Minor Physiological Response to Elevated CO(2) by the CAM Plant Agave vilmoriniana

The Agavoideae: an emergent model clade for CAM evolutionary biology

Crassulacean acid metabolism - or CAM photosynthesis - was described in the early to mid-20th century, and our understanding of this metabolic pathway was later expanded upon through detailed biochemical analyses of carbon balance. Soon after, scientists began to study the ecophysiological implications of CAM, and a large part of this early work was conducted in the genus Agave, in the subfamily Agavoideae of the family Asparagaceae. Today, the Agavoideae continues to be important for the study of CAM photosynthesis, from the ecophysiology of CAM species, to the evolution of the CAM phenotype and to the genomics underlying CAM traits. Here we review past and current work on CAM in the Agavoideae, in particular highlighting the work of Park Nobel in Agave, and focusing on the powerful comparative system the Agavoideae has become for studying the origins of CAM. We also highlight new genomics research and the potential for studying intraspecific variation within species of the Agavoideae, particularly species in the genus Yucca. The Agavoideae has served as an important model clade for CAM research for decades, and undoubtedly will continue to help push our understanding of CAM biology and evolution in the future.

Diel shifts in carboxylation pathway and metabolite dynamics in the CAM bromeliad Aechmea 'Maya' in response to elevated CO2

BACKGROUND AND AIMS

The deployment of temporally separated carboxylation pathways for net CO(2) uptake in CAM plants provides plasticity and thus uncertainty on how species with this photosynthetic pathway will respond to life in a higher-CO(2) world. The present study examined how long-term exposure to elevated CO(2) influences the relative contributions that C(3) and C(4) carboxylation make to net carbon gain and to establish how this impacts on the availability of carbohydrates for export and growth and on water use efficiency over the day/night cycle.

METHODS

Integrated measurements of leaf gas exchange and diel metabolite dynamics (e.g. malate, soluble sugars, starch) were made in leaves of the CAM bromeliad Aechmea 'Maya' after exposure to 700 micromol mol(-1) CO(2) for 5 months.

KEY RESULTS

There was a 60 % increase in 24-h carbon gain under elevated CO(2) due to a stimulation of daytime C(3) and C(4) carboxylation in phases II and IV where water use efficiency was comparable with that measured at night. The extra CO(2) taken up under elevated CO(2) was largely accumulated as hexose sugars during phase IV and net daytime export of carbohydrate was abolished. Under elevated CO(2) there was no stimulation of dark carboxylation and nocturnal export and respiration appeared to be the stronger sinks for carbohydrate.

CONCLUSIONS

Despite the increased size of the soluble sugar storage pool under elevated CO(2), there was no change in the net allocation of carbohydrates between provision of substrates for CAM and export/respiration in A. 'Maya'. The data imply the existence of discrete pools of carbohydrate that provide substrate for CAM or sugars for export/respiration. The 2-fold increase in water-use efficiency could be a major physiological advantage to growth under elevated CO(2) in this CAM bromeliad.

Ecophysiology of Crassulacean Acid Metabolism (CAM)

BACKGROUND AND SCOPE

Crassulacean Acid Metabolism (CAM) as an ecophysiological modification of photosynthetic carbon acquisition has been reviewed extensively before. Cell biology, enzymology and the flow of carbon along various pathways and through various cellular compartments have been well documented and discussed. The present attempt at reviewing CAM once again tries to use a different approach, considering a wide range of inputs, receivers and outputs.

INPUT

Input is given by a network of environmental parameters. Six major ones, CO(2), H(2)O, light, temperature, nutrients and salinity, are considered in detail, which allows discussion of the effects of these factors, and combinations thereof, at the individual plant level ('physiological aut-ecology').

RECEIVERS

Receivers of the environmental cues are the plant types genotypes and phenotypes, the latter including morphotypes and physiotypes. CAM genotypes largely remain 'black boxes', and research endeavours of genomics, producing mutants and following molecular phylogeny, are just beginning. There is no special development of CAM morphotypes except for a strong tendency for leaf or stem succulence with large cells with big vacuoles and often, but not always, special water storage tissues. Various CAM physiotypes with differing degrees of CAM expression are well characterized.

