Effect of Neotyphodium endophyte infection on growth and leaf gas exchange of Arizona fescue under contrasting water availability regimes

We assessed how Neotyphodium infection influenced the biomass production and growth of Arizona fescue (Festuca arizonica Vasey), a dominant understory grass in Ponderosa pine (Pinus ponderosa) forests of the southwest USA, by growing potted infected (E+) and uninfected (E-) plants under a high and a low water availability regime for 87 days. We measured growth analysis parameters, leaf net photosynthesis (P_n), chlorophyll fluorescence parameters, conductance to water vapor (g_l) and water potential (Ψ), to provide explanations for differences in biomass production under these treatments. Under high water availability, E- plants produced more biomass and had greater relative growth rates (RGR; rate of biomass gain per biomass); higher RGR of E- plants was correlated with higher P_n as well as production of less dense, presumably thinner leaves, which provided more leaf area per leaf biomass, and greater LAR (leaf area ratio; leaf area per total plant biomass). Under low water availability, E+ plants produced more aboveground biomass and had greater RGR; higher RGR of E+ plants was correlated with higher net assimilation rates, as well as production of less dense leaves and greater LAR. Infected plants tended to have lower midday P_n and g_l in both water availability regimes. Lower P_n in E+ plants appeared primarily due to stomatal, rather than biochemical, limitations to photosynthesis. When a more severe water stress was imposed in the low water availability treatment over the last 61 days of the experiment, E+ plants tended to have higher midday P_n and g_l. Infected plants also tended to have less negative leaf ψ regardless of water availability regime. Lower g_l and transpirational losses of E+ plants probably conserved soil moisture, such that when a more severe water stress was subsequently imposed, higher soil moisture availability allowed E+ plants to maintain higher P_n and g_l. Neotyphodium infection appears beneficial to Arizona fescue performance under low water availability and detrimental under ample water availability.