Guard cell zeaxanthin tracks photosynthetically active radiation and stomatal apertures in Vicia faba leaves*

Zeaxanthin, antheraxanthin and violaxanthin concentrations in guard cells from sonicated abaxial epidermal peels of Vicia faba were measured from dawn to dusk, and compared with concentrations in mesophyll tissue of the same leaves. Measured changes in guard cell zeaxanthin and violaxanthin concentrations indicate that guard cells operate the xanthophyll cycle throughout the day. Mesophyll tissue had no detectable zeaxanthin at dawn, whereas guard cells had 30–50 mmol mol^?1 chlorophyll a+b. On a chlorophyll basis, maximal zeaxanthin levels were 3–4 fold higher in guard cells than in mesophyll cells. Zeaxanthin concentrations tracked levels of photosynthetically active radiation (PAR) in both mesophyll and guard cells. In the mesophyll, most of the zeaxanthin changes occurred in mid-morning and mid-afternoon. In guard cells, zeaxanthin concentrations changed nearly linearly with PAR in the early morning and late afternoon, and closely tracked PAR levels throughout the day. Guard cell zeaxanthin concentrations were also closely correlated with stomatal apertures. The close relationship between zeaxanthin concentrations and PAR levels in guard cells indicates that zeaxanthin is well suited to function as a molecular photosensor in stomatal movements.     

The cellular basis of guard cell sensing of rising CO2

Numerous studies conducted on both whole plants and isolated epidermes have documented stomatal sensitivity to CO₂. In general, CO₂ concentrations below ambient stimulate stomatal opening, or an inhibition of stomatal closure, while CO₂ concentrations above ambient have the opposite effect. The rise in atmospheric CO₂ concentrations which has occurred since the industrial revolution, and which is predicted to continue, will therefore alter rates of transpirational water loss and CO₂ uptake in terrestrial plants. An understanding of the cellular basis for guard cell CO₂ sensing could allow us to better predict, and perhaps ultimately to manipulate, such vegetation responses to climate change. However, the mechanisms by which guard cells sense and respond to the CO₂ signal remain unknown. It has been hypothesized that cytosolic pH and malate levels, cytosolic Ca²⁺ levels, chloroplastic zeaxanthin levels, or plasma-membrane anion channel regulation by apoplastic malate are involved in guard cell perception and response to CO₂. In this review, these hypotheses are discussed, and the evidence for guard cell acclimation to prevailing CO₂ concentrations is also considered.     

The response of soil CO2 flux to changes in atmospheric CO2, nitrogen supply and plant diversity

We measured soil CO₂ flux over 19 sampling periods that spanned two growing seasons in a grassland Free Air Carbon dioxide Enrichment (FACE) experiment that factorially manipulated three major anthropogenic global changes: atmospheric carbon dioxide (CO₂) concentration, nitrogen (N) supply, and plant species richness. On average, over two growing seasons, elevated atmospheric CO₂ and N fertilization increased soil CO₂ flux by 0.57 µmol m^?2 s^?1 (13% increase) and 0.37 µmol m^?2 s^?1 (8% increase) above average control soil CO₂ flux, respectively. Decreases in planted diversity from 16 to 9, 4 and 1 species decreased soil CO₂ flux by 0.23, 0.41 and 1.09 µmol m^?2 s^?1 (5%, 8% and 21% decreases), respectively. There were no statistically significant pairwise interactions among the three treatments. During 19 sampling periods that spanned two growing seasons, elevated atmospheric CO₂ increased soil CO₂ flux most when soil moisture was low and soils were warm. Effects on soil CO₂ flux due to fertilization with N and decreases in diversity were greatest at the times of the year when soils were warm, although there were no significant correlations between these effects and soil moisture. Of the treatments, only the N and diversity treatments were correlated over time; neither were correlated with the CO₂ effect. Models of soil CO₂ flux will need to incorporate ecosystem CO₂ and N availability, as well as ecosystem plant diversity, and incorporate different environmental factors when determining the magnitude of the CO₂, N and diversity effects on soil CO₂ flux.     

