Photoinhibitory responses of the epiphytic bromeliad Guzmania monostachia during the dry season in Trinidad maintain photochemical integrity under adverse conditions

The C₃–CAM epiphytic bromeliad Guzmania monostachia var. monostachia may be exposed to high incident photosynthetically active radiation (PAR) during the dry season in Trinidad, and resultant variations in photochemical efficiency have been investigated for ‘exposed’ (receiving ～100% incident PAR), ‘semi-exposed’ (～60% PAR) and shaded populations under natural conditions. The more succulent leaves of the plants growing fully exposed within the canopy had higher overall CAM activity (measured as ΔH⁺, the dawn-dusk titratable acidity), a smaller proportion of chlorenchyma and lower total chlorophyll content. There was a gradation of morphological and physiological characteristics between these and shaded leaves. Diurnal time-courses of photosynthetic light responses (as O₂ evolution) showed marked variations in apparent quantum yield (AQY) and light-saturated rates for both exposed and semi-exposed populations, dependent on incident PAR during the day. Similar measurements of photosystem II fluorescence characteristics showed that F_v/F_m declined from 0·70 to 0·42 at midday for exposed plants (on a day when total incident PAR was 44 mol photon m^?2), indicating non-photochemical quenching (qNP) of photosynthesis. However, in contrast to AQY determinations, F_v/F_m recovered during the afternoon. The decrease in F_v/F_m was reduced from 0·72 to 0·64 under 24 mol photon m^?2 d^?1. The long–term recovery of photo-synthetic efficiency was assessed for exposed plants placed under three shading regimes (60, 30 and 3% of incident PAR) over a 17-d period. During this time, total chlorophyll content increased from 228 to 515 and 585 μg g^?1 fresh weight (for 3 and 30%, respectively) and chlorophyll a:b declined. While AQY recovery was much longer under the lowest PAR (17d), under 30% PAR both AQY and F_v/F_m had recovered after 2d shading. The differences between timing of recovery for F_v/F_m during diurnal time courses and in the long term suggest that, while quenching associated with PSII recovers rapidly, enzyme activation and/or protein synthesis of other photosynthetic components may be limiting under low PAR. However, it is suggested that the occurrence of qNP on a daily basis may preclude long-term photoinhibitory damage under natural conditions during the dry season.