Studies of the relationship between isoprene emission rate and CO2 or photon-flux density using a real-time isoprene analyser

Abstract. Studies of the isoprene emission rate in response to changes in photon-flux density and CO₂ partial pressure were conducted using a recently developed on-line isoprene analyser combined with a gas exchange system and chlorophyll fluorometer. Upon darkening, the isoprene emission rate from leaves of aspen ( Populus tremuloides Michaux.) began to decline immediately, demonstrating that the internal pool of isoprene, or its precursors, is small and that the instantaneous emission rate is tightly coupled to the rate of synthesis. A post-illumination burst of isoprene was observed within 5 min after darkening and lasted for 15–20 min in four isoprene-emitting species that were examined. In leaves of eucalyptus ( Eucalyptus globulus Labill.), the magnitude of the post-illumination burst was dependent on the photon-flux density that existed before darkening, but not on ambient CO₂ partial pressure. The dependence of the post-illumination burst on photon-flux density paralleled that for the steady-state rate of isoprene emission. A step-wise increase in intercellular CO₂ partial pressure from 24.5 to 60 Pa resulted in an immediate decrease in isoprene emission rate and non-photochemical fluorescence quenching, but an increase in CO₂ assimilation rate. Given the several recent studies that link isoprene emission to chloroplastic processes, the results of this study indicate that the linkage is not dependent on the rate of CO₂ flux through the reductive pentose phosphate pathway, but rather on more complex relationships involving metabolites not appreciably influenced by CO₂ partial pressure.     

Leaf isoprene emission rate as a function of atmospheric CO2 concentration

There is considerable interest in modeling isoprene emissions from terrestrial vegetation, because these emissions exert a principal control over the oxidative capacity of the troposphere. We used a unique field experiment that employs a continuous gradient in CO₂ concentration from 240 to 520 ppmv to demonstrate that isoprene emissions in Eucalyptus globulus were enhanced at the lowest CO₂ concentration, which was similar to the estimated CO₂ concentrations during the last Glacial Maximum, compared with 380 ppmv, the current CO₂ concentration. Leaves of Liquidambar styraciflua did not show an increase in isoprene emission at the lowest CO₂ concentration. However, isoprene emission rates from both species were lower for trees grown at 520 ppmv CO₂ compared with trees grown at 380 ppmv CO₂. When grown in environmentally controlled chambers, trees of Populus deltoides and Populus tremuloides exhibited a 30–40% reduction in isoprene emission rate when grown at 800 ppmv CO₂, compared with 400 ppmv CO₂. P. tremuloides exhibited a 33% reduction when grown at 1200 ppmv CO₂, compared with 600 ppmv CO₂. We used current models of leaf isoprene emission to demonstrate that significant errors occur if the CO₂ inhibition of isoprene is not taken into account. In order to alleviate these errors, we present a new model of isoprene emission that describes its response to changes in atmospheric CO₂ concentration. The model logic is based on assumed competition between cytosolic and chloroplastic processes for pyruvate, one of the principal substrates of isoprene biosynthesis.     

The response of isoprene emission rate and photosynthetic rate to photon flux and nitrogen supply in aspen and white oak trees

Isoprene is the primary biogenic hydrocarbon emitted from temperate deciduous forest ecosystems. The effects of varying photon flux density (PFD) and nitrogen growth regimes on rates of isoprene emission and net photosynthesis in potted aspen and white oak trees are reported. In both aspen and oak trees, whether rates were expressed on a leaf area or dry mass basis, (1) growth at higher PFD resulted in significantly higher rates of isoprene emission, than growth at lower PFD, (2) there is a significant positive relationship between isoprene emission rate and leaf nitrogen concentration in both sun and shade trees, and (3) there is a significant positive correlation between isoprene emission rate and photosynthetic rate in both sun and shade trees. The greater capacity for isoprene emission in sun leaves was due to both higher leaf mass per unit area and differences in the biochemical and/or physiological properties that influence isoprene emission. Positive correlations between isoprene emission rate and leaf nitrogen concentration support the existence of mechanisms that link leaf nitrogen status to isoprene synthase activity. Positive correlations between isoprene emission rate and photosynthesis rate support previous hypotheses that isoprene emission plays a role in protecting photosynthetic mechanisms during stress.     