Elevated CO2 concentrations and stomatal density: observations from 17 plant species growing in a CO2 spring in central Italy

Stomatal density (SD) and stomatal conductance ( g _s) can be affected by an increase of atmospheric CO₂ concentration. This study was conducted on 17 species growing in a naturally enriched CO₂ spring and belonging to three plant communities. Stomatal conductance, stomatal density and stomatal index (SI) of plants from the spring, which were assumed to have been exposed for generations to elevated [CO₂], and of plants of the same species collected in a nearby control site, were compared. Stomatal conductance was significantly lower in most of the species collected in the CO₂ spring and this indicated that CO₂ effects on g _s are not of a transitory nature but persist in the long term and through plant generations. Such a decrease was, however, not associated with changes in the anatomy of leaves: SD was unaffected in the majority of species (the decrease was only significant in three out of the 17 species examined), and also SI values did not vary between the two sites with the exception of two species that showed increased SI in plants grown in the CO₂-enriched area. These results did not support the hypothesis that long-term exposure to elevated [CO₂] may cause adaptive modification in stomatal number and in their distribution.     

Elevated atmospheric CO2 alters stomatal responses to variable sunlight in a C4 grass

Native tallgrass prairie in NE Kansas was exposed to elevated (twice ambient) or ambient atmospheric CO₂ levels in open-top chambers. Within chambers or in adjacent unchambered plots, the dominant C₄ grass, Andropogon gerardii, was subjected to fluctuations in sunlight similar to that produced by clouds or within canopy shading (full sun > 1500 μmol m⁻² s⁻¹ versus 350 μmol m⁻² s⁻¹ shade) and responses in gas exchange were measured. These field experiments demonstrated that stomatal conductance in A. gerardii achieved new steady state levels more rapidly after abrupt changes in sunlight at elevated CO₂ when compared to plants at ambient CO₂. This was due primarily to the 50% reduction in stomatal conductance at elevated CO₂, but was also a result of more rapid stomatal responses. Time constants describing stomatal responses were significantly reduced (29–33%) at elevated CO₂. As a result, water loss was decreased by as much as 57% (6.5% due to more rapid stomatal responses). Concurrent increases in leaf xylem pressure potential during periods of sunlight variability provided additional evidence that more rapid stomatal responses at elevated CO₂ enhanced plant water status. CO₂-induced alterations in the kinetics of stomatal responses to variable sunlight will likely enhance direct effects of elevated CO₂ on plant water relations in all ecosystems.     

Effects of atmospheric CO2 enrichment on early growth of Vivia faba, a plant with large cotyledons

Seedlings of Vicia faba L. were grown in open-top growth chambers at present (P=350μmol^?1) and at elevated (E=700μmol mol^?1) atmospheric CO₂ concentration. The effects of CO₂ enrichment on the first phase of growth after germination were examined over 45 d. There were no positive effects of CO₂ enrichment on growth of the seedlings during this early phase. No differences were observed in leaf area or in total dry weight. No differences were found in morphology or anatomy of the leaves. The numbers of stomatal and epidermal cells, thickness of leaf, of epidermis and of mesophyll cell-layers were unaffected by CO₂ enrichment. Also, no differences were observed in leaf concentrations of chlorophyll, reducing carbohydrates or starch. These results contrast markedly with results from similar experiments on poplar hybrids and Phaseolus vulgaris obtained in the same growth facility. It seems that the intitial growth is under internal control such that the atmospheric CO₂ concentration has no effects. The lack of response in this case may be attributed to the presence and longevity of the large cotyledons which provided available substrate for growth.     

Stomatal sensitivity to vapour pressure difference over a subambient to elevated CO2 gradient in a C3/C4 grassland

In the present study the response of stomatal conductance (g_s) to increasing leaf‐to‐air vapour pressure difference (D) in early season C₃ (Bromus japonicus) and late season C₄ (Bothriochloa ischaemum) grasses grown in the field across a range of CO₂ (200–550 µmol mol^?1) was examined. Stomatal sensitivity to D was calculated as the slope of the response of g_s to the natural log of externally manipulated D (dg_s/dlnD). Increasing D and CO₂ significantly reduced g_s in both species. Increasing CO₂ caused a significant decrease in stomatal sensitivity to D in Br. japonicus, but not in Bo. ischaemum. The decrease in stomatal sensitivity to D at high CO₂ for Br. japonicus fit theoretical expectations of a hydraulic model of stomatal regulation, in which g_s varies to maintain constant transpiration and leaf water potential. The weaker stomatal sensitivity to D in Bo. ischaemum suggested that stomatal regulation of leaf water potential was poor in this species, or that non‐hydraulic signals influenced guard cell behaviour. Photosynthesis (A) declined with increasing D in both species, but analyses of the ratio of intercellular to atmospheric CO₂ (C_i/C_a) suggested that stomatal limitation of A occurred only in Br. japonicus. Rising CO₂ had the greatest effect on g_s and A in Br. japonicus at low D. In contrast, the strength of stomatal and photosynthetic responses to CO₂ were not affected by D in Bo. ischaemum. Carbon and water dynamics in this grassland are dominated by a seasonal transition from C₃ to C₄ photosynthesis. Interspecific variation in the response of g_s to D therefore has implications for predicting seasonal ecosystem responses to CO₂.     

