Seasonal changes in photosynthesis for the epiphytic bromeliad Tillandsia brachycaulos in a tropical dry deciduous forest

Abstract

Background: Sunlight stress and drought affect plants by inducing various biochemical and physiological responses, which reduce growth. Seasonal changes in light and water availability that occur in forest canopies, where epiphytes occur, are extreme.
 Questions: What are the seasonal changes in photosynthesis for an abundant epiphytic bromeliad in contrasting microenvironments? Is Crassulacean acid metabolism (CAM) an important feature of photoprotection for this epiphyte?
 Studied species: Tillandsia brachycaulos Schltdl. (Bromeliaceae)
 Study site and dates: Canopy of the tropical dry deciduous forest of Dzibilchaltún National Park, Yucatan, Mexico during the rainy season 2008 and dry season 2009.
 Methods: Diurnal measurements of photosystem II efficiency, titratable acidity, leaf water potential, and photosynthetic pigment concentration were measured during the dry and rainy seasons in adult plants of T. brachycaulos in shaded and exposed microenvironments. The prevailing environmental conditions (photon flux density, precipitation, air temperature and relative humidity) were also seasonally characterized.
 Results: The highest irradiance occurred during the dry season caused photo-inactivation, a decrease of the quantum efficiency of photosystem II and a reduction in CAM activity of about 40 % in leaves of exposed plants of T. brachycaulos. During the rainy season, the leaf water potential of exposed and shaded plants of T. brachycaulos was lower at midday than at predawn, indicating water loss during the day.
 Conclusions: Individuals of T. brachycaulos reduced CAM activity during the dry season; and, during the rainy season, increased carbon gain by stomata opening during phase II and IV of CAM.

Physiological ecology of Mexican CAM plants: history, progress, and opportunities

“…The accumulation of acids by CAM or solutes compatible with the cytoplasm to reduce the osmotic potential are mechanisms used by terrestrial desert species and could be advantageous for epiphytes with access to aerial soil. For instance, during the dry season, water potential values of T. brachycaulos were not significantly different between dawn and noon, but pre-dawn water potential values were lower than midday water potential ones during the rainy season in a tropical dry deciduous forest of Yucatán, Mexico (González-Salvatierra et al 2021). However, osmotic potential changes are subtle because the water obtained by epiphytes is water with high water potentials (Hernández-Robinson et al 2020).…”

“…The quantum yield (Φ PSII ) of plants of the epiphytic cactus R. baccifera from the cloud forest under irrigation and low light (20 % of sunlight, 1.81 mol m -2 d -1 PPF) was maximum (Φ PSII pre-dawn = 0.8, Φ PSII noon = 0.7) and decreased (Φ PSII pre-dawn = 0.7, Φ PSII noon = 0.3) at high light (65 % of sunlight, 9.8 mol m -2 d -1 PPF); the electron transport rate (ETR) increases linearly with the increase of light (maximum values of ETR at low light with irrigation = 160-180 μmol m -2 s -1 , ETR after one month without irrigation = 120-140 μmol m -2 s -1 ), and at high light it reaches satura-O n l i n e f i r s t tion (ETR with irrigation = 138.9 μmol m -2 s -1 , ETR after one month without irrigation = 126.6 μmol m -2 s -1 ) (de la Rosa Manzano 2008). In the bromeliad T. brachycaulos, Φ PSII values differ between shaded and exposed plants and between the dry and the rainy seasons; the lowest average of Φ PSII (0.57) is in the leaves of exposed T. brachycaulos plants during the dry season and the highest value of Φ PSII (0.80) is in leaves of shaded plants during the rainy season (González-Salvatierra et al 2021). Additionally, the maximum ETR of this epiphytic bromeliad is higher during the rainy season, with values of 23 and 32 μmol m -2 s -1 in exposed and shaded plants, respectively, and the light saturation point of 800 μmol m -2 s -1 in both light conditions; in contrast, during the dry season the maximum rate of electron transport is 12 and 18 μmol m -2 s -1 in exposed and shaded plants, respectively, and the light saturation point decreases to less than 465 μmol m -2 s -1 in both light conditions (González-Salvatierra et al 2021).…”

“…In the bromeliad T. brachycaulos, Φ PSII values differ between shaded and exposed plants and between the dry and the rainy seasons; the lowest average of Φ PSII (0.57) is in the leaves of exposed T. brachycaulos plants during the dry season and the highest value of Φ PSII (0.80) is in leaves of shaded plants during the rainy season (González-Salvatierra et al 2021). Additionally, the maximum ETR of this epiphytic bromeliad is higher during the rainy season, with values of 23 and 32 μmol m -2 s -1 in exposed and shaded plants, respectively, and the light saturation point of 800 μmol m -2 s -1 in both light conditions; in contrast, during the dry season the maximum rate of electron transport is 12 and 18 μmol m -2 s -1 in exposed and shaded plants, respectively, and the light saturation point decreases to less than 465 μmol m -2 s -1 in both light conditions (González-Salvatierra et al 2021). On the other hand, individuals of the epiphytic T. utriculata under a similar light environment (a coastal scrub) have a higher electron transport rate than those individuals of the same species from a population of a broadleaf forest transplanted to a coastal scrub site; after 20 d without irrigation, plants from the coastal scrub reduce ETR to 30 %, while those from the forest reduce ETR to 50 % (Rosado-Calderón et al 2018).…”

“…Individuals of the epiphyte T. utriculata from a coastal scrub population maintain relatively high NPQ values (around 1.5 on average) under experimental conditions of irrigation or drought and with high and low PPF, while individuals from a forest population had higher values (2.4 on average) with irrigation and high PPF and showed values between 1 and 1.5 with drought and high or low PPF conditions (Rosado-Calderón et al 2018). During the rainy and dry seasons, NPQ values in T. brachycaulos are consistently low (< 1) in a shaded environment; however, in an exposed environment during the rainy season, NPQ values increase close to 2 during the morning, decreasing to low levels during the day in a tropical dry deciduous forest (González-Salvatierra et al 2021). Plants of the epiphytic cactus R. baccifera that grow with constant irrigation or after one month without irrigation increase two-fold the NPQ (about 2.8) when exposed to 65 % compared to 20 % of total PPF (de la Rosa Manzano 2008).…”

“…The epiphytic CAM T. usneoides shows an optimum carbon dioxide assimilation between temperatures of 15 and 20 °C, but assimilation decreases at temperatures outside this range (Medina 1987). In response to high air temperature at night, which is above 23 °C in the dry and wet seasons in the tropical dry deciduous forest were T. brachycaulos inhabits (Hernández-Robinson et al 2020), individuals reduce CAM activity, as measured by change in tissue acidity, during the dry season, but increased it by opening stomata in the II and IV phases of CAM during the rainy season in Yucatán, Mexico (González-Salvatierra et al 2021).…”