Photosynthesis Inhibition During Gas Exchange Oscillations in ABA-Treated Helianthus annuus: Relative Role of Stomatal Patchiness and Leaf Carboxylation Capacity

Environmental factors that induce spatial heterogeneity of stomatal conductance, g _s, called stomatal patchiness, also reduce the photochemical capacity of CO₂ fixation, yet current methods cannot distinguish between the relative effect of stomatal patchiness and biochemical limitations on photosynthetic capacity. We evaluate effects of stomatal patchiness and the biochemical capacity of CO₂ fixation on the sensitivity of net photosynthetic rate (P _N) to stomatal conductance (g _s), θ (θ = δP _N/g _s). A qualitative model shows that stomatal patchiness increases the sensitivity θ while reduced biochemical capacity of CO₂ fixation lowers θ. We used this feature to distinguish between stomatal patchiness and mesophyll impairments in the photochemistry of CO₂ fixation. We compared gas exchange of sunflower (Helianthus annuus L.) plants grown in a growth chamber and fed abscisic acid, ABA (10⁻⁵ M), for 10 d with control plants (-ABA). P _N and g _s oscillated more frequently in ABA-treated than in control plants when the leaves were placed into the leaf chamber and exposed to a dry atmosphere. When compared with the initial CO₂ response measured at the beginning of the treatment (day zero), both ABA and control leaves showed reduced P _N at particular sub-stomatal CO₂ concentration (c _i) during the oscillations. A lower reduction of P _N at particular g _s indicated overestimation of c _i due to stomatal patchiness and/or omitted cuticular conductance, g _c. The initial period of damp oscillation was characterised by inhibition of chloroplast processes while stomatal patchiness prevailed at the steady state of gas exchange. The sensitivity θ remained at the original pre-treatment values at high g _s in both ABA and control plants. At low g _s, θ decreased in ABA-treated plants indicating an ABA-induced impairment of chloroplast processes. In control plants, g _c neglected in the calculation of g _s was the likely reason for apparent depression of photosynthesis at low g _s. This revised version was published online in August 2006 with corrections to the Cover Date.     

RESPONSE OF BUSH BEAN EXPOSED TO ACID MIST

Bush bean plants (Phaseolus vulgaris L. cv. Contender) were treated once a week for six weeks with simulated acid mist at five pH levels ranging from 5.5 to 2.0. Leaf injury developed on plants exposed to acid concentrations below pH 3 and many leaves developed a flecking symptom similar to that caused by ozone. An adaxial, interveinal bleached area resembling SO₂ injury also developed on some trifoliate leaves at the low pH treatments. Microscopic observation of injured trifoliates indicated that the palisade cells were plasmolyzed and that the chloroplasts lost structural integrity. Reductions in plant weight and chlorophyll content were detected across the pH gradient. Seed and pod growth were reduced at some intermediate acid depositions even though no visible foliar injury developed. Foliar losses of nitrogen, calcium, magnesium and phosphorous increased with decreases in acid mist pH, whereas foliar potassium concentrations were unaffected by acid mist treatment.     

On the relationship between leaf anatomy and CO2 diffusion through the mesophyll of hypostomatous leaves