Seasonal CO2 exchange patterns of developing peach (Prunus persica) fruits in response to temperature, light and CO2 concentration

CO₂ exchange rates per unit dry weight, measured in the field on attached fruits of the late-maturing Cal Red peach cultivar, at 1200 μmol photons m^?2S^?1 and in dark, and photosynthetic rates, calculated by the difference between the rates of CO₂ evolution in light and dark, declined over the growing season. Calculated photosynthetic rates per fruit increased over the season with increasing fruit dry matter, but declined in maturing fruits apparently coinciding with the loss of chlorophyll. Slight net fruit photosynthetic rates ranging from 0. 087 ± 0. 06 to 0. 003 ± 0. 05 nmol CO₂ (g dry weight)^?1 S^?1 were measured in midseason under optimal temperature (15 and 20°C) and light (1200 μmol photons m^?2 S^?1) conditions. Calculated fruit photosynthetic rates per unit dry weight increased with increasing temperatures and photon flux densities during fruit development. Dark respiration rates per unit dry weight doubled within a temperature interval of 10°C; the mean seasonal O₁₀ value was 2. 03 between 20 and 30°C. The highest photosynthetic rates were measured at 35°C throughout the growing season. Since dark respiration rates increased at high temperatures to a greater extent than CO₂ exchange rates in light, fruit photosynthesis was apparently stimulated by high internal CO₂ concentrations via CO₂ refixation. At 15°C, fruit photosynthetic rates tended to be saturated at about 600 μmol photons m^?2 S^?1. Young peach fruits responded to increasing ambient CO₂ concentrations with decreasing net CO₂ exchange rates in light, but more mature fruits did not respond to increases in ambient CO₂. Fruit CO₂ exchange rates in the dark remained fairly constant, apparently uninfluenced by ambient CO₂ concentrations during the entire growing season. Calculated fruit photosynthetic rates clearly revealed the difference in CO₂ response of young and mature peach fruits. Photosynthetic rates of younger peach fruits apparently approached saturation at 370 μl CO₂1^?2. In CO₂ free air, fruit photosynthesis was dependent on CO₂ refixation since CO₂ uptake by the fruits from the external atmosphere was not possible. The difference in photosynthetic rates between fruits in CO₂-free air and 370 μl CO₂ 1^?1 indicated that young peach fruits were apparently able to take up CO₂ from the external atmosphere. CO₂ uptake by peach fruits contributed between 28 and 16% to the fruit photosynthetic rate early in the season, whereas photosynthesis in maturing fruits was supplied entirely by CO₂ refixation.     

Physiological effects of sulphur dioxide. 1. The effect of SO2 on photosynthesis and stomatal regulation of Vicia faba L.

Abstract. The effect of short-term SO₂ fumigation on photosynthesis and transpiration of Vicia faba L. was measured at different irradiances and SO₂ concentrations. At high irradiances photosynthetic rates were reduced when leaves were exposed to SO₂ and the magnitude of the reduction was linearly related to the rate of SO₂ uptake through the stomata. Photosynthetic rates stabilized within 2 h after the start of fumigation.
The effect of SO₂ on photosynthesis was measured at different CO₂ concentrations to analyse the contribution of stomatal and non-stomatal factors to photosynthetic inhibition. Mesophyll resistance to CO₂ diffusion increased as a result of SO₂ exposure and caused a rapid reduction in photosynthesis after the start of fumigation. Stomatal resistance was not affected directly by SO₂ fumigation, but indirectly as a result of a feedback loop between net photosynthesis and internal CO₂ concentration.
Analysis of gas-exchange measurements in biochemical terms indicated that photosynthetic inhibition during SO₂ exposure can be explained by a stronger reduction in the affinity of RBP carboxylase/oxygenase for CO₂ than for O₂.     

