Temperature and Water Relations for Sun and Shade Leaves of a Desert Broadleaf, Hyptis emoryi

The temperature and water relations of sun versus shade leavesof Hyptis emoryi Torr. were evaluated from field measurementsmade in late summer. Throughout most of the day sun leaves hadhigher temperatures and higher resistances to water vapour diffusion,but lower transpiration rates and lower stem water potentials,than did shade leaves. Leaf absorptivity to solar irradiationwas less for 1.5-cm-long sun leaves (0.44) than for 4.0-cm shadeleaves (0.56). For both leaf types the stomatal resistance increasedas the water vapour concentration drop from the leaf to theair increased. Energy balance equations were used together with the measuredtemperature dependence of photosynthesis to predict the effectof variations in leaf absorptivity, length, and resistance onnet photosynthesis. The influence of leaf dimorphism on wholeplants was determined by calculating daily photosynthesis andtranspiration for plants with various percentages of sun andshade leaves. A hypothetical plant with all sun leaves in thesun had about twice the photosynthesis and half the transpirationratio as did plants with sun leaves in the shade or shade leavesin the sun or shade. Plants with both sun and shade leaves hadthe highest predicted photosynthesis per unit ground area. Thepossible adaptive significance of the seasonal variation insun and shade leaf percentages observed for individual H. emoryibushes is discussed in terms of water economy and photosynthesi     

Relation between Mesophyll Surface Area, Photosynthetic Rate, and Illumination Level during Development for Leaves of Plectranthus parviflorus Henckel

The influence of illumination level during leaf development on the mesophyll cell surface area per unit leaf area (A^mes/A), CO₂ resistances, and the photosynthetic rate was determined for leaves of Plectranthus parviflorus Henckel. The relative importance of A^mes/A versus CO₂ resistances in accounting for observed changes in photosynthesis was quantitatively evaluated using equations based on analogies to electrical circuits.     

Photosynthesis and Carbon Partitioning in Transgenic Tobacco Plants Deficient in Leaf Cytosolic Pyruvate Kinase

Whole-plant diurnal C exchange analysis provided a noninvasive estimation of daily net C gain in transgenic tobacco (Nicotiana tabacum L.) plants deficient in leaf cytosolic pyruvate kinase (PK_c−). PK_c− plants cultivated under a low light intensity (100 μmol m⁻² s⁻¹) were previously shown to exhibit markedly reduced root growth, as well as delayed shoot and flower development when compared with plants having wild-type levels of PK_c (PK_c+). PK_c− and PK_c+ source leaves showed a similar net C gain, photosynthesis over a range of light intensities, and a capacity to export newly fixed ¹⁴CO₂ during photosynthesis. However, during growth under low light the nighttime, export of previously fixed ¹⁴CO₂ by fully expanded PK_c− leaves was 40% lower, whereas concurrent respiratory ¹⁴CO₂ evolution was 40% higher than that of PK_c+ leaves. This provides a rationale for the reduced root growth of the PK_c− plants grown at low irradiance. Leaf photosynthetic and export characteristics in PK_c− and PK_c+ plants raised in a greenhouse during winter months resembled those of plants grown in chambers at low irradiance. The data suggest that PK_c in source leaves has a critical role in regulating nighttime respiration particularly when the available pool of photoassimilates for export and leaf respiratory processes are low.     

Determination of mesophyll diffusion resistance in Chamaerion angustifolium by the method of three-dimensional reconstruction of the leaf cell packing

