Early detection of Kentucky bluegrass and perennial ryegrass responses to drought stress by measuring chlorophyll fluorescence parameters

Kentucky bluegrass (Poa pratensis L.; drought resistant) and perennial ryegrass (Lolium perenne L.; drought sensitive) are economically important grass species contrasting in drought stress resistance. This study determined the optimal chlorophyll fluorescence parameters to indicate drought incidence and whether the parameters differentiate intra- and interspecies variations in drought stress. For each species, nine cultivars were exposed to well-watered or drought (water withheld) conditions in growth chambers containing high-throughput photosynthetic imagers to track real-time responses of maximum quantum efficiency of photosystem II (Fv/Fm), quantum yield of photosystem II (phi II), non-photochemical quenching (NPQ), energy-dependent quenching (qE), and photoinhibition-associated quenching (qI). Soil moisture content and relative water content of leaf tissues were evaluated. Due to drought, Fv/Fm and phi II decreased for cultivars of both species but earlier for perennial ryegrass compared to Kentucky bluegrass. The NPQ, qI, and qE values exhibited more dynamic and earlier changes due to drought compared to Fv/Fm and phi II and allowed for early, mid, and late drought responses to be illustrated. Drought induced an increase in NPQ, qI, and qE values, which corresponded to activation of photoprotection mechanisms, and was exhibited earlier for perennial ryegrass compared to Kentucky bluegrass. As the drought treatment progressed, a decrease or stabilization of low values of NPQ, qI, and qE was observed, and the lowest values were associated with the most drought-sensitive cultivars. These results indicate important stress tolerance protection mechanisms for grass species and will broadly impact basic and applied grass research as a nondestructive phenotyping tool. Kentucky bluegrass and perennial ryegrass contrast in drought resistance.Chlorophyll fluorescence parameters during drought responses reveal novel insight into resistance mechanisms.Plant non-photochemical protection strategies were activated by drought stress in both grass species.Measurements of non-photochemical quenching parameters indicated drought responses earlier than quantum efficiency.Non-photochemical parameters are good, early indicators of drought stress incidence in grass species.