Growth and physiological plasticity among differentially adapted genotypes of a widespread C4 grass under altered precipitation

Background/Question/Methods

Variation in precipitation expected with climate change may impact plant fitness and alter ecosystem dynamics by modifying species phenology, productivity, and physiology. Species responses to varied precipitation will depend in part on plastic responses of genotypes adapted to local climate.  Here, we examined the effects of variable precipitation on genotype reproductive phenology, aboveground net primary productivity (ANPP), leaf area index (LAI), and leaf functional traits in Panicum virgatum L. (switchgrass), an ecologically dominant tallgrass prairie species.  We hypothesized that plastic responses of genotypes (genotype plasticity index) to varied precipitation would depend upon genotype climate of origin.  To test this hypothesis, we collected nine P. virgatum genotypes adapted to different climates and grew them under rainout shelters located at two sites in Central Texas, differing in soil depth (deep, shallow).  The genotypes received six experimental precipitation treatments, representing the driest to wettest years (based on mean annual precipitation) for each site, in a randomized complete block design.  Days to flowering (DF), LAI, and ANPP were measured in all treatments, and leaf water potentials (Ψ), net photosynthetic rates (A_CO2), leaf nitrogen (N), and leaf mass area (LMA) were measured in the low, mean, and high precipitation treatments during June and August.

Results/Conclusions

Decreased precipitation delayed DF up to 21 days (P<0.001), reduced LAI 9% - 37% (0.5<P<0.04), and reduced ANPP 8% - 144% (P<0.001).  Genotypes differed in DF by up to 134 days (P<0.0001), and showed substantial differences in LAI (0.7–5.2 m² m^-2; P<0.0001) and ANPP (39–2870 g m^-2; P<0.0001).  Predawn Ψ and A_CO2 increased with increasing precipitation, with higher values in June and at the deeper soil site.  Precipitation × genotype effects were significant for DF (P<0.01) and ANPP at both sites (P<0.001), and LAI at the deep soil site (P=0.04).  Genotypes showed substantial variation in leaf traits with few significant precipitation × genotype effects.  In terms of plasticity, genotypes from warmer climates showed lower ANPP and LAI plasticity at the deep soil site.  Genotypes from climates with warm dry summers also showed lower LMA plasticity at the deep soil site.  Our results indicate that adaptation to local climate may influence genotypic plasticity to variable precipitation.  Such plasticity may have important implications for species and ecosystem responses to climate change.