A detailed quantitative analysis of the three-dimensional organization of the mesophyll was performed, and mesophyll diffusion resistance to CO₂ in the leaves of Chamaerion angustifolium formed under different irradiance was calculated using an original method of stereometric cellular packing. For each type of leaves (sun and shade), we determined structural components of gas exchange: the volume of mesophyll per unit leaf area (V _mes), the volume of the intercellular space in the mesophyll (V _is), the area of the total mesophyll surface (S), the area of the free mesophyll surface facing the intercellular spaces (S _mes), and the ratios of the total and the free mesophyll surfaces to its volume (S/V and S _mes/V). As compared with sun leaves, in the shade leaves of Ch. angustifolium, S and V _mes decreased twofold, tissue density was reduced twofold, and the share of the intercellular space in the mesophyll rose from 49 to 72%. In shade, the diffusion resistance of the mesophyll increased by 1.8 times because of changes in the leaf structure. At the same time, the ratio S _mes/V was found to increase by 1.4 times, which facilitated the diffusion of CO₂. In the shade leaves of Ch. angustifolium, the diffusion resistance of the intercellular air spaces was reduced twofold as a result of an increase in their share in the leaf mesophyll and simplification of their geometry. Thus, the method of three-dimensional reconstruction of sun and shade leaves of Ch. angustifolium showed a comprehensive rearrangement of the mesophyll spatial organization in shade and revealed the structural mechanisms of changes in the resistance to CO₂ diffusion within the leaf.     

Effects of magnesium deficiency on the photosynthesis and respiration of leaves of sugar beet

The effects of Mg deficiency on the photosynthesis and respiration of sugar beets (Beta vulgaris L. cv. F58-554H1) were studied by withholding Mg from the culture solution and by following changes in CO₂ and water vapor exchange of attached leaves. Leaf blade Mg concentration decreased from about 1200 to less than 200 meq kg⁻¹ dry matter without change in the rate of photosynthetic CO₂ uptake per unit leaf area, while from 200 to 50 meq kg⁻¹ the rate decreased to one-third. Rates of photorespiratory evolution of CO₂ into CO₂-free air responded to Mg like those of photosynthetic CO₂ uptake, the rates decreasing to one-half, below 200 meq kg⁻¹. Respiratory CO₂ evolution in the dark increased almost 2-fold in low Mg leaves. Magnesium deficiency had less effect on leaf (mainly stomatal) diffusion resistance (r₁) than on mesophyll resistance (r_m); in Mg-deficient plants r_m increased from 2.9 to 7.1 sec cm⁻¹, whereas r₁ became significantly greater than the control value only in the most severe instances of Mg deficiency.     

Comparison of Fungal Activities on Wood and Leaf Litter in Unaltered and Nutrient-Enriched Headwater Streams

Fungi are the dominant organisms decomposing leaf litter in streams and mediating energy transfer to other trophic levels. However, less is known about their role in decomposing submerged wood. This study provides the first estimates of fungal production on wood and compares the importance of fungi in the decomposition of submerged wood versus that of leaves at the ecosystem scale. We determined fungal biomass (ergosterol) and activity associated with randomly collected small wood (<40 mm diameter) and leaves in two southern Appalachian streams (reference and nutrient enriched) over an annual cycle. Fungal production (from rates of radiolabeled acetate incorporation into ergosterol) and microbial respiration on wood (per gram of detrital C) were about an order of magnitude lower than those on leaves. Microbial activity (per gram of C) was significantly higher in the nutrient-enriched stream. Despite a standing crop of wood two to three times higher than that of leaves in both streams, fungal production on an areal basis was lower on wood than on leaves (4.3 and 15.8 g C m⁻² year⁻¹ in the reference stream; 5.5 and 33.1 g C m⁻² year⁻¹ in the enriched stream). However, since the annual input of wood was five times lower than that of leaves, the proportion of organic matter input directly assimilated by fungi was comparable for these substrates (15.4 [wood] and 11.3% [leaves] in the reference stream; 20.0 [wood] and 20.2% [leaves] in the enriched stream). Despite a significantly lower fungal activity on wood than on leaves (per gram of detrital C), fungi can be equally important in processing both leaves and wood in streams.     

Effects of Light, Carbon Dioxide, and Temperature on Photosynthesis, Oxygen Inhibition of Photosynthesis, and Transpiration in Solanum tuberosum

Individual leaves of potato (Solanum tuberosum L. W729R), a C₃ plant, were subjected to various irradiances (400-700 nm), CO₂ levels, and temperatures in a controlled-environment chamber. As irradiance increased, stomatal and mesophyll resistance exerted a strong and some-what paralleled regulation of photosynthesis as both showed a similar decrease reaching a minimum at about 85 neinsteins·cm⁻²·sec⁻¹ (about ½ of full sunlight). Also, there was a proportional hyperbolic increase in transpiration and photosynthesis with increasing irradiance up to 85 neinsteins·cm⁻²·sec⁻¹. These results contrast with many C₃ plants that have a near full opening of stomata at much less light than is required for saturation of photosynthesis.     