Translocation of labelled assimilates following photosynthesis of 14CO2 by the field bean, Vicia faba

When whole plants were exposed to ¹⁴CO₂, almost the same amount of radioactivity was taken up initially by each leaf regardless of its position on the stem and of the presence of beans at that node. Thus, although developing beans are a powerful sink for assimilated carbon, they do not increase the CO₂ uptake by adjoining leaves.
The distribution of labelled assimilates 6 hours after feeding ¹⁴CO₂ to a single leaf for 1 hour varied with both the position of the treated leaf and the stage of development of the plant. Before any flowers were set most of the radioactivity from all expanded leaves moved downwards to the roots and the stem below the treated leaf (lower stem). Later, during pod-fill, the upper leaves maintained this supply to the roots and lower stem, whilst most of the carbon translocated from the lower and mid-stem leaves went to the beans. However, we found no exclusive relationship between a leaf and the supply to beans developing on the same node.
The amount of radioactivity moving out of a source leaf at a fruiting node increased over successive samplings up to 48 h; the pattern of distribution of the ¹⁴CO₂ however remained virtually unchanged.     

Stomatal acclimation over a subambient to elevated CO2 gradient in a C3/C4 grassland

An investigation to determine whether stomatal acclimation to [CO₂] occurred in C₃/C₄ grassland plants grown across a range of [CO₂] (200–550 µmol mol^?1) in the field was carried out. Acclimation was assessed by measuring the response of stomatal conductance (g_s) to a range of intercellular CO₂ (a g_s–C_i curve) at each growth [CO₂] in the third and fourth growing seasons of the treatment. The g_s–C_i response curves for Solanum dimidiatum (C₃ perennial forb) differed significantly across [CO₂] treatments, suggesting that stomatal acclimation had occurred. Evidence of non–linear stomatal acclimation to [CO₂] in this species was also found as maximum g_s (g_smax; g_s measured at the lowest C_i) increased with decreasing growth [CO₂] only below 400 µmol mol^?1. The substantial increase in g_s at subambient [CO₂] for S. dimidiatum was weakly correlated with the maximum velocity of carboxylation (V_cmax; r² = 0·27) and was not associated with CO₂ saturated photosynthesis (A_max). The response of g_s to C_i did not vary with growth [CO₂] in Bromus japonicus (C₃ annual grass) or Bothriochloa ischaemum (C₄ perennial grass), suggesting that stomatal acclimation had not occurred in these species. Stomatal density, which increased with rising [CO₂] in both C₃ species, was not correlated with g_s. Larger stomatal size at subambient [CO₂], however, may be associated with stomatal acclimation in S. dimidiatum. Incorporating stomatal acclimation into modelling studies could improve the ability to predict changes in ecosystem water fluxes and water availability with rising CO₂ and to understand their magnitudes relative to the past.     

Stomatal behaviour, photosynthesis and transpiration under rising CO2

Definitions of the variables used and the units are given in Table 1

The literature reports enormous variation between species in the extent of stomatal responses to rising CO₂. This paper attempts to provide a framework within which some of this diversity can be explained. We describe the role of stomata in the short-term response of leaf gas exchange to increases in ambient CO₂ concentration by developing the recently proposed stomatal model of Jarvis & Davies (1998 ). In this model stomatal conductance is correlated with the functioning of the photosynthetic system so that the effects of increases in CO₂ on stomata are experienced through changes in the rate of photosynthesis in a simple and mechanistically transparent way. This model also allows us to consider the effects of evaporative demand and soil moisture availability on stomatal responses to photosynthesis and therefore provides a means of considering these additional sources of variation. We emphasize that the relationship between the rate of photosynthesis and the internal CO₂ concentration and also drought will have important effects on the relative gains to be achieved under rising CO₂.  

  Table 1 . Abbreviations     

19.

Soil CO₂ efflux in a boreal pine forest under atmospheric CO₂ enrichment and air warming   总被引：3，自引：0，他引：3  

Sini Maaria Niinistö  Jouko Silvola†  Seppo Kellomäki 《Global Change Biology》2004,10(8):1363-1376