Internal conductances to CO₂ transfer from the stomatal cavity to sites of carboxylation (g_i) in hypostomatous sun-and shade-grown leaves of citrus, peach and Macadamia trees (Lloyd et al. 1992) were related to anatomical characteristics of mesophyll tissues. There was a consistent relationship between absorptance of photosynthetically active radiation and chlorophyll concentration (mmol m^?2) for all leaves, including sclerophyllous Macadamia, whose transmittance was high despite its relatively thick leaves. In thin peach leaves, which had high g_i, the chloro-plast volume and mesophyll surface area exposed to intercellular air spaces (ias) per unit leaf area were similar to those in the thicker leaves of the evergreen species. Peach leaves, however, had the lowest leaf dry weight per area (D/a), the lowest tissue density (T_d) and the highest chloro-plast surface area (S_c) exposed to ias. There were negative correlations between g_i and leaf thickness or D/a, but positive correlations between g_i and S_c or S_c/T_d. We developed a one-dimensional diffusion model which partitioned g_i into a gaseous diffusion conductance through the ias (g_ias) plus a liquid-phase conductance through mesophyll cell walls (g_cw). The model accounted for a significant amount of variation (r₂=0.80) in measured g_i by incorporating both components. The g_ias component was related to the one-dimensional path-length for diffusion across the mesophyll and so was greater in thinner peach leaves than in leaves of evergreen species. The g_cw component was related to tissue density and to the degree of chloroplast exposure to the ias. Thus the negative correlations between g_i and leaf thickness or D/a related to g_ias whereas positive correlations between g_i and S_c or S_c/T_d, related to g_cw. The g_cw was consistently lower than g_ias, and thus represented a greater constraint on CO₂ diffusion in the mesophylls of these hypostomatous species.     

Regulation of transpiration in coffee hedgerows: covariation of environmental variables and apparent responses of stomata to wind and humidity

Stomatal regulation of transpiration was studied in hedgerow coffee (Coffea arabica L.) at different stages of canopy development encompassing a range of leaf area indices (L) from 0·7 to 6·7. Stomatal (g_s) and crown (g_c) conductance attained maximum values early during the day and then declined as both leaf-to-bulk air water vapour mole fraction difference (V_a) and photosynthetically active photon flux density (I) continued to increase. Covariation of environmental variables during the day, particularly V, I, and wind speed (u), obscured stomatal responses to individual variables. This also caused diurnal hysteresis in the relationship between g_c and individual variables. Normalization of g_s and g_c by I removed the hysteresis and revealed a strong stomatal response to humidity. At the crown scale, transpiration (E) increased linearly with net radiation (R_n) and seemed to increase with increasing wind speed. Increasing wind speed imposed higher leaf interior to leaf surface water vapour mole fraction differences (V_s) at given levels of V_a. However, strong relationships between declining g_c and E and increasing wind speed were obtained when g_c and E were normalized by I and R_n, respectively, without invoking additional potential interactions involving temperature or CO₂ concentration at the leaf surface. Apparent stomatal responses to wind were thus at least partially a reflection of the stomatal response to humidity.     

Determinant of photosynthetic capacity in rice leaves under ambient air conditions

At the grain-filling stage, net photosynthetic rate (P _N), stomatal conductance (g _s), and ribulose-1,5-bisphosphate carboxylation efficiency (CE) were correlated in order to find the determinant of photosynthetic capacity in rice leaves. For a flag leaf, P _N in leaf middle region was higher than in its upper region, and leaf basal region had the lowest P _N value. The differences in g _s and CE were similar. P _N, g _s, and CE gradually declined from upper to basal leaves, showing a leaf position gradient. The correlation coefficient between P _N and CE was much higher than that between P _N and g _s in both cases, and P _N was negatively correlated with intercellular CO₂ concentration (C _i). Hence the carboxylation activity or activated amount of ribulose-1,5-bisphosphate carboxylase/oxygenase rather than g_s was the determinant of the photosynthetic capacity in rice leaves. In addition, in flag leaves of different tillers P _N was positively correlated with g _s, but negatively correlated with C _i. Thus g _s is not the determinant of the photosynthetic capacity in rice leaves.The study was supported by the State Key Basic Research and Development Plan (No.G1998010100).     

