Kate Maxwell

Hydrophobic trichome layers and epicuticular wax powders in Bromeliaceae

The distinctive foliar trichome of Bromeliaceae has promoted the evolution of an epiphytic habit in certain taxa by allowing the shoot to assume a significant role in the uptake of water and mineral nutrients. Despite the profound ecophysiological and taxonomic importance of this epidermal structure, the functions of nonabsorbent trichomes in remaining Bromeliaceae are not fully understood. The hypothesis that light reflection from these trichome layers provides photoprotection was not supported by spectroradiometry and fluorimetry in the present study; the mean reflectance of visible light from trichome layers did not exceed 6.4% on the adaxial surfaces of species representing a range of ecophysiological types nor was significant photoprotection provided by their presence. Several reports suggesting water repellency in some terrestrial Bromeliaceae were investigated. Scanning electron microscopy (SEM) and a new technique-fluorographic dimensional imaging (FDI)-were used to assess the interaction between aqueous droplets and the leaf surfaces of 86 species from 25 genera. In the majority of cases a dense layer of overlapping, stellate or peltate trichomes held water off the leaf epidermis proper. In the case of hydrophobic tank-forming tillandsioideae, a powdery epicuticular wax layer provided water repellency. The irregular architecture of these indumenta resulted in relatively little contact with water droplets. Most mesic terrestrial Pitcairnioideae examined either possessed glabrous leaf blades or hydrophobic layers of confluent trichomes on the abaxial surface. Thus, the present study indicates that an important ancestral function of the foliar trichome in Bromeliaceae was water repellency. The ecophysiological consequences of hydrophobia are discussed.

Regulation of Rubisco activity in crassulacean acid metabolism plants: better late than never

This paper originates from a presentation at the IIIrd International Congress on Crassulacean Acid Metabolism, Cape Tribulation, Queensland, Australia, August 2001. The diurnal regulation of Rubisco was compared for a range of crassulacean acid metabolism (CAM) species in the context of high carboxylation and electron transport capacities, which may be an order of magnitude greater than rates of net CO2 uptake. Early in the light period, Rubisco activity and electron transport were limited when phosphoenolpyruvate carboxylase (PEPC) may have been operating, and maximal extractable activities and activation state for Rubisco were achieved at the end of Phase III, prior to the direct atmospheric uptake of CO2 during Phase IV. The delayed activation was associated with levels of Rubisco activase protein, which reached a maximum at midday, and may account for this pattern of Rubisco activation. This regulation may be modified by environmental conditions - processes that tend to restrict PEPC activity, such as drought stress or incubation of leaves overnight in an oxygen-free atmosphere, release Rubisco from inhibition early in the light period. The quantum yield of light use also tracks Rubisco carboxylation, being particularly low at dawn when PEPC is active. The plasticity in expression of the CAM cycle is therefore matched by the regulation of key carboxylases, with extractable Rubisco activity maximal when drawdown of atmospheric CO2 to cells in succulent CAM tissues is most likely to limit photon utilization shortly after midday, during Phase IV.

Resistance is useful: diurnal patterns of photosynthesis in C3 and crassulacean acid metabolism epiphytic bromeliads

This paper originates from a presentation at the IIIrd International Congress on Crassulacean Acid Metabolism, Cape Tribulation, Queensland, Australia, August 2001 Diurnal patterns of photosynthesis in response to environmental variables were investigated in an obligate C3 and a facultative C3-crassulacean acid metabolism (CAM) bromeliad species. A midday depression of photosynthesis occurred in both C3 groups, mediated as a decrease in stomatal conductance in response to increased vapour pressure difference. The response was associated with a reduction in Rubisco activation state during the period of maximum photon flux density. In contrast, the switch to CAM resulted in a strong shift in the pattern of Rubisco carbamylation, with full enzyme activation delayed until the midday period. For the first time it is demonstrated that the pattern of Rubisco activation differs between C3 and CAM plants of the same species under identical conditions. Despite large differences in Rubisco content between C3 and CAM plants, neither the amount of Rubisco or enzyme activity is thought to be limiting for photosynthesis, and it is suggested that Rubisco may function as a nitrogen store. Extreme CO2 diffusion limitation resulted in low rates of atmospheric CO2 assimilation that were associated with high rates of photosynthetic electron transport, and it is likely that photorespiration constitutes a significant electron sink over the entire diurnal course. Leaf morphological and physiological adaptations to drought stress are necessary for the epiphytic lifestyle but limit CO2 assimilation and confound the likelihood of high productivity.