OUTPUT

Output is the shaping of habitats, ecosystems and communities by CAM. A number of systems are briefly surveyed, namely aquatic systems, deserts, salinas, savannas, restingas, various types of forests, inselbergs and paramós.

CONCLUSIONS

While quantitative census data for CAM diversity and biomass are largely missing, intuition suggests that the larger CAM domains are those systems which are governed by a network of interacting stress factors requiring versatile responses and not systems where a single stress factor strongly prevails. CAM is noted to be a strategy for variable, flexible and plastic niche occupation rather than lush productivity. 'Physiological syn-ecology' reveals that phenotypic plasticity constitutes the ecophysiological advantage of CAM.

Activities of carboxylating enzymes in the CAM species Opuntia ficus-indica grown under current and elevated CO2 concentrations

Responses of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPCase) to an elevated atmospheric CO2 concentration were determined along with net CO2 uptake rates for the Crassulacean acid metabolism species Opuntia ficus-indica growing in open-top chambers. During the spring 13 months after planting, total daily net CO2 uptake of basal and first-order daughter cladodes was 28% higher at 720 than at 360 μl CO2 l(-1). The enhancement, caused mainly by higher CO2 assimilation during the early part of the night, was also observed during late summer (5 months after planting) and the following winter. The activities of Rubisco and PEPCase measured in vitro were both lower at the elevated CO2 concentration, particularly under the more favorable growth conditions in the spring and late summer. Enzyme activity in second-order daughter cladodes increased with cladode age, becoming maximal at 6 to 10 days. The effect ofelevated CO2 on Rubisco and PEPCase activity declined with decreasing irradiance, especially for Rubisco. Throughout the 13-month observation period, O. ficus-indica thus showed increased CO2 uptake when the atmospheric CO2 concentration was doubled despite lower activities of both carboxylating enzymes.

CO2 Exchange and Growth of the Crassulacean Acid Metabolism Plant Opuntia ficus-indica under Elevated CO2 in Open-Top Chambers

CO2 uptake, water vapor conductance, and biomass production of Opuntia ficus-indica, a Crassulacean acid metabolism species, were studied at CO2 concentrations of 370, 520, and 720 [mu]L L-1 in open-top chambers during a 23-week period. Nine weeks after planting, daily net CO2 uptake for basal cladodes at 520 and 720 [mu]L L-1 of CO2 was 76 and 98% higher, respectively, than at 370 [mu]L L-1. Eight weeks after daughter cladodes emerged, their daily net CO2 uptake was 35 and 49% higher at 520 and 720 [mu]L L-1 of C02, respectively, than at 370 [mu]L L-1. Daily water-use efficiency was 88% higher under elevated CO2 for basal cladodes and 57% higher for daughter cladodes. The daily net CO2 uptake capacity for basal cladodes increased for 4 weeks after planting and then remained fairly constant, whereas for daughter cladodes, it increased with cladode age, became maximal at 8 to 14 weeks, and then declined. The percentage enhancement in daily net CO2 uptake caused by elevated CO2 was greatest initially for basal cladodes and at 8 to 14 weeks for daughter cladodes. The chlorophyll content per unit fresh weight of chlorenchyma for daughter cladodes at 8 weeks was 19 and 62% lower in 520 and 720 [mu]L L-1 of CO2, respectively, compared with 370 [mu]L L-1. Despite the reduced chlorophyll content, plant biomass production during 23 weeks in 520 and 720 [mu]L L-1 of CO2 was 21 and 55% higher, respectively, than at 370 [mu]L L-1. The root dry weight nearly tripled as the C02 concentration was doubled, causing the root/shoot ratio to increase with CO2 concentration. During the 23-week period, elevated CO2 significantly increased CO2 uptake and biomass production of O. ficus-indica.