Ian E. Woodrow

A Model Predicting Stomatal Conductance and its Contribution to the Control of Photosynthesis under Different Environmental Conditions

In the past, stomatal responses have generally been considered in relation to single environmental variables in part because the interactions between factors have appeared difficult to quantify in a simple way. A linear correlation between stomatal conductance (g) and CO2 assimilation rate (A) has been reported when photon fluence was varied and when the photosynthetic capacity of leaves was altered by growth conditions, provided CO2, air humidity and leaf temperature were constant (1). Temperature and humidity are, however, not consistent in nature. Lack of a concise description of stomatal responses to combinations of environmental factors has limited attempts to integrate these responses into quantitative models of leaf energy balance, photosynthesis, and transpiration. Moreover, this lack has hindered progress toward understanding the stomatal mechanism. We have taken a multi-variant approach to the study of stomatal conductance and we show that under many conditions the responses of stornata can be described by a set of linear relationships. This model can be linked to models of leaf carbon metabolism and the environment to predict fluxes of CO2, H2O and energy. In this paper, we show how the model of conductance can be linked to a description of CO2 assimilation as a function of intercellular CO2 (whether empirical or the output of a model) to predict the distribution of flux control between the stornata and leaf “biochemistry” under conditions in a gas-exchange cuvette.

The effects of elevated CO2 atmospheres on the nutritional quality of Eucalyptus foliage and its interaction with soil nutrient and light availability

Abstract Seedlings of Eucalyptus tereticornis (Smith) were grown under two levels of availability each of CO2 (352 and 793 µmol mol−1), soil nutrients (1/24 and 1/4 Hoagland’s solution) and light (full and 30% sunlight). Low soil nutrient availability or high light increased the C:N ratio of leaves, leading to lower leaf nitrogen concentrations, higher leaf specific weights and higher levels of both total phenolics and condensed tannins. These results were consistent with other studies of the effect of environmental resource availability on foliage composition. Similar results were observed when the C:N ratio of leaves was increased under elevated CO2. The changes in leaf chemistry induced by the treatments affected the performance of 4th-instar larvae of Chrysophtharta flaveola (Chapuis) fed on the leaves. Increased C:N ratios of leaves reduced digestive efficiencies and pupal body sizes and increased mortality. Below a threshold nitrogen concentration of approximately 1% dry mass, severe reductions in the performance of larvae were recorded. Such changes may have significant consequences for herbivores of Eucalyptus, particularly in view of projected increases in atmospheric CO2.

Mini-Review: Constraints on Effectiveness of Cyanogenic Glycosides in Herbivore Defense

Cyanogenesis is the process by which hydrogen cyanide is released from endogenous cyanide containing compounds. Many cyanogenic plants release HCN in sufficient quantities to be toxic and, as a result, tend to be avoided by herbivores. However, there are many exceptions with some herbivores either immune to the cyanogenic status of the plant, or in some cases attracted to cyanogenic plants. This has led to a certain degree of scepticism regarding the role of cyanogenic glycosides as defense compounds. In this review, we examine evidence showing that differences in the effectiveness of cyanogenic glycosides in deterring herbivory can usually be reconciled when the morphology, physiology, and behavior of the animals, together with the concentration of cyanogenic glycosides in the host plant, are taken into account. Cyanogenic glycosides are not effective against all herbivores, and not all cyanogenic plants release enough cyanide to be considered toxic. Nevertheless, they do form part of the broad spectrum of toxic and distasteful compounds that herbivores must accommodate if they are to feed on cyanogenic plants.

Plant chemical defense: at what cost?

Plants are sessile organisms and dependent on deployment of secondary metabolites for their response to biotic and abiotic challenges. A trade-off is envisioned between resources allocated to growth, development, and reproduction and to the biosynthesis, storage, and maintenance of secondary metabolites. However, increasing evidence suggests that secondary metabolites serve auxiliary roles, including functions associated with primary metabolism. In this opinion article, we examine how the costs of plant chemical defense can be offset by multifunctional biosynthesis and the optimization of primary metabolism. These additional benefits may negate the trade-off between primary and secondary metabolism, and provide plants with an innate plasticity required for growth, development, and interactions with their environment.

Optimal acclimation of the C3 photosynthetic system under enhanced CO2.