INFLUENCE OF IRRADIATION,SOIL WATER POTENTIAL,AND LEAF TEMPERATURE ON LEAF MORPHOLOGY OF A DESERT BROADLEAF,ENCELIA FARINOSA GRAY (COMPOSITAE)

Laboratory experiments were performed to evaluate observed seasonal changes in leaf morphology of the desert perennial shrub, Encelia farinosa Gray. Plants were grown under low or high conditions of photosynthetically active irradiation, soil water potential (Ψ^soil), and leaf temperature (8 different experimental regimes). The relative growth rate, leaf water vapor conductance, leaf water potential, and leaf length were all greater for the high Ψ^soil regimes, the largest leaves occurring at low irradiation. High irradiation during growth led to thicker leaves with a higher internal to external leaf area ratio (A^mes/A); low Ψ^soil tended to increase A^mes/A somewhat. High irradiation also led to decreased absorptance to solar irradiation caused by increased pubescence. High leaf temperature during development resulted in slightly smaller, thicker leaves with higher A^mes/A. Thus, irradiation appeared to have its major influence on leaf thickness, A^mes/A, and absorptance, with a secondary effect on leaf length; Ψ^soil affected primarily leaf length, growth rate, and water status, and secondarily A^mes/A. Results are discussed with regard to recent ecophysiological studies on the observed seasonal changes in leaf morphology of E. farinosa.     

Do fruit sunburn control measures affect leaf photosynthetic rate and stomatal conductance in ‘Royal Gala’ apple?

A field study was conducted on a 5-year-old orchard of ‘Royal Gala’ apple (Malus domestica Borkh.) in Stellenbosch, South Africa, to investigate whether the measures employed to control sunburn in fruit, viz., evaporative cooling, Surround WP and 20% black shade net affect leaf photosynthetic gas exchange attributes in comparison to untreated control during the 2003/2004 season. Shade net significantly reduced midday leaf net photosynthetic rate (A) compared to evaporative cooling. Furthermore, shade net and Surround WP significantly reduced midday leaf stomatal conductance (g_s) compared to evaporative cooling and control. Evaporative cooling increased light saturated photosynthetic rate by 27 and 24% compared to shade net and Surround WP, respectively. Light compensation point and dark respiration of shaded leaves were about a third of the other treatments and about 50% less than the control leaves, respectively. Shade net down-regulated photosynthetic capacity of the leaves as evidenced by lower maximum rate of carboxylation and light saturated rate of electron transport compared to control leaves. Sunburn control treatments reduced day respiration by 60–70% compared to the control. Response of A and g_s to increasing temperature showed only slight increase in both A and g_s with increasing temperature from 20 to 30 °C. A declined at 35 °C in Surround WP and shade net leaves while it declined at 40 °C in evaporatively cooled and control leaves. Evaporative cooling and control had higher g_s than shade net and Surround WP at all leaf temperatures. In conclusion, shade net down-regulated photosynthetic reactions and Surround WP and shade net reduced leaf g_s and increased the vulnerability of leaf A and g_s to high temperature compared to evaporative cooling and control.     

Water Relations and Photosynthesis of a Desert CAM Plant, Agave deserti

The water relations and photosynthesis of Agave deserti Engelm., a plant exhibiting Crassulacean acid metabolism, were measured in the Colorado desert. Although no natural stomatal opening of A. deserti occurred in the summer of 1975, it could be induced by watering. The resistance for water vapor diffusion from a leaf (R_WV) became less than 20 sec cm⁻¹ when the soil water potential at 10 cm became greater than −3 bars, as would occur after a 7-mm rainfall. As a consequence of its shallow root system (mean depth of 8 cm), A. deserti responded rapidly to the infrequent rains, and the succulent nature of its leaves allowed stomatal opening to continue for up to 8 days after the soil became drier than the plant. When the leaf temperature at night was increased from 5 to 20 C, R_WV increased 5-fold, emphasizing the importance of cool nighttime temperatures for gas exchange by this plant. Although most CO₂ uptake occurred at night, a secondary light-dependent rise in CO₂ influx generally occurred after dawn. The transpiration ratio (mass of water transpired/mass of CO₂ fixed) had extremely low values of 18 for a winter day, and approximately 25 for an entire year.     