The response of forest soil CO₂ efflux to the elevation of two climatic factors, the atmospheric concentration of CO₂ (↑CO₂ of 700 μmol mol⁻¹) and air temperature (↑ T with average annual increase of 5°C), and their combination (↑CO₂+↑ T ) was investigated in a 4-year, full-factorial field experiment consisting of closed chambers built around 20-year-old Scots pines ( Pinus sylvestris L.) in the boreal zone of Finland. Mean soil CO₂ efflux in May–October increased with elevated CO₂ by 23–37%, with elevated temperature by 27–43%, and with the combined treatment by 35–59%. Temperature elevation was a significant factor in the combined 4-year efflux data, whereas the effect of elevated CO₂ was not as evident. Elevated temperature had the most pronounced impact early and late in the season, while the influence of elevated CO₂ alone was especially notable late in the season. Needle area was found to be a significant predictor of soil CO₂ efflux, particularly in August, a month of high root growth, thus supporting the assumption of a close link between whole-tree physiology and soil CO₂ emissions. The decrease in the temperature sensitivity of soil CO₂ efflux observed in the elevated temperature treatments in the second year nevertheless suggests the existence of soil response mechanisms that may be independent of the assimilating component of the forest ecosystem. In conclusion, elevated atmospheric CO₂ and air temperature consistently increased forest soil CO₂ efflux over the 4-year period, their combined effect being additive, with no apparent interaction.     

20.

Natural CO₂ springs in Italy: a resource for examining long-term response of vegetation to rising atmospheric CO₂ concentrations   总被引：1，自引：1，他引：1  

F. MIGLIETTA  A. RASCHI  I. BETTARINI  R. RESTI  F. SELVI 《Plant, cell & environment》1993,16(7):873-878

It is estimated that more than 100 geothermal CO₂ springs exist in central-western Italy. Eight springs were selected in which the atmospheric CO₂ concentrations were consistently observed to be above the current atmospheric average of 354μmol mol^-1. CO₂ concentration measurements at some of the springs are reported. The springs are described, and their major topographic and vegetational features are reported. Preliminary observations made on natural vegetation growing around the gas vents are then illustrated. An azonal pattern of vegetation distribution occurs around every CO₂ spring regardless of soil type and phytoclimatic areas. This is composed of pioneer populations of a Northern Eurasiatic species (Agrostis canina L.) which is often associated with Scirpus lacustris L. The potential of these sites for studying the long-term response of vegetation to rising atmospheric CO₂ concentrations is discussed.     

Soil CO2 efflux in a boreal pine forest under atmospheric CO2 enrichment and air warming

The response of forest soil CO₂ efflux to the elevation of two climatic factors, the atmospheric concentration of CO₂ (↑CO₂ of 700 μmol mol⁻¹) and air temperature (↑ T with average annual increase of 5°C), and their combination (↑CO₂+↑ T ) was investigated in a 4-year, full-factorial field experiment consisting of closed chambers built around 20-year-old Scots pines ( Pinus sylvestris L.) in the boreal zone of Finland. Mean soil CO₂ efflux in May–October increased with elevated CO₂ by 23–37%, with elevated temperature by 27–43%, and with the combined treatment by 35–59%. Temperature elevation was a significant factor in the combined 4-year efflux data, whereas the effect of elevated CO₂ was not as evident. Elevated temperature had the most pronounced impact early and late in the season, while the influence of elevated CO₂ alone was especially notable late in the season. Needle area was found to be a significant predictor of soil CO₂ efflux, particularly in August, a month of high root growth, thus supporting the assumption of a close link between whole-tree physiology and soil CO₂ emissions. The decrease in the temperature sensitivity of soil CO₂ efflux observed in the elevated temperature treatments in the second year nevertheless suggests the existence of soil response mechanisms that may be independent of the assimilating component of the forest ecosystem. In conclusion, elevated atmospheric CO₂ and air temperature consistently increased forest soil CO₂ efflux over the 4-year period, their combined effect being additive, with no apparent interaction.     

Natural CO2 springs in Italy: a resource for examining long-term response of vegetation to rising atmospheric CO2 concentrations

It is estimated that more than 100 geothermal CO₂ springs exist in central-western Italy. Eight springs were selected in which the atmospheric CO₂ concentrations were consistently observed to be above the current atmospheric average of 354μmol mol^-1. CO₂ concentration measurements at some of the springs are reported. The springs are described, and their major topographic and vegetational features are reported. Preliminary observations made on natural vegetation growing around the gas vents are then illustrated. An azonal pattern of vegetation distribution occurs around every CO₂ spring regardless of soil type and phytoclimatic areas. This is composed of pioneer populations of a Northern Eurasiatic species (Agrostis canina L.) which is often associated with Scirpus lacustris L. The potential of these sites for studying the long-term response of vegetation to rising atmospheric CO₂ concentrations is discussed.