What does the RuBisCO activity tell us about a C3-CAM plant?

Plants that perform the Crassulacean acid metabolism (CAM), which obtain CO2 overnight and convert it mainly in malic acid, successfully grow in environments with water and nutrient shortages, that is partly associated with their higher water- and nitrogen-use efficiencies. Water and nutrient limitations can impair photosynthesis through the reduction of RuBisCO and increment of photorespiration, disturbing the plant carbon balance. In this context, we conducted a controlled experiment with the epiphytic C3-CAM bromeliad Guzmania monostachia to investigate how the combined water and nutritional deficits affect the activity of RuBisCO and its activation state (RAS), and to evaluate the efficiency of photosynthesis during the transition from C3 to CAM. Apart from an increase in CAM activity, bromeliads submitted to both water and nutritional deficits showed higher RAS values and unaltered RuBisCO activity compared to C3 bromeliads and, surprisingly, the maximum quantum efficiency of photosynthesis increased. Glucose, fructose and starch levels were maintained, while sucrose concentrations increased over time. These results, combined with the high RAS values, suggest an increased efficiency of RuBisCO functioning. Our results reinforce the ability of epiphytic bromeliads to deal with stressful habitats by a higher efficiency of RuBisCO during the transition to CAM, another feature that may allow their evolution in the epiphytic environment.

The contribution of weak CAM to the photosynthetic metabolic activities of a bromeliad species under water deficit.

The Crassulacean acid metabolism (CAM) can be a transitory strategy for saving water during unfavourable conditions, like a dry season. In some cases, CAM can also contribute to the maintenance of photosynthetic integrity, even if carbon gain and growth are impaired. CAM occurs in different intensities, being stronger or weaker depending on the degree of nocturnal malic acid accumulation. For example, Guzmania monostachia is an epiphytic tank bromeliad that shows an increase in its nocturnal organic acid accumulation and a variable CAM behaviour when exposed to water deficit. In this context, this study aimed at investigating whether the weak CAM displayed by this species may mitigate the harmful effects of water limitation on its photosynthetic activity. To this, bromeliads were submitted to well-watered and water deficit conditions. Guzmania monostachia plants under water deficiency conditions showed a reduction on atmospheric carbon assimilation without exhibiting changes in PSII integrity and carbohydrate production while showed an increase in nocturnal malic acid accumulation. Additionally, spots with high PSII efficiency in the leaf portion with a greater nocturnal malic acid accumulation were observed in plants exposed to water shortage conditions. These high-efficiency spots might be associated with a greater malate decarboxylation capacity. Also, the malic acid contributed to approximately 50% of the total carbon assimilated under water deficit. These results suggest that weak CAM may participate in photo-protection and it appears to meaningfully contribute to the overall carbon balance, being an important metabolic strategy to maintain plant fitness during water deficit periods.

Tropical forest light regimes in a human-modified landscape.

Light is the key energy input for all vegetated systems. Forest light regimes are complex, with the vertical pattern of light within canopies influenced by forest structure. Human disturbances in tropical forests impact forest structure and hence may influence the light environment and thus competitiveness of different trees. In this study, we measured vertical diffuse light profiles along a gradient of anthropogenic disturbance, sampling intact, logged, secondary, and fragmented sites in the biodiversity hot spot of the Atlantic forest, southeast Brazil, using photosynthetically active radiation sensors and a novel approach with estimations of vertical light profiles from hemispherical photographs. Our results show clear differences in vertical light profiles with disturbance: Fragmented forests are characterized by rapid light extinction within their low canopies, while the profiles in logged forests show high heterogeneity and high light in the mid-canopy despite decades of recovery. The secondary forest showed similar light profiles to intact forest, but with a lower canopy height. We also show that in some cases the upper canopy layer and heavy liana infestations can severely limit light penetration. Light extinction with height above the ground and depth below the canopy top was highest in fragmented forest and negatively correlated with canopy height. The novel, inexpensive, and rapid methods described here can be applied to other sites to quantify rarely measured vertical light profiles.