Involvement of Abscisic Acid in Regulating Water Status in Phaseolus vulgaris L. during Chilling

During the first hours of chilling, bean (Phaseolus vulgaris L., cv Mondragone) seedlings suffer severe water stress and wilt without any significant increase in leaf abscisic acid (ABA) content (P. Vernieri, A. Pardossi, F. Tognoni [1991] Aust J Plant Physiol 18: 25-35). Plants regain turgor after 30 to 40 h. We hypothesized that inability to rapidly synthesize ABA at low temperatures contributes to chilling-induced water stress and that turgor recovery after 30 to 40 h is mediated by changes in endogenous ABA content. Entire bean seedlings were subjected to long-term (up to 6 d) chilling (3°C, 0.2-0.4 kPa vapor pressure deficit, 100 μmol·m⁻²·s⁻¹ photosynthetic photon flux density, continuous fluorescent light). During the first 24 h, stomata remained open, and plants rapidly wilted as leaf transpiration exceeded root water absorption. During this phase, ABA did not accumulate in leaves or in roots. After 24 h, ABA content increased in both tissues, leaf diffusion resistance increased, and plants rehydrated and regained turgor. No osmotic adjustment was associated with turgor recovery. Following turgor recovery, stomata remained closed, and ABA levels in both roots and leaves were elevated compared with controls. The application of ABA (0.1 mm) to the root system of the plants throughout exposure to 3°C prevented the chilling-induced water stress. Excised leaves fed 0.1 mm ABA via the transpiration stream had greater leaf diffusion resistance at 20 and 3°C compared with non-ABA fed controls, but the amount of ABA needed to elicit a given degree of stomatal closure was higher at 3°C compared with 20°C. These findings suggest that endogenous ABA may play a role in ameliorating plant water status during chilling.     

Effects of Phenylmercuric Acetate on Stomatal Movement and Transpiration of Excised Retula papyrifera Marsh. Leaves

Effects of 10⁻³m, 10⁻⁴m, and 10⁻⁵m phenylmercuric acetate (PMA) on stomatal movement and transpiration of excised Betula papyrifera leaves were investigated. Duco cement leaf prints and transpiration decline curves were used for the analysis of stomatal condition. PMA induced stomatal closure and decreased transpiration. Stomata of leaves treated with any of the 3 PMA concentrations closed earlier and at a higher relative water content than did stomata of untreated leaves. As determined from transpiration decline curves, PMA at 10⁻³m caused an increase in apparent “cuticular” transpiration. However, the increase appeared to result largely from some PMA-poisoned stomata which remained open for prolonged periods. Considerable PMA toxicity was observed, with 10⁻³m and 10⁻⁴m concentrations causing browning of leaves. PMA treatment caused a decrease in chlorophyll content, even at a low PMA concentration (10⁻⁵m) which influenced stomatal response only slightly and did not cause evident browning of leaves. The time and degree of stomatal opening varied with stomatal size. Large stomata tended to open earlier and close later than small stomata. Hence, in Betula papyrifera stomata of various size classes were considered as physiologically different populations.     

