stress resilience and ecological distribution across diverse environments.