Comparison of Whole System of Carbon and Nitrogen Accumulation Between Two Maize Hybrids Differing in Leaf Senescence

A field experiment was conducted to investigate the carbon (C) and nitrogen (N) balance in relation to grain formation and leaf senescence in two different senescent types of maize (Zea mays L.), one stay-green (cv. P3845) and one earlier senescent (cv. Hokkou 55). In comparison with Hokkou 55, P3845 had a higher N concentration (N_c) in the leaves and a higher specific N absorption rate by roots (SAR_N), which indicated that a large amount of N was supplied to the leaves from the roots during maturation. This resulted in a higher photosynthetic rate, which supports saccharide distribution to roots. Thus, stay-green plants maintained a more balanced C and N metabolism between shoots and roots. Moreover, the coefficients of the relationship between the relative growth rate (RGR) and N_c, and between the photon-saturated photo-synthetic rate (P _sat) and N_c were lower in P3845. The P _sat per unit N_c in leaves was lower in the stay-green cultivars, which indicated that high yield was attained by longer green area duration and not by a high P _sat per unit N_c in the leaf. Consequently, a high P_sat caused a high leaf senescence rate because C and N compounds will translocate actively from the leaves.     

Drought‐introduced variability of mesophyll conductance in Gossypium and its relationship with leaf anatomy

Mesophyll conductance (g_m) is one of the major determinants of photosynthetic rate, for which it has an impact on crop yield. However, the regulatory mechanisms behind the decline in g_m of cotton (Gossypium. spp) by drought are unclear. An upland cotton (Gossypium hirsutum) genotype and a pima cotton (Gossypium barbadense) genotype were used to determine the gas exchange parameters, leaf anatomical structure as well as aquaporin and carbonic anhydrase gene expression under well‐watered and drought treatment conditions. In this study, the decrease of net photosynthetic rate (A_N) under drought conditions was related to a decline in g_m and in stomatal conductance (g_s). g_m and g_s coordinate with each other to ensure optimum state of CO₂ diffusion and achieve the balance of water and CO₂ demand in the process of photosynthesis. Meanwhile, mesophyll limitations to photosynthesis are equally important to the stomatal limitations. Considering g_m, its decline in cotton leaves under drought was mostly regulated by the chloroplast surface area exposed to leaf intercellular air spaces per leaf area (S_c/S) and might also be regulated by the expression of leaf CARBONIC ANHYDRASE (CA1). Meanwhile, cotton leaves can minimize the decrease in g_m under drought by maintaining cell wall thickness (T_cw). Our results indicated that modification of chloroplasts might be a target trait in future attempts to improve cotton drought tolerance.     

Variations in photosynthetic rate and associated parameters with age of oil palm leaves under irrigation

Net photosynthetic rate (P _N), transpiration rate (E), and stomatal conductance (g _s) in an adult oil palm (Elaeis guineensis) canopy were highest in the 9^th leaf and progressively declined with leaf age. Larger leaf area (LA) and leaf dry mass (LDM) were recorded in middle leaves. P _N showed a significant positive correlation with g _s and a negative relationship with leaf mass per area (ALM). The oil palm leaf remains photosynthetically active for a longer time in the canopy which contributes significantly to larger dry matter production in general and greater fresh fruit bunch yields in particular.     