Stomata of trees growing in CO2-enriched air show reduced sensitivity to vapour pressure deficit and drought

Stomatal conductance ( g _s) and photosynthetic rate ( A ) were measured in young beech ( Fagus sylvatica ), chestnut ( Castanea sativa ) and oak ( Quercus robur ) growing in ambient or CO₂-enriched air. In oak, g _s was consistently reduced in elevated CO₂. However, in beech and chestnut, the stomata of trees growing in elevated CO₂ failed to close normally in response to increased leaf-to-air vapour pressure deficit (LAVPD). Consequently, while g _s was reduced in elevated CO₂ on days with low LAVPD, on warm sunny days (with correspondingly high LAVPD) g _s was unchanged or even slightly higher in elevated CO₂. Furthermore, during drought, g _s of beech and chestnut was unresponsive to [CO₂], over a wide range of ambient LAVPD, whereas in oak g _s was reduced by an average of 50% in elevated CO₂. Stimulation of A by elevated CO₂ in beech and chestnut was restricted to days with high irradiance, and was greatest in beech during drought. Hence, most of the additional carbon gain in elevated CO₂ was made at the expense of water economy, at precisely those times (drought, high evaporative demand) when water conservation was most important. Such effects could have serious consequences for drought tolerance, growth and, ultimately, survival as atmospheric [CO₂] increases.     

Adaptation to shade of the light-harvesting apparatus in Silene dioica

Abstract. The physiological characteristics and photo-system composition of the photosynthetic apparatus of Silene dioica , a woodland plant, grown in sun and natural shade are examined. As expected, shade leaves exhibited lower chlorophyll a/b ratios, light saturated rates of CO₂ assimilation (A_sat), dark respiration (R_d,) and light compensation points ( Г ), with both sun and shade leaves having similar absorptances and quantum yields of CO₂ assimilation (φ). Shade leaves were able to utilize far-red light for electron transport and carbon assimilation and reach the compensation point. Sun leaves in far-red light had a rate of carbon assimilation equivalent to their dark respiration rate. Chlorophyll fluorescence kinetics from leaves at 77 K together with analyses of thylakoid contents of photosystems (PS) I and II and the light-harvesting cholorphyll a/b protein complex associated with PSII (LHCII) demonstrated that the antenna size of PSII was similar in thylakoids of sun and shade leaves, but shade leaves contained ca. 20% more PSII and ca. 12% less PSI complexes. The increased PSII/PSI ratio in shade leaves accounted for their ability to achieve the compensation point in far-red light. An important feature of photosynethic shade adaptation in S. dioica is an increase in the PSII/PSI ratio and not an increase in the antenna size of PSII. The adaptive response of sun leaves when placed in a shade environment was rapid and had a half-time of ca. 18h.     

Ecophysiological responses to simulated canopy gaps of two tree species of contrasting shade tolerance in elevated CO2

1. One-year-old seedlings of shade tolerant Acer rubrum and intolerant Betula papyrifera were grown in ambient and twice ambient (elevated) CO₂, and in full sun and 80% shade for 90 days. The shaded seedlings received 30-min sun patches twice during the course of the day. Gas exchange and tissue–water relations were measured at midday in the sun plants and following 20 min of exposure to full sun in the shade plants to determine the effect of elevated CO₂ on constraints to sun-patch utilization in these species.
2. Elevated CO₂ had the largest stimulation of photosynthesis in B. papyrifera sun plants and A. rubrum shade plants.
3. Higher photosynthesis per unit leaf area in sun plants than in shade plants of B. papyrifera was largely owing to differences in leaf morphology. Acer rubrum exhibited sun/shade differences in photosynthesis per unit leaf mass consistent with biochemical acclimation to shade.
4. Betula papyrifera exhibited CO₂ responses that would facilitate tolerance to leaf water deficits in large sun patches, including osmotic adjustment and higher transpiration and stomatal conductance at a given leaf-water potential, whereas A. rubrum exhibited large increases in photosynthetic nitrogen-use efficiency.
5. Results suggest that species of contrasting successional ranks respond differently to elevated CO₂, in ways that are consistent with the habitats in which they typically occur.     