Physiological responses of the CAM epiphyteTillandsia usneoides L. (Bromeliaceae) to variations in light and water supply

In an effort to understand the mechanisms that sustain rootless atmospheric plants, the modulation of Crassulacean acid metabolism (CAM) in response to variations in irradiance and water supply was investigated in the epiphyte Tillandsia usneoides. Plants were acclimated to three light regimes, i.e. high, intermediate and low, with integrated photon flux densities (PFD) of 14.40, 8.64 and 4.32 mol m-2 d-1 equivalent to an instantaneous PFD of 200, 100, and 50 mumol m-2 s-1, respectively. Daily watering was then withdrawn from half of the plants at each PFD for 7 d prior to sampling. In response to the three PFD treatments, chlorophyll content increased in plants acclimated to lower irradiances. Light response curves using non-invasive measurements of chlorophyll fluorescence demonstrated that photosystem II efficiency (phi PSII) was maintained in high PFD acclimated plants, as they exhibited a larger capacity for non-photochemical dissipation (NPQ) of excess light energy than low PFD acclimated plants. Net CO2 uptake increased in response to higher PFD, reflecting enhanced carboxylation capacity in terms of phosphoenolpyruvate carboxylase (PEPc) and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activities. After water was withdrawn, nocturnal net CO2 uptake and accumulated levels of acidity declined in all PFD treatments, concomitant with increased respiratory recycling of malate. Examining the strategies employed by epiphytes such as T. usneodies to tolerate extreme light and water regimes has demonstrated the importance of physiological mechanisms that allow flexible carboxylation capacity and continued carbon cycling to maintain photosynthetic integrity.

Ecophysiology of Plants with Crassulacean Acid Metabolism

The taxonomic and ecological diversity of CAM plants is testament to a suite of morphological and metabolic attributes. These have evolved under contrasting selective pressures in up to 7% of all plant species many times over the past 10–100 Ma. The water and carbon conserving features of CAM impose: i) morphological constraints in terms of the diffusive limitations of succulence in cells or organs and ii) metabolic constraints in terms of maintaining two temporally separated carboxylation systems (usually within the same cell) and a reciprocating pool of carbohydrates which are unavailable for growth. Despite these limitations, the expression of CAM is characterized by a highly plastic response to environmental perturbations which not only permits survival under variable adverse conditions but can also result in high annual productivities of some species. This chapter will consider the biochemical components and physiological consequences of this photosynthetic plasticity. The implications for plant carbon balance, photosynthetic integrity and water use efficiency will be considered in both constitutive and facultative CAM species in response to daily, seasonal and possible future changes in environmental conditions. By integrating field and laboratory-based approaches to the characterization of CAM expression, we seek to demonstrate that studies on the ecophysiology of this photosynthetic pathway can provide a key to understanding the components of metabolic plasticity in plants.

Turning the land green: Inferring photosynthetic physiology and diffusive limitations in early bryophytes

Publisher Summary The tremendous interest in the form and function of the earliest land plants mirrors the enormous effects that such plants had on the early climate, increasing the drawdown of CO2 directly through photosynthesis and indirectly via weathering. Ultimately, without a better understanding of the genes and regulatory processes leading to the expression (or suppression) of the pyrenoid or multiplastidic cells, one cannot make any detailed inferences on the selective processes that shape the earliest terrestrial bryophytes. A definitive phylogenetic tree would help clarify the paraphyletic development of the bryophytes in the context of the protracheophytes and help resolve how the dramatic change in reproductive life cycle was accomplished during the progression toward vascular plants. However, it would not necessarily resolve the occurrence of the pyrenoid in hornworts because in the genus Megaceros, there is a gradual loss of the pyrenoid associated with the development of the multiplastidic condition, which seemingly represents the derived condition. The increasing ventilation of thalli—in a progression seen in extant liverworts as well as in the fossil record through increasing stomatal densities—would undoubtedly increase internal conductance to CO2 but at the cost of higher water loss. Gas exchange characteristics of extant bryophytes provide some insight into the likely benefits of a CO2 concentrating mechanism (CCM) for hornworts as compared to the non-ventilated Pellia. As long as oxygenase products could be detoxified via photorespiration, the higher light intensities available to drive electron transport (and not the CCM) could then have led to additional carbon reserves for creating structural material and conducting tissues for competing in the developing land–plant canopy.