Carbon gain and photosynthetic response of chrysanthemum to photosynthetic photon flux density cycles

Most models of carbon gain as a function of photosynthetic irradiance assume an instantaneous response to increases and decreases in irradiance. High- and low-light-grown plants differ, however, in the time required to adjust to increases and decreases in irradiance. In this study the response to a series of increases and decreases in irradiance was observed in Chrysanthemum × morifolium Ramat. “Fiesta” and compared with calculated values assuming an instantaneous response. There were significant differences between high- and low-light-grown plants in their photosynthetic response to four sequential photosynthetic photon flux density (PPFD) cycles consisting of 5-minute exposures to 200 and 400 micromoles per square meter per second (μmol m⁻²s⁻¹). The CO₂ assimilation rate of high-light-grown plants at the cycle peak increased throughout the PPFD sequence, but the rate of increase was similar to the increase in CO₂ assimilation rate observed under continuous high-light conditions. Low-light leaves showed more variability in their response to light cycles with no significant increase in CO₂ assimilation rate at the cycle peak during sequential cycles. Carbon gain and deviations from actual values (percentage carbon gain over- or underestimation) based on assumptions of instantaneous response were compared under continuous and cyclic light conditions. The percentage carbon gain overestimation depended on the PPFD step size and growth light level of the leaf. When leaves were exposed to a large PPFD increase, the carbon gain was overestimated by 16 to 26%. The photosynthetic response to 100 μmol m⁻² s⁻¹ PPFD increases and decreases was rapid, and the small overestimation of the predicted carbon gain, observed during photosynthetic induction, was almost entirely negated by the carbon gain underestimation observed after a decrease. If the PPFD cycle was 200 or 400 μmol m⁻² s⁻¹, high- and low-light leaves showed a carbon gain overestimation of 25% that was not negated by the underestimation observed after a light decrease. When leaves were exposed to sequential PPFD cycles (200-400 μmol m⁻² s⁻¹), carbon gain did not differ from leaves exposed to a single PPFD cycle of identical irradiance integral that had the same step size (200-400-200 μmol m⁻² s⁻¹) or mean irradiance (200-300-200 μmol m⁻² s⁻¹).     

Effect of cold hardening on sensitivity of winter and spring wheat leaves to short-term photoinhibition and recovery of photosynthesis

Photoinhibition of photosynthesis and its recovery were studied in wheat (Triticum aestivum L.) leaves grown at nonhardening (20°C) and cold-hardening (5°C) temperatures. Cold-hardened wheat leaves were less susceptible to photoinhibition at 5°C than nonhardened leaves, and the winter cultivars, Kharkov and Monopol, were less susceptible than the spring cultivar, Glenlea. The presence of chloramphenicol, a chloroplastic protein synthesis inhibitor, increased the susceptibility to photoinhibition, but cold-hardened leaves still remained less susceptible to photoinhibition than nonhardened leaves. Recovery at 50 μmol m⁻² s⁻¹ photosynthetic photon flux density and 20°C was at least biphasic, with a fast and a slow phase in all cultivars. Cold-hardened leaves recovered maximum fluorescence and maximum variable fluorescence in the dark-adapted state during the fast phase at a rate of 42% h⁻¹ compared with 22% h⁻¹ for nonhardened leaves. The slow phase occurred at similar rates (2% h⁻¹) in cold-hardened and nonhardened leaves. Full recovery required up to 30 h. Fast-recovery phase was not reduced by either lowering the recovery temperature to 5°C or by the presence of chloramphenicol. Slow-recovery phase was inhibited by both treatments. Hence, the fast phase of recovery does not require de novo chloroplast protein synthesis. In addition, only approximately 60% of the photochemical efficiency lost through photoinhibition at 5°C was associated with lost [¹⁴C]atrazine binding and, hence, with damage to the secondary quinone electron acceptor for photosystem II-binding site. We conclude that the decrease in susceptibility to photoinhibition exhibited following cold hardening of winter and spring cultivars is not due to an increased capacity for repair of photoinhibitory damage at 5°C but reflects intrinsic properties of the cold-hardened photosynthetic apparatus. A model to account for the fast component of recovery is discussed.     