Heat resistance and energy budget in different Scandinavian plants

• 1 Alpine vascular plants seem to use other strategies in surviving a cold environment than reducing the reflectance in level leaves. Pubescence in alpine plants has small effect upon total reflectance, but may increase the amount of photosynthetic active radiation within the sheltered canopy. Alpine cushion plants like Silene acaulis, Diapensia lapponica and Loiseleuria procumbens maximize the absorptance of radiant energy with minimum heat losses, probably as an effect of the dense canopy structure. The young inflorescences of Eriophorum vaginatum were found to be extremely efficient absorbators, while the reflectance in Salix catkins was close to that of green leaves.
• 2 In lichens, a great variation both in visible (400–700 nm) and infrared (700–1400 nm) reflectance was found: (A) The Pseudephebe pubescence group consists of species with very low reflectance at all measured wavelengths. The species are chionophobous, probably because of the high absorptance which makes growth possible during the cold season. (B) The Sticta sylvatica group, characterized by very low visible reflectance and very high infrared reflectance, is well adapted to shade. (C) The Cetraria nivalis group consists of fruticose species with high reflectance both in the visible and the near infrared. The intense visible reflectance probably makes net photosynthesis possible in well protected layers of the canopy. (D) The Nephroma arcticum group with spectral properties resembling green leaves in vascular plants. (E) The Haematomma ventosum group and the Parmelia perlata group with spectral properties intermediate between group C and D.
• 3 A modified method determining lethal temperatures and energies of activation in the process leading to death during a heat shock, is described. The two parameters are rather species specific in many of the 118 Scandinavian plants investigated. The lethal temperatures completely overlap the values in hotter parts of the world. However, habitat specific lethal temperatures were found; low values in wet- or shade-growing species and high values in dry-growing species. In Picea abies lethal temperatures and energies of activation showed pronounced, but diverging, year cycles in 12 ecotypes from different parts of Europe. Only negligible differences between the ecotypes exist, and cycles are probably photoperiodically determined.
• 4 Heat hardening can be achieved quickly, both in an active and dormant stage, by increasing the temperature. A linear correlation between hardening temperature, both in the optimal and supraoptimal temperature range, and hardening capability was found. In most species, but especially in cold adapted species, the hardening capability at supraoptimal temperatures decreases with increasing cultivation temperature.
• 5 Diffusion resistances with open (r_s) and closed stomata (r_c) are measured on excised leaf samples in 72 species. A positive correlation between r_s and r_c was found. r_c ranges from 2.2-62.5 s cm⁻¹ in mesophytes and from 19-425 s cm⁻¹ in xerophytes, the highest values were found in succulents. Some of the alpine species had extremely low r_c, falling within the r_s-range. Some habitat specific differences in r_c were found, but the relatively few significant differences in r_s between different habitats indicate that a lot of different drought avoidance mechanisms exist. The greatest variation in r_c between different species was found in periodically dry habitats, though a few species like Epilobium alsinifolium (r_c= 62.5 s cm⁻¹) growing in constantly wet habitats had remarkably high r_c.
• 6 In Saussurea alpina leaf size increases with improved moisture conditions. Calculations of leaf temperatures with closed stomata and somewhat extreme meteorological conditions showed that the mean leaf size in the wettest part of the transect was below, but very close to the size giving lethal leaf temperatures. In Rubus chamaemorus leaf size increases with increasing artificial shading. The leaves growing in sunexposed sites will be only 0.5°C below the lethal limit when the stomata are closed. All the shade-leaves would exceed the lethal limit if the screen was removed and closing of stomata occurred. The northern distribution of this species is probably due to its low ability to avoid heat stress.
• 7 In Silene acaulis heat damage was observed under natural conditions at an air temperature of only 21°C. Leaf temperatures about 20°C above air temperature was often found in prostrate alpine vascular plants during sunny periods. The highest overtemperature (25.5°C) was observed in the broad leaves of Rubus chamaemorus. A comparison with maximum leaf temperatures measured in different parts of the world revealed rather uniform maximum leaf temperatures in spite of very contrasting air temperatures. Thus, vascular plants seem to control the leaf temperatures to a great extent by means of morphological modifications.
• 8 Leaf temperatures in a hot and dry period were calculated and compared with the heat resistance in 69 Scandinavian, mainly alpine, plants. In 14 wet growing species the lethal limits were exceeded if closing of the stomata occurred. In the remaining species calculated temperatures in single leaves never exceeded the lethal limit. Most of these species have leaves densely crowded in cushions or prostrate rosettes. Hence they get warmer than indicated by the calculated temperatures in single leaves, and will probably be heated close to the lethal limit. A highly significant correlation between lethal temperatures and cuticular diffusion resistances was found, probably illustrating the importance of transpirational cooling during a hot period. A combination of cuticular diffusion resistances and lethal temperatures segregates the species better in natural groups than only one parameter alone.
• 9 Factors involved in limiting the downward distribution of alpine plants are discussed. Some species avoid lowlands since they are drought sensitive (low cuticular diffusion resistance), others, mainly cushion plants with low heat exchange capacity, are probably overheated in lowlands.