A range of studies of C3 plants have shown that there is a change in both the carbon flux and the pattern of nitrogen allocation when plants are grown under enhanced CO2. This paper examines evidence that allocation of nitrogen both to and within the photosynthetic system is optimised with respect to the carbon flux. A model is developed which predicts the optimal relative allocation of nitrogen to key enzymes of the photosynthetic system as a function of CO2 concentration. It is shown that evidence from flux control analysis is broadly consistent with this model, although at high nitrogen and under certain conditions at low nitrogen experimental data are not consistent with the model. Acclimation to enhanced CO2 is also assessed in terms of resource allocation between photosynthate sources and sinks. A means of assessing the optimisation of this source-sink allocation is proposed, and several studies are examined within this framework. It is concluded that C3 plants probably possess the genetic feedback mechanisms required to efficiently ‘smooth out” any imbalance within the photosynthetic system caused by a rise in atmospheric CO2.

Distribution and accumulation of ultraviolet-radiation-absorbing compounds in leaves of tropical mangroves

Ultraviolet (UV)-absorbing phenolic compounds that have been shown to be protective against the damaging: effects of UV-B radiation (Tevini et al., 1991, Photochem. Photobiol. 53, 329–333) were found in the leaf epidermis of tropical mangrove tree species. These UV-absorbing phenolic compounds and leaf succulence function as selective filters, removing short and energetic wavelengths. A field survey showed that the concentration of UV-absorbing compounds varied between species, between sites that would be experiencing similar levels of UV radiation, and between sun and shade leaves. Sun leaves have greater contents of phenolic compounds than shade leaves, and more saline sites have plants with greater levels in their leaves than less saline sites. In addition, increases in leaf nitrogen contents and quantum yields did not correlate with increasing levels of UV-absorbing compounds. It was concluded from these results that although UV-absorbing compounds form a UV-screen in the epidermis of mangrove leaves, UV radiation may not be the only factor influencing the accumulation of phenolic compounds, thus an experiment which altered the level of UV radiation incident on mangrove species was done. Near ambient levels of UVA and UV-B radiation resulted in a greater content of UV-absorbing compounds in Bruguiera parviflora (Roxb.) Wight and Arn. ex Griff., but did not result in increases in B. gymnorrhiza (L.) Lamk or Rhizophora apiculata Blume. Total chlorophyll contents were lower in R. apiculata when it was grown under near-ambient levels of UV radiation than when it was grown under conditions of UV-A and UV-B depletion, but no differences were observed between the UV radiation treatments in the other two species. There was no difference in leaf morphology, carotenoid/chlorophyll ratios, or chlorophyll a/b ratios between UV treatments, although these varied among species; B. parviflora had the highest carotenoid/chlorophyll ratio and R. apiculata had the lowest. Thus it is proposed that differences in species response tu UV radiation may be influenced by their ability to dissipate excess visible solar radiation.

Lateral heterogeneity in the distribution of thylakoid membrane lipid and protein components and its implications for the molecular organisation of photosynthetic membranes

Abstract The lateral distribution of all the major components of photosynthetic thylakoid membranes, i.e., acyl lipids, chlorophylls and proteins, has been examined by comparing the composition of whole thylakoids with that of appressed membrane regions derived therefrom. Separations were effected by means of French-press treatment followed by aqueous polymer two-phase partition. Considerable lateral heterogeneities in the distribution of all major membrane components were found. Membrane preparations highly enriched in appressed (granal) thylakoids contained less than 10% of the total thylakoid Photosystem I complex, but contained twice the amount of total chlorophyll relative to protein (mg/ml or ml/mol) and only 50–60% of the amount of total lipid relative to protein compared with whole thylakoids. Appressed thylakoids were substantially depleted in both major galactolipids in comparison with both whole thylakoids and non-appressed thylakoids. The ratio of diacylglycerophosphoglycerol:protein showed little variation between membrane fractions. A molar ratio of lipid:protein in appressed regions of only 13.5:1 was deduced, which implies a very considerable protein-protein proximity in chloroplast granal membranes. The results are discussed in terms of their implications for the molecular organisation of photosynthetic membranes.

