Neotyphodium endophytes and cool-season grasses are mutualistic symbionts. The host grasses provide necessary photosynthates for the fungi. The endophytes often enhance the hosts′ growth and protect them from biotic and abiotic stresses, contributing to their widespread adaptability. Abiotic attributes affected by Neotyphodium endophytes include drought resistance, light, temperature and mineral stresses. Studies on endophyte-related responses of grasses to nutrient acquisition have focused upon the influence of soil nitrogen (N) availability, since this element is a constituent of alkaloids in infected plants. Similar to N nutrition, phosphorus (P) availability influences ergot alkaloid production in EI (endophyte-infected) grasses. But reports of endophyte-related responses of grasses to P nutrition are relatively limited. In this paper, Lolium perenne L. cv. SR4000 infected by Neotyphodium lolii (originally from Beijing Clover Seed Company, China) was employed to establish EF (endophyte-free) and EI populations. EI and EF ryegrasses were grown in the field and tested for their ecophysiological response to P deficiency.
The experiment was carried out at the campus experiment site of Nankai University, Tianjin. There were two separate 2-month periods of P stress treatment. The first began in mid July. The second began in late Sept. Two-factor randomized-block design was used. The first factor was P treatment and two levels of P treatments were imposed, i.e. P supply (P+) and P deficiency (P-); the other factor was endophyte status, i.e. EI and EF. Each treatment was replicated five times. P+ treatment was achieved by addition of P to the soil in the form of Hoagland nutrient solution. For P- treatment, 2000μM KCl was added instead of KH2PO4. Two liters nutrient solution per pot was added once a month, and four times in total.
The results showed that endophyte infection did not have significant effect on leaf growth but did improve root development of perennial ryegrass under P deficiency. In response to P deficiency, EI roots were longer and had higher mass than EF roots. The root: shoot ratio was also greater in EI individuals. The content of total phenolics and organic acids was greater in EI roots than in EF roots at low P supply. However, the concentration of both did not increase with endophyte infection. The results suggest that higher root dry weight contributes to higher content of total phenolics and organic acids for EI plants, and endophyte infection might have negligible effects on chemical modification of perennial ryegrass. Endophyte infection did not increase the rate of P uptake but did improve P use efficiency when P was limited. Higher P use efficiency of EI ryegrass might result from higher acid phosphatase activity (APA) for EI ryegrass. Higher APA may contribute to ryegrass ability to reuse a limited P source, which is further beneficial to the development of ryegrass roots.