Inhibition of photosynthesis and export in geranium grown at two CO2 levels and infected with Xanthomonas campestris pv. Pelargonii

The effects of CO₂ enrichment on growth of Xanthomonas campestris pv. pelargonii and the impact of infection on the photosynthesis and export of attached, intact, 'source' leaves of geranium ( Pelargonium x domesticum, 'Scarlet Orbit Improved' ) are reported. Two experiments were performed, one with plants without flower buds, and another with plants which were flowering. Measurements were made on healthy and diseased leaves at the CO₂ levels (35 Pa or 90 Pa) at which the plants were grown. There were no losses of chlorophyll, or any signs of visible chlorosis or necrosis due to infection. Lower numbers of bacteria were found in leaves at high CO₂, suggesting growth at elevated CO₂ created a less favourable condition in the leaf for bacterial growth. Although high CO₂ lowered the bacterial number in infected leaves, reductions in photosynthesis and export were greater than at ambient CO₂. The capacity of infected source leaves to export photoassimilates at rates observed in the controls was reduced in both light and darkness. In summary, the severity of infection on source leaf function by the bacteria was increased, rather than reduced by CO₂ enrichment, underscoring the need for further assessment of plant diseases and bacterial virulence in plants growing under varying CO₂ levels.     

Photosynthesis, Soluble and Structural Carbon Compounds in Two Mediterranean Oak Species (Quercus pubescens and Q. ilex) after Lifetime Growth at Naturally Elevated CO2 Concentrations

Abstract: To study physiological responses of mature forest trees to elevated CO₂ after lifetime growth under elevated atmospheric CO₂ concentrations ( p CO₂), photosynthesis, Rubisco content, foliar concentrations of soluble sugars and starch, sugar concentrations in transport tissues (phloem and xylem), structural biomass, and lignin in leaves and branches were investigated in 30- to 50-year-old Quercus pubescens and Q. ilex trees grown at two naturally elevated CO₂ springs in Italy. Ribulose-1,5-bisphosphate carboxylase/oxygenase content was decreased in Q. pubescens grown under elevated CO₂ concentrations, but not in Q. ilex. Photosynthesis was consistently higher in Q. pubescens grown at elevated CO₂ as compared with "control" sites, whereas the response in Q. ilex was less pronounced. Stomatal conductance was lower in both species leading to decreased transpiration and increased instantaneous water use efficiency in Q. pubescens. Overall mean sugar + starch concentrations of the leaves were not affected by elevated p CO₂, but phloem exudates contained higher concentrations of soluble sugars. This finding suggests increased transport to sinks. Qualitative changes in major carbon-bearing compounds, such as structural biomass and lignins, were only found in bark but not in other tissues. These results support the concept that the maintenance of increased rates of photosynthesis after long-term acclimation to elevated p CO₂ provides a means of optimization of water relations under arid climatic conditions but does not cause an increase in aboveground carbon sequestration per unit of tissue in Mediterranean oak species.     

