Plants with varying life histories may respond to seasonal stress with alternative physiological mechanisms. For example, at high irradiance, plants may face oxidative stress from absorbing light beyond what they can use for photosynthesis. Drought can exacerbate this condition by forcing stomatal closure, further limiting photosynthetic rates. Plants can protect themselves from oxidative damage through non-photochemical quenching (NPQ) facilitated by the xanthophyll cycle. Species with high water-use efficiency (WUE) may have lower need for photoprotection because they experience less drought-induced photosynthesis limitation. Alternatively, high WUE and photoprotective mechanisms may be correlated, as part of a suite of traits through which plants adapt to high light environments with seasonal drought stress.
We tested for associations between WUE and total xanthophyll concentration, and examined the implications of variation in xanthophyll pigment concentrations for light reflectance spectra and remote sensing. We repeatedly measured carotenoids, chlorophyll, and instantaneous intrinsic WUE in 15 phylogenetically diverse prairie grasses and forbs across summer 2014 in Cedar Creek Ecosystem Science Reserve, MN. We also took visible-range light reflectance spectra at the plot (9×9 m) and leaf scale, and used them to calculate the photochemical reflectance index (PRI), which is commonly used to measure light-use efficiency and xanthophyll cycle activity. We used Blomberg's K to test for phylogenetic conservatism in traits.
After phylogenetic correction, we found a negative correlation between water-use efficiency and the total pool size of xanthophyll cycle pigments at the species level. Leaf-scale PRI also correlated with the total pool size of xanthophyll cycle pigments, and the extent of seasonal change in PRI also increased with the extent of seasonal change in xanthophyll cycle pigments. Furthermore, the leaf-level concentration of all pigments, the total xanthophyll pool size, and xanthophyll de-epoxidation state, were strongly conserved across the phylogeny, although species mean PRI was not. At the plot scale, PRI showed strong changes throughout the season in response to soil moisture and precipitation.
The negative association between total xanthophyll pool size and water-use efficiency suggests that these traits define a spectrum of drought and light adaptation strategies among prairie plants: Species with low water-use efficiency have a greater need for photoprotective responses to high irradiance. The resulting variation in photoprotective pigment concentrations creates variation in the spectral characteristics of plant species. Furthermore, phylogenetic conservatism in plant photoprotective pigments hints at the possibility of using evolutionary history to aid in remote sensing of drought or light stress.