     

Net CO2 assimilation of taro and cocoyam as affected by shading and leaf age

Taro and cocoyam were grown outdoors in either full sun or under 40% shade. Leaves were tagged as they emerged and the effect of leaf age on net CO₂ assimilation rate (A) was determined. The effects of shading on A, transpiration (E), stomatal conductance for CO₂ (g_c) and H₂O (g_s), and water use efficiency (WUE) were also determined for leaves of a single age for each species. The effect of leaf age on A was similar for both species. Net CO₂ assimilation rates increased as leaf age increased up to 28 days with the exception of a sharp decline in A for 21 day-old leaves which corresponded to unusually low temperatures during the period of leaf expansion. A generally decreased as leaves aged beyond 28 days. Cocoyam had higher A rates than taro. Leaves of shade-grown plants had higher rates of A and E for both species at photosynthetic photon flux densities (PPFD) up to 1600 mol s^–1 m^–2. Shade-grown leaves of cocoyam had greater leaf dry weights per area (LW/A) and a trend toward higher g_c and g_s than sun-grown leaves. Shade leaves of taro had greater g_c and g₃ rates than sun-grown leaves. The data suggest that taro and cocoyam are highly adapted to moderate shade conditions.     

Leaf internal diffusion conductance limits photosynthesis more strongly in older leaves of Mediterranean evergreen broad-leaved species

Leaf age-dependent changes in structure, nitrogen content, internal mesophyll diffusion conductance (g_m), the capacity for photosynthetic electron transport (J_max) and the maximum carboxylase activity of Rubisco (V_cmax) were investigated in mature non-senescent leaves of Laurus nobilis L., Olea europea L. and Quercus ilex L. to test the hypothesis that the relative significance of biochemical and diffusion limitations of photosynthesis changes with leaf age. The leaf life-span was up to 3 years in L. nobilis and O. europea and 6 years in Q. ilex. Increases in leaf age resulted in enhanced leaf dry mass per unit area (M_A), larger leaf dry to fresh mass ratio, and lower nitrogen contents per dry mass (N_M) in all species, and lower nitrogen contents per area (N_A) in L. nobilis and Q. ilex. Older leaves had lower g_m, J_max and V_cmax. Due to the age-dependent increase in M_A, mass-based g_m, J_max and V_cmax declined more strongly (7- to 10-fold) with age than area-based (5- to 7-fold) characteristics. Diffusion conductance was positively associated with foliage photosynthetic potentials. However, this correlation was curvilinear, leading to lower ratio of chloroplastic to internal CO₂ concentration (C_c/C_i) and larger drawdown of CO₂ from leaf internal air space to chloroplasts (Δ_C) in older leaves with lower g_m. Overall the age-dependent decreases in photosynthetic potentials were associated with decreases in N_M and in the fraction of N in photosynthetic proteins, whereas decreases in g_m were associated with increases in M_A and the fraction of cell walls. These age-dependent modifications altered the functional scaling of foliage photosynthetic potentials with M_A, N_M, and N_A. The species primarily differed in the rate of age-dependent modifications in foliage structural and functional characteristics, but also in the degree of age-dependent changes in various variables. Stomatal openness was weakly associated with leaf age, but due to species differences in stomatal openness, the distribution of total diffusion limitation between stomata and mesophyll varied among species. These data collectively demonstrate that in Mediterranean evergreens, structural limitations of photosynthesis strongly interact with biochemical limitations. Age-dependent changes in g_m and photosynthetic capacities do not occur in a co-ordinated manner in these species such that mesophyll diffusion constraints curb photosynthesis more in older than in younger leaves.     