Gas exchange studies in two Portuguese grapevine cultivars

Gas exchange characteristics of leaves of Vitis vinifera L. cvs Tinta Amarela and Periquita, two grapevine cultivars grown in distinct climatic regions of Portugal, were studied under natural and controlled conditions. Daily time courses of gas exchange were measured on both a hot, sunny day and a cooler, partly cloudy day. Responses of net photosynthesis to irradiance and internal partial pressure of CO₂, were also obtained. A strong correlation between net photosynthesis (P_N) and leaf conductance (g_s) was found during the diurnal time courses of gas exchange, as well as a relatively constant internal partial pressure of CO₂ (P_i), even under non-steady-state conditions. On the cloudless day, both P_N and g_s were lower in the afternoon than in the morning, despite similar conditions of leaf temperature, air to leaf water vapor deficit and irradiance. The response curves of net photosynthesis to internal CO₂ showed linearity up to p_i values of 50 Pa, possibly indicating a substantial excess of photosynthetic capacity. When measured at low partial pressures of O₂ (1 kPa), P_N became inhibited at high CO₂ levels. Inhibition of P_N at high CO₂ was absent under normal levels of O₂ (21 kPa). Significant differences in gas exchange characteristics were found between the two cultivars, with T. Amarela having higher rates under similar measurement conditions. In particular, the superior performance of T. Amarela at high temperatures may represent adaptation to the warmer conditions at its place of origin.     

Mycorrhiza formation and elevated CO2 both increase the capacity for sucrose synthesis in source leaves of spruce and aspen

The effects of mycorrhiza formation in combination with elevated CO₂ concentrations on carbon metabolism of Norway spruce ( Picea abies ) seedlings and aspen ( Populus tremula × Populus tremuloides ) plantlets were analysed. Plants were inoculated for 6 wk with the ectomycorrhizal fungi Amanita muscaria and Paxillus involutus (aspen only) in an axenic Petri-dish culture at 350 and 700 μl l⁻¹ CO₂ partial pressure. After mycorrhiza formation, a stimulation of net assimilation rate was accompanied by decreased activities of sucrose synthase, an increased activation state of sucrose-phosphate synthase, decreased fructose-2,6-bisphosphate and starch, and slightly elevated glucose-6-phosphate contents in source leaves of both host species, independent of CO₂ concentration. Exposure to elevated CO₂ generally resulted in higher net assimilation rates, increased starch as well as decreased fructose-2,6-bisphosphate (aspen only) content in source leaves of both mycorrhizal and nonmycorrhizal plants. Our data indicate only slightly improved carbon utilization by mycorrhizal plants at elevated CO₂. They demonstrate however, that both factors which modulate the sink-source properties of plants increase the capacity for sucrose synthesis in source leaves mainly by allosteric enzyme regulation.     

Effect of carbon dioxide enrichment on chlorophyll content, starch content and starch grain structure in Trifolium subterraneum leaves

Trifolium subterraneum (cv. Dinninup) responds to enriched atmospheric CO₂ in a manner similar to that described by Madsen (1968 and 1976) for tomato. In immature leaves, the total chlorophyll content per unit dry weight and the chlorophyll a:b ratio are significantly lower in plants grown at 0.10 vol% CO₂. Although fully expanded mature leaves partially overcome the deficit in chlorophyll content, the chlorophyll a:b ratio remains substantially lower in these high CO₂ grown plants. The large amount of starch accumulated as irregularly shaped grains appears to disrupt normal chloroplast structure in clover plants grown in enriched atmospheric CO₂. These results indicate the chlorotic appearance of leaves from high CO₂ grown clover plants is due to a decrease in chlorophyll content per dry weight possibly resulting from large starch grains and starch accumulation altering normal chloroplast structure and function.     