Characterization of a Manganese Superoxide Dismutase from the Higher Plant Pisum sativum

A manganese-containing superoxide dismutase (EC 1.15.1.1) was fully characterized from leaves of the higher plant Pisum sativum L., var. Lincoln. The amino acid composition determined for the enzyme was compared with that of a wide spectrum of superoxide dismutases and found to have a highest degree of homology with the mitochondrial manganese superoxide dismutases from rat liver and yeast. The enzyme showed an apparent pH optimum of 8.6 and at 25°C had a maximum stability at alkaline pH values. By kinetic competition experiments, the rate constant for the disproportionation of superoxide radicals by pea leaf manganese superoxide dismutase was found to be 1.61 × 10⁹ molar⁻¹·second⁻¹ at pH 7.8 and 25°C. The enzyme was not sensitive to NaCN or to H₂O₂, but was inhibited by N₃⁻. The sulfhydryl reagent p-hydroxymercuribenzoate at 1 mm concentration produced a nearly complete inhibition of the manganese superoxide dismutase activity. The metal chelators o-phenanthroline, EDTA, and diethyldithiocarbamate all inhibited activity slightly in decreasing order of intensity. A comparative study between this higher plant manganese superoxide dismutase and other dismutases from different origins is presented.     

Adenosine 5'-triphosphate-sulfurylase in corn roots and its partial purification

ATP-sulfurylase (ATP-sulfate adenyltransferase, EC 2.7.7.4) was found in nonparticulate fractions of both roots and leaves of Zea mays L. seedlings using two detection methods. Addition of exogenous pyrophosphatase was essential for maximum rates of conversion of ³⁵SO₄²⁻ to labeled adenosine phosphosulfate in unpurified root extracts, but not in unpurified leaf extracts. In the presence of exogenous pyrophosphatase, the enzyme from roots exhibited specific activities as high as those obtained with the leaf enzyme. The root enzyme was purified 33-fold by centrifugation and column chromatography procedures. Its molecular weight obtained by Sephadex gel filtration was about 42,000. Its Km for pyrophosphate was 7 μm, while for adenosine phosphosulfate, the Km was 1.35 μm. None of the enzyme fractions studied converted adenosine phosphosulfate into detectable amounts of 3′-phosphoadenosine-5′-phosphosulfate. ATP-sulfurylase was also found in roots of corn seedlings grown aseptically. The data suggest that at least the first reaction in sulfate reduction might proceed as effectively in roots as in shoots.     

Influence of Benzyladenine, Leaf Darkening, and Ringing on Movement of C-labeled Assimilates Into Expanded Leaves of Vitis vinifera L

Leaves of Vitis vinifera L., nearly fully expanded, imported only trace amounts of ¹⁴C following assimilation of ¹⁴CO₂ by a lower leaf on the same shoot, but benzyladenine (BA) application at 4.4 × 10⁻³m caused a marked increase in the movement of ¹⁴C into these leaves. Older leaves near the shoot base were less responsive; BA treatment alone had little effect on import of labeled assimilates from adjacent leaves but when the BA-treated leaves were darkened there was an increased import of labeled materials. When these 2 treatments were combined and applied to leaves on shoots with ringed bases, relatively high levels of radioactivity were detected in the BA-treated leaves but under these conditions darkening, without the application of BA, also resulted in an increased import of ¹⁴C. Accumulation of imported ¹⁴C was found to be restricted to the area of the leaf blade treated with BA. Separation of labeled compounds in ethanol extracts of treated leaves showed a lower percentage of radioactivity present in the sugar fraction from BA-treated leaves and an increased percentage present in the amino acid fraction.     

Surface properties of right side-out plasma membrane vesicles isolated from barley roots and leaves

