Investigating genotype specific response in photosynthetic behavior under drought stress and nitrogen limitation in Brassica rapa.

Abstract:

Challenges in terrestrial ecosystem modeling include characterizing the impact of stress on vegetation and the heterogeneous behavior of different species within the environment. In an effort to address these challenges the impacts of drought and nutrient limitation on the CO₂ assimilation of multiple genotypes of Brassica rapa was investigated using the Farquhar Model (FM) of photosynthesis following a Bayesian parameterization and updating scheme. Leaf gas exchange and chlorophyll fluorescence measurements from an unstressed group (well-watered/well-fertilized) and two stressed groups (drought/well-fertilized and well-watered/nutrient limited) were used to estimate FM model parameters. Unstressed individuals were used to initialize Bayesian parameter estimation. Posterior mean estimates yielded a close fit with data as observed assimilation (A_n) closely matched predicted (A_p) with mean standard error for all individuals ranging from 0.8 to 3.1 μmol CO₂ m^-2 s^-1. Posterior parameter distributions of the unstressed individuals were combined and fit to distributions to establish species level Bayesian priors of FM parameters for testing stress responses. Species level distributions of unstressed group identified mean maximum rates of carboxylation standardized to 25° (V_cmax25) as 101.8 μmol m^-2 s^-1 (± 29.0) and mean maximum rates of electron transport standardized to 25° (J_max25) as 319.7 μmol m^-2 s^-1 (± 64.4). These updated priors were used to test the response of drought and nutrient limitations on assimilation. In the well-watered/nutrient limited group a decrease of 28.0 μmol m^-2 s^-1 was observed in mean estimate of V_cmax25, a decrease of 27.9 μmol m^-2 s^-1 in Jmax25 and a decrease in quantum yield from 0.40 mol photon/mol e^- in unstressed individuals to 0.14 in the nutrient limited group. In the drought/well-fertilized group a decrease was also observed in V_cmax25 and J_max25. The genotype specific unstressed and stressed responses were then used to parameterize an ecosystem process model with application at the field scale to investigate mechanisms of stress response in B. rapa by testing a variety of functional forms to limit assimilation in hydraulic or nutrient limited conditions.