Control of the rate of photosynthetic carbon dioxide fixation

Abstract A simplified model of the reductive pentose phosphate pathway of photosynthesis is analysed in order to quantify the degree to which each of the constituent reactions controls the rate of CO 2 fixation (given by the control coefficient). The analysis focuses on the four largely irreversible reactions of the cycle together with the first irreversible reaction in the sucrose and starch synthetic pathways. The model assumes that the other reactions are at equilibrium. The photorespiratory and electron transport systems are not included in the model. The analysis demonstrates that: (1) an analytical approach can be used to investigate the distribution of flux control in autocatalytic and moiety-conserved cycles; (2) measurements of enzyme kinetic parameters and certain fluxes and substrate concentrations can be used to solve the equations defining the enzyme control coefficients; (3) the conservation of total stromal phosphate and the intricate regulatory mechanisms of the photosynthetic system result in a relationship between the control coefficients that is complex and may defy any intuitive assessment of ‘rate limitation’; (4) ribulose-1,5-bisphosphate carboxylase / oxygenase may, under certain conditions, be a major controller of the rate of CO 2 fixation and, by regulating the concentration of ribulose 1,5-bisphosphate, may be important in governing the ratio of organic to inorganic phosphate in the stroma; (5) the other enzymes may also serve an important role in determining the distribution of phosphate between organic and inorganic species because they catalyze reactions at the branch points between starch and sucrose synthesis and ribulose 1,5-bisphosphate regeneration; (6) these enzymes that catalyze ‘branch-pint’ reactions may have negative control coefficients because of their ability to reduce the total concentration of cycle intermediates; (7) an approach combining the use of the equations presented in this paper and flux and substrate concentration measurements may be adequate for determining the control coefficients of several enzymes of the reductive pentose phosphate pathway.

Characterization of foliar manganese (Mn) in Mn (hyper)accumulators using X-ray absorption spectroscopy

Plant hyperaccumulation of the essential nutrient manganese (Mn) is a rare phenomenon most evident in the Western Pacific region, and differs from hyperaccumulation of other elements. Mn hyperaccumulators employ a variety of species-dependent spatial distribution patterns in sequestering excess foliar Mn, including primary sequestration in both nonphotosynthetic and photosynthetic tissues. This investigation employed synchrotron X-ray absorption spectroscopy (XAS) in a comparative study of Mn (hyper)accumulators, to elucidate in situ the chemical form(s) of foliar Mn in seven woody species from Australia, New Caledonia and Japan. Foliar Mn was found to predominate as Mn(II) in all samples, with strong evidence of the role of carboxylic acids, such as malate or citrate, as complexing ligands. Overall, the X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine-structure spectroscopy (EXAFS) data appeared weighted against previous observations that oxalate binds excess Mn in Mn-(hyper)accumulating species.

Phytostabilisation of arsenical gold mine tailings using four Eucalyptus species: Growth, arsenic uptake and availability after five years

Arsenic (As) contamination is a worldwide problem. Where arsenic is highly concentrated and confined within a limited area, such as in many mine tailings facilities, phytostabilisation is an attractive technology for long-term remediation. Important characteristics of a plant to be useful for phytostabilisation include As tolerance and low levels of As accumulation, as well as the ability to limit As availability. Performance needs to be monitored over the long term to ensure an ongoing vegetation community, though this is rarely done. In this study, the suitability of four Eucalyptus species (E. cladocalyx, E. melliodora, E. polybractea, E. viridis) for the phytostabilisation of arsenical, sulphidic gold mine tailings was assessed after five years. All four species accumulated low As concentrations, the highest being recorded in mature leaves, ranging from 0.29 to 5.14 microg g(-1) As. E. polybractea had significantly higher foliar As than the other three species but there was also great variation within the species. Between 5-10 times lower concentrations were recorded in stem samples and no As was detected in young leaf tips. There was also significant variation in the growth of trees upon the site. Eucalyptus cladocalyx grew significantly taller than other species although greater variation was detected within the species than between. The variation in tree heights was not correlated with As concentrations in either stems or leaves. Arsenic availability was determined to depths of 2.2 m and found to be low when compared to total As in the tailings. Importantly, no effect of trees on As availability or soil pH was detected. We conclude that E. cladocalyx, in particular is an ideal candidate for the long-term phytostabilisation of As-contaminated land and mine tailings. The variation detected in both As accumulation and growth is also promising for the selection of desirable traits.