Highly purified plasma membrane vesicles were obtained from roots and leaves of 7-day-old light-grown barley (Hordeum vulgare L. cv Kristina) seedlings by partitioning of crude microsomal fractions in a dextran-polyethylene glycol two-phase system. Sodium dodecylsulfate polyacrylamide gel electrophoresis showed the polypeptide composition of plasma membranes from the two organs to be qualitatively similar, but with different relative amounts of some of the polypeptides. Between 80 and 100% of the K⁺,Mg²⁺-ATPase activity was latent indicating that the vesicles were sealed and right side-out. The isoelectric points of the outer surface of root and leaf plasma membranes as determined by cross-partitioning were similar and quite acidic—about pH 3.6. In contrast, the net negative surface charge density at pH 7.0 as measured by 9-aminoacridine fluorescence differed significantly, being −29 mC·m⁻² for the leaf plasma membrane and only −19 mC·m⁻² for the root plasma membrane. As isolated, both types of plasma membrane vesicles had Ca²⁺ and Mg²⁺ bound to the outer surface as shown by the combined use of chelators and 9-aminoacridine fluorescence; however, the leaf plasma membrane had a relatively higher proportion of Ca²⁺ bound (0.57) than did the root plasma membrane (0.45). This difference probably reflects differences in the in vivo conditions as no chelator was present during the isolation procedure. Also Ni²⁺ could bind to the root vesicles as indicated by the effect of Ni²⁺ on 9-aminoacridine fluorescence, and by the binding of ⁶³Ni²⁺ (44 nanomoles bound per milligram protein) at 100 micromolar NiCl₂.     

Influence of differences in leaf anatomy on net photosynthetic rates of some cultivars of Cassava

Gas exchange measurements and leaf anatomy of 10 cassava cultivars were conducted to study the interrelationship between the relatively high photosynthetic rates and the factors limiting internal CO₂ diffusion. The internal mesophyll surface area per unit leaf surface area (A^mes/A) and the intracellular components of CO₂ diffusion and fixation resistance (R^cellCO₂) were determined. Among the group of cultivars tested net CO₂ exchange rates were 26±2.5 mol CO₂ m^–2 s^–1 in normal air and intense light and A^mes/A ranged from 14 to 38. Estimated R^cellCO₂ ranged from 4300 to 13,000 s m^–1. The combined and compensating effects of A^mes/A and R^cellCO₂ accounted for both the high net photosynthetic rates (P_n) and the lack of large differences in P_n among cultivars.     

Contributions of diffusional limitation, photoinhibition and photorespiration to midday depression of photosynthesis in Arisaema heterophyllum in natural high light

Diurnal changes in photosynthetic gas exchange and chlorophyll fluorescence were measured under full sunlight to reveal diffusional and non‐diffusional limitations to diurnal assimilation in leaves of Arisaema heterophyllum Blume plants grown either in a riparian forest understorey (shade leaves) or in an adjacent deforested open site (sun leaves). Midday depressions of assimilation rate (A) and leaf conductance of water vapour were remarkably deeper in shade leaves than in sun leaves. To evaluate the diffusional (i.e. stomatal and leaf internal) limitation to assimilation, we used an index [1–A/A₃₅₀], in which A₃₅₀ is A at a chloroplast CO₂ concentration of 350 μ mol mol ^{? 1}. A₃₅₀ was estimated from the electron transport rate (J_T), determined fluorometrically, and the specificity factor of Rubisco (S), determined by gas exchange techniques. In sun leaves under saturating light, the index obtained after the ‘peak’ of diurnal assimilation was 70% greater than that obtained before the ‘peak’, but in shade leaves, it was only 20% greater. The photochemical efficiency of photosystem II ( Δ F/F_m ′ ) and thus J_T was considerably lower in shade leaves than in sun leaves, especially after the ‘peak’. In shade leaves but not in sun leaves, A at a photosynthetically active photon flux density (PPFD) > 500 μ mol m ^{? 2} s ^{? 1} depended positively on J_T throughout the day. Electron flows used by the carboxylation and oxygenation (J_O) of RuBP were estimated from A and J_T. In sun leaves, the J_O/J_T ratio was significantly higher after the ‘peak’, but little difference was found in shade leaves. Photorespiratory CO₂ efflux in the absence of atmospheric CO₂ was about three times higher in sun leaves than in shade leaves. We attribute the midday depression of assimilation in sun leaves to the increased rate of photorespiration caused by stomatal closure, and that in shade leaves to severe photoinhibition. Thus, for sun leaves, increased capacities for photorespiration and non‐photochemical quenching are essential to avoid photoinhibitory damage and to tolerate high leaf temperatures and water stress under excess light. The increased Rubisco content in sun leaves, which has been recognized as raising photosynthetic assimilation capacity, also contributes to increase in the capacity for photorespiration.