Elevated atmospheric CO2 lowers leaf litter nutritional quality for stream ecosystem food webs

Up to 99% of the carbon fuelling the food webs of temperate woodland streams is derived from inputs of terrestrial leaf litter. Aquatic bacteria, fungi, and detritivore invertebrates directly utilize these inputs, transferring this energy to other components of the food web. Increases in atmospheric CO₂ could indirectly impact woodland stream food webs by chemically altering leaf litter. This study evaluated CO₂-induced chemical changes in aspen ( Populus tremuloides ) leaf litter, and the corresponding effects on stream bacteria, fungi and leaf-shredding cranefly larvae ( Tipula abdominalis : Diptera). Leaf litter from plants grown under elevated CO₂ had decreased nutritional value to aquatic decomposers and detritivores because of higher levels of structural compounds and lower nitrogen content. Consequently, elevated CO₂-grown leaf litter supported 59% lower bacterial production in a stream than litter grown at ambient CO₂ levels, while not affecting fungal biomass. Larval craneflies fed elevated CO₂-grown microbially colonized leaves consumed less, assimilated less, and grew 12 times slower than their ambient fed counterparts.     

Kinetics of leaf oxygen uptake represent in planta activities of respiratory electron transport and terminal oxidases

We present, for the first time, the oxygen response kinetics of mitochondrial respiration measured in intact leaves (sunflower and aspen). Low O₂ concentrations in N₂ (9–1500 ppm) were preset in a flow-through gas exchange measurement system, and the decrease in O₂ concentration and the increase in CO₂ concentration as result of leaf respiration were measured by a zirconium cell O₂ analyser and infrared-absorption CO₂ analyser, respectively. The low O₂ concentrations little influenced the rate of CO₂ evolution during the 60-s exposure. The initial slope of the O₂ uptake curve on the dissolved O₂ concentration basis was relatively constant in leaves of a single species, 1.5 mm s⁻¹ in sunflower and 1.8 mm s⁻¹ in aspen. The apparent K _0.5(O₂) values ranged from 0.33 to 0.67 μ M in sunflower and from 0.33 to 1.1 μ M in aspen, mainly because of the variation of the maximum rate, V _max (leaf temperature 22°C). The initial slope of the O₂ response of respiration characterizes the catalytic efficiency of terminal oxidases, an important parameter of the respiratory machinery in leaves. The plateau of the response characterizes the activity of the mitochondrial electron transport chain and is subject to regulations in accordance with the necessity for ATP production. The relatively low oxygen conductivity of terminal oxidases means that in leaves, less than 10% of the photosynthetic oxygen can be reassimilated by mitochondria.     

A relationship between carbon dioxide, photosynthetic efficiency and shade tolerance

Net photosynthesis and transpiration of seedlings from shade tolerant, moderately tolerant and intolerant tree species were measured in ambient carbon dioxide (CO₂) concentrations ranging from 312 to 734 ppm. The species used, Fagus grandifolia Ehrh. (tolerant), Quercus alba L., Q. rubra L., Liriodendron tulipifera L. (moderately tolerant), Liquidambar styraciflua L. and Pinus taeda L. (intolerant), are found co-occurring in the mixed pine-hardwood forests of the Piedmont region of the southeastern United States. When seedlings were grown in shaded conditions, photosynthetic CO₂ efficiency was significantly different in all species with the highest efficiency in the most shade tolerant species, Fagus grandifolia , and progressively lower efficiencies in moderately tolerant and intolerant species. Photosynthetic CO₂ efficiency was defined as the rate of increase in net photosynthesis with increase in ambient CO₂ concentration. When plants which had grown in a high light environment were tested, the moderately tolerant and intolerant deciduous species had the highest photosynthetic CO₂ efficiencies but this capacity was reduced when these species grew in low light. The lowest CO₂ efficiency and apparent quantum yield occurred in Pinus taeda in all cases. Water use efficiency was higher for all species in enriched CO₂ environments but transpiration rate and leaf conductance were not affected by CO₂ concentration. High photosynthetic CO₂ efficiency may be advantageous for maintaining a positive carbon balance in the low light environment under a forest canopy.     

