Characteristics of sun- and shade-adapted populations of an endangered plant Primulina tabacum Hance

Primulina tabacum Hance is an endangered perennial herb distributed in calcium-rich and nitrogen-limited soil of the karst limestone areas in southern China. The morphological, ultrastructural, and physiological traits were determined for P. tabacum populations growing in three different environment conditions: twilight zone of a cave (site TZ, extremely low light intensity), at a cave entrance (site EZ, low light intensity), and in an open area (site OA, high light intensity). At site OA, P. tabacum plants were exposed to high light (635 μmol m⁻² s⁻¹ of mean daily photosynthetically active radiation) with drought stress, and expressed traits to minimize light capture and water loss. Compared to plants at sites EZ and TZ, those at site OA had thicker leaves with higher densities of stomata and pubescence, higher palisade/spongy ratio, higher light-saturated rate of net photosynthetic rate (P _max), higher biomass, higher non-photochemical quenching coefficient (NPQ), and higher light saturation point (LSP) but fewer grana per chloroplast and less thylakoid stacking per granum. In contrast, P. tabacum growing at the cave vicinities: EZ (mean daily irradiance 59 μmol m⁻² s⁻¹) and TZ (mean daily irradiance 11 μmol m⁻² s⁻¹) showed typical shade-adapted characteristics for optimum light capture. The presence of sun- and shade-adapted characteristics indicates that P. tabacum has different strategies to cope with different environments but whether these strategies reflect genetic selection or phenological plasticity is yet to be determined. Such variability in physiological and morphological traits is important for the survival of P. tabacum in heterogeneous light conditions.