Effects of <i>Epichloë</i> <i>sinensis</i> Endophyte and Host Ecotype on Physiology of <i>Festuca sinensis</i> under Different Soil Moisture Conditions

This study explored the effects of the <i>Epichloë</i> <i>sinensis</i> endophyte on growth, photosynthesis, ionic content (K⁺ and Ca²⁺), phytohormones (abscisic acid—ABA, cytokinin—CTK, indolE−3-acetic acid—IAA, and gibberellin—GA), and elements—C, N, P (in the shoot and root) in two ecotypes of <i>Festuca sinensis</i> (ecotypes 111 and 141) under different soil water conditions (35% and 65% relative saturation moisture content (RSMC)). The results showed that 35% RSMC inhibited the plants’ growth, and compared with 65% RSMC, there was a significant (<i>p</i> < 0.05) decrease in the growth and photosynthesis indices, the contents of CTK and GA, Ca²⁺ concentration, and the contents of C, N, and P (in both the aboveground and underground parts) under 35% RSMC. <i>E.</i> <i>sinensis</i> had beneficial effects on host growth and stress tolerance. Under both 35% and 65% RSMC, the presence of <i>E.</i> <i>sinensis</i> significantly (<i>p</i> < 0.05) increased host plant height, tiller number, root length, root volume, shoot dry weight, chlorophyll content, and the rate of photosynthesis of both ecotypes. Furthermore, the shoot C, N, and P contents in plants infected with <i>E.</i> <i>sinensis</i> (E+) from the two ecotypes, under both conditions of RSMC, were significantly (<i>p</i> < 0.05) higher than those in corresponding plants that were not infected with <i>E.</i> <i>sinensis</i> (E−). Under 35% RSMC, the contents of ABA, K⁺, Ca²⁺, and root P contents in E+ plants were significantly (<i>p</i> < 0.05) higher than those in corresponding E− plants in both ecotypes. However, under 65% RSMC, root C, N, and P contents in E+ plants of ecotype 111 and 141 were significantly (<i>p</i> < 0.05) higher than those in corresponding E− plants. In addition, the host ecotype also had effects on host growth and stress tolerance; the growth and photosynthetic indices of ecotype 141 were significantly (<i>p</i> < 0.05) higher than those of ecotype 111 under 35% RSMC, which suggested that ecotype 141 is more competitive than ecotype 111 under water deficiency conditions. These findings suggest that the endophyte improved the host plant resistance to water deficiency by maintaining the growth of the plant, improving photosynthesis, accumulating K⁺ and Ca²⁺, promoting nutrient absorption, and adjusting the metabolism of plant hormones.