Contrasting photosynthetic capacity and carbon allocation under increasing irradiance in two Paris polyphylla...

Shade-tolerant herbs occurring across contrasting radiation environments may differ not only in photosynthetic capacity but also in how assimilated carbon is stored and used. We compared two varieties of Paris polyphylla associated with lower- and higher-elevation habitats: P. polyphylla var. chinensis (PPC) and P. polyphylla var. yunnanensis (PPY), respectively. Plants were grown for 40 d under light intensity of 50, 200, and 400 μmol m^-² s^-¹, and we measured leaf structure, gas exchange, chlorophyll fluorescence, carboxylation traits, non-structural carbohydrates, biomass allocation, and Paris saponin VII (PS VII). At 400 μmol m^-² s^-¹, PPY had higher net photosynthesis and light-saturated photosynthetic capacity than PPC. Rubisco and Rubisco activase activities, maximum rate of carboxylation and maximum rate of electron transport increased across the irradiance gradient in PPY, even though its stomatal and mesophyll conductance remained lower than in PPC. PPY also maintained a higher relative electron transport rate and a higher photorespiration rate, with little change in non-photochemical quenching (NPQ) or dark respiration. Increased irradiance shifted PPY biomass allocation belowground and increased its rhizome/leaf soluble sugar ratio. Foliar PS VII concentration and expression of PpUGT80A2 and PpSmt2-1 also increased more strongly in PPY. In PPC, light-saturated photosynthetic capacity peaked at 200 μmol m^-² s^-¹, whereas NPQ and dark respiration were higher and the leaf soluble sugar/starch ratio was lower at 400 μmol m^-² s^-¹. These contrasts indicate that the response of PPY to increased growth irradiance was associated with greater biochemical capacity for CO₂ assimilation, greater belowground allocation, and a larger foliar saponin pool. Whether saponin synthesis directly alleviates sink limitation or contributes to photoprotection requires direct measurements of carbon flux and oxidative damage.