Diurnal regulation of photosynthesis in understory saplings

Photosynthetic rates of plants grown in natural systems exhibit diurnal patterns often characterized by an afternoon decline, even when measured under constant light and temperature conditions. Since we thought changes in the carbohydrate status could cause this pattern through feedback from starch and sucrose synthesis, we studied the natural fluctuations in photosynthesis rates of plants grown at 36 and 56 Pa CO₂ at a FACE (free-air-CO₂-enrichment) research site. Light-saturated photosynthesis varied by 40% during the day and was independent of the light-limited quantum yield of photosynthesis, which varied little through the day. Photosynthesis did not correspond with xylem water potential or leaf carbohydrate build-up, but rather with diurnal changes in air vapor-pressure deficit and light. The afternoon decline in photosynthesis also corresponded with decreased stomatal conductance and decreased Rubisco carboxylation efficiency which in turn allowed leaf-airspace CO₂ partial pressure to remain constant. Growth at elevated CO₂ did not affect the afternoon decline in photosynthesis, but did stimulate early-morning photosynthesis rates relative to the rest of the day. Plants grown at 56 Pa CO₂ had higher light-limited quantum yields than those at 36 Pa CO₂ but, there was no growth–CO₂ effect on quantum yield when measured at 2 kPa O₂. Therefore, understory plants have a high light-limited quantum yield that does not vary through the day. Thus, the major diurnal changes in photosynthesis occur under light-saturated conditions which may help understory saplings maximize their sunfleck-use-efficiency.     

Influence of leaf chemistry of Lotus corniculatus (Fabaceae) on larval development of Polyommatus icarus (Lepidoptera, Lycaenidae): effects of elevated CO2 and plant genotype

1. Four Lotus corniculatus genotypes differing in cyanoglycoside and condensed tannin concentrations were grown in either low (350 ppm) or high (700 ppm) atmospheric CO₂ environments. Larval performance, consumption and conversion efficiency of Polyommatus icarus feeding on this plant material were measured.
2. Plants grown under elevated CO₂ contained less cyanoglycosides, more condensed tannins and more starch than control plants. However, water concentration, nitrogen and protein as well as nitrogen concentration in relation to carbon concentration did not differ between CO₂ treatments.
3. The four genotypes differed significantly in condensed tannins, cyanoglucoside, leaf water and leaf nitrogen but no genotype–CO₂ interaction was detected, except for total phenolics and condensed tannins in which two plant genotypes showed stronger increases under elevated CO₂ than the other two.
4. Larvae of P . icarus consumed more plant material and used and converted it more efficiently from plants grown at high atmospheric CO₂.
5. Larvae developed significantly faster and were significantly heavier when fed plant material grown under elevated CO₂. The observed difference in mass disappeared in the pupal and adult stages. However, lipid concentration of adults from the elevated CO₂ treatment was marginally significantly higher than of controls.
6. It is concluded that the higher carbohydrate concentration of L . corniculatus plants grown at elevated CO₂ renders leaves more suitable and better digestible to P . icarus . Furthermore, differences in allelochemicals might influence the palatability of L . corniculatus leaves for this specialist on Fabaceae.     

Effects of manganese toxicity on photosynthesis of white birch (Betula platyphylla var. japonica) seedlings

The effects of manganese (Mn) toxicity on photosynthesis in white birch ( Betula platyphylla var. japonica ) leaves were examined by the measurement of gas exchange and chlorophyll fluorescence in hydroponically cultured plants. The net photosynthetic rate at saturating light and ambient CO₂ (C_a) of 35 Pa decreased with increasing leaf Mn concentrations. The carboxylation efficiency, derived from the difference in CO₂ assimilation rate at intercellular CO₂ pressures attained at C_a of 13 Pa and O Pa, decreased with greater leaf Mn accumulation. Net photosynthetic rate at saturating light and saturating CO₂ (5%) also declined with leaf Mn accumulation while the maximum quantum yield of O₂ evolution at saturating CO₂ was not affected. The maximum efficiency of PSII photochemistry (F_v/F_m) was little affected by Mn accumulation in white birch leaves over a wide range of leaf Mn concentrations (2–17 mg g₋₁ dry weight). When measured in the steady state of photosynthesis under ambient air at 430 μmol quanta m₋₂ s₋₁, the levels of photochemical quenching (qP) and the excitation capture efficiency of open PSII (F'^v/F'^m) declined with Mn accumulation in leaves. The present results suggest that excess Mn in leaves affects the activities of the CO² reduction cycle rather than the potential efficiency of photochemistry, leading to increases in Q_A reduction state and thermal energy dissipation, and a decrease in quantum yield of PSII in the steady state.     

