ABA treatment increases both the desiccation tolerance of photosynthesis, and nonphotochemical quenching in the moss Atrichum undulatum

Pulse amplitude modulation fluorescence was used to investigate whether abs cisic acid (ABA) pretreatment increases the desiccation tolerance of photos ynthesis in the moss Atrichum undulatum. In unstressed plants, ABA pretreat ment decreased the F-v/F-m ratio, largely as a result of an increase in F-o . This indicated a reduction in energy transfer between LHCII and PSII, pos sibly hardening the moss to subsequent stress by reducing the production of the reactive oxygen species near PSII. During desiccation, F-0, F-m, F-v/F -m, Phi PSII, and NPQ and Fo quenching declined in ABA-treated and nontreat ed mosses. However, during rehydration. F-0, F-m, F-v/F-m, and Phi PSII rec overed faster in ABA-treated plants, suggesting that ABA improved the toler ance of photosystem II to desiccation. NPQ increased upon rehydration in mo sses from both treatments, but much more rapidly in ABA-treated plants: dur ing the first hour of rehydration, NPQ was two-fold greater in plants treat ed with ABA. Fo quenching followed a similar pattern, indicating that ABA t reatment stimulated zeaxanthin-based quenching, The implications of these r esults For the mechanisms of ABA-induced desiccation tolerance in A. undula tum are discussed.