Effects of elevated temperature and carbon dioxide on the nutritional quality of leaves of oak (Quercus robur L.) as food for the Winter Moth (Operophtera brumata L.)

1. Pedunculate Oak trees were grown in ambient and elevated temperatures and CO₂. Leaves were fed to Winter Moth caterpillars reared either in constant conditions or with the trees (caged or on-tree).
2. Caterpillars in constant conditions ate the same mass and produced the same mass of faeces whether fed elevated or ambient temperature leaves. However, less was assimilated from elevated leaves, resulting in lighter pupae and fewer, lighter eggs.
3. Caterpillars in constant conditions ate more and produced more faeces when fed elevated CO₂ leaves than when fed ambient CO₂ leaves, but the mass assimilated and pupal mass were unchanged.
4. Caged caterpillars reared with the trees from which they were fed had constant pupal mass in all treatments, but pupated earlier at elevated temperature. Pupal mass was also unaffected when caterpillars fed on the trees.
5. Nitrogen was reduced in both elevated temperature and elevated CO₂ leaves. Increased fibre in the former prevented increased consumption and resulted in reduced pupal mass and fecundity. Reduced fibre in the latter allowed increased consumption, resulting in pupae of normal mass.
6. Despite the clear effect of nutrient quality, experiments rearing caterpillars and trees together suggest that anticipated climatic change will have no nutritional effect on Winter Moth development.     

Stomatal and photosynthetic responses to variable sunlight

Most plants experience many fluctuations in sunlight from full sun to shade throughout the day. Under these conditions, stomatal and photosynthetic responses vary dramatically among species depending on water status and growth form. Many herbaceous, fast-growing species rapidly reduce stomatal opening during short-term shade periods. Rapid stomatal closure during shade conserves water, but may also reduce CO₂ uptake. Because periods of alternating sun and shade can reduce accumulative water stress that would otherwise severely curtail carbon gain, some herbs are restricted to habitats with intermittent periods of shade. In contrast to herbaceous growth forms, woody species maintain relatively constant stomatal opening during both sun and shade periods. This results in greater CO₂ uptake, but with greater water loss. These two generalized response patterns for woody and herbaceous species to natural variations in sunlight conflict with conventional ideas of water use and carbon gain based on measurements made under constant light.     

Different regulation of leaf respiration between Spinacia oleracea, a sun species, and Alocasia odora, a shade species

Mature leaves of shade species exhibit lower respiratory rates than those of sun species. To elucidate the mechanism underlying different respiratory rates between sun and shade species, we examined respiratory properties of leaves in Spinacia oleracea L., a sun species, and Alocasia odora (Lodd.) Spach, a shade species, with special reference to changes in the respiratory rate throughout the night. In S. oleracea , rates of both CO₂ efflux and O₂ uptake decreased with time during the night, whereas in A. odora both rates were virtually constant at lower levels. The rates of O₂ uptake in S . oleracea increased upon addition of sucrose, and the rates attained were virtually identical throughout the night. However, the addition of an uncoupler [carbonyl cyanide p -(trifluoromethoxy)-phenylhydrazone; FCCP] did not alter the rates. In contrast, the rates of O₂ uptake in A. odora were enhanced by the addition of FCCP, but not by sucrose. The concentrations of carbohydrates in the tissue decreased throughout the night in both species and the ATP/ADP ratio was always greater in A. odora. These results indicate that, in S. oleracea , the availability of respiratory substrate determines the respiratory rate, while the low respiratory rate in A. odora is ascribed to its low demand for ATP.