Injured and uninjured leaf photosynthetic responses after mechanical injury on <Emphasis Type="Italic">Nerium oleander</Emphasis> leaves,and <Emphasis Type="Italic">Danaus plexippus</Emphasis> herbivory on <Emphasis Type="Italic">Asclepias curassavica</Emphasis> leaves

Insect herbivory has variable effects on plant physiology; so greater understanding is needed about how injury alters photosynthesis on individual injured and uninjured leaves. Gas exchange and light-adapted leaf chlorophyll fluorescence measurements were collected from uninjured and mechanical partial leaf defoliation in two experiments with Nerium oleander (Apocynaceae) leaves, and one experiment with Danaus plexippus herbivory on Asclepias curassavica (Asclepiadaceae) leaves. Gas exchange impairment (lower photosynthetic rate (P _n ), stomatal conductance (g _s)) indicates water stress in a leaf, suggests stomatal limitations causing injury P _n impairment. The same pattern of gas exchange impairment also occurred on uninjured leaves opposite from injured leaves in both N. oleander experiments. This is an interesting result because photosynthetic impairment is rarely reported on injured leaves near injured leaves. No photosynthetic changes occurred in uninjured A. curassavica leaves opposite from D. plexippus-fed leaves. Partially defoliated leaves that had P _n and g _s reductions lacked any significant changes in intercellular leaf [CO₂], C _i. These results neither support, nor are sufficient to reject, stomatal limitations to photosynthesis. Manually imposed midrib vein severance in N. oleander experiment #1 significantly increased leaf C _i, indicating mesophyll limitations to photosynthesis. Maximal light-adapted leaf photochemical efficiency () and also non-photochemical quenching (q _N) were reduced by mechanical or insect herbivory to both study species, suggesting leaf trouble handling excess light energy not used for photochemistry. Midrib injury on N. oleander leaves and D. plexippus herbivory on A. curassavica leaves also reduced effective quantum yield (ΦPSII) and photochemical quenching (q _P); so reduced plastoquinone pools could lead to additional PSII reaction center closure.     

The Effect of Downy and Powdery Mildew on Grapevine (Vitis vinifera L.) Leaf Gas Exchange

The impact of powdery (Uncinula necator) and downy mildew (Plasmopara viticola) on grapevine leaf gas exchange was analysed. Gas exchange measurements (assimilation A, transpiration E, stomatal conductance g_s, intercellular concentration of CO₂C_i) were made on three different leaf materials: (i) healthy tissue of diseased leaves, (ii) infected tissue of diseased leaves, (iii) healthy tissue of healthy leaves (control treatment). Using the same source of leaf tissue, photosynthetic pigment concentration (chlorophyll a, b) and fluorescence levels (minimal fluorescence F₀, maximal fluorescence F_m and the optimal quantum yield [F_m ? F₀]/F_m) were determined to explain the mechanism of action of the two diseases on leaf assimilation. The results indicated that powdery and downy mildew reduced the assimilation rates, not only through a reduction in green leaf area (visual lesions), but also through an influence on gas exchange of the remaining green leaf tissues, determining a ‘virtual lesion’. The ratios between virtual and visual lesions were higher in powdery mildewed leaves than in the downy mildewed leaves. The photosynthetic fluorescence level (F_v/F_m) was affected by neither of the two pathogens. The reduction in intercellular concentration of CO₂ and photosynthetic pigment may explain the lower assimilation rates in the healthy tissues of powdery and downy mildewed leaves respectively.     

A critical appraisal of a combined stomatal-photosynthesis model for C3 plants

Gas-exchange measurements on Eucalyptus grandis leaves and data extracted from the literature were used to test a semi-empirical model of stomatal conductance for CO₂ g_Sc=g_o+a₁A/(c_s-I) (1+D_s/D_o)] where A is the assimilation rate; D_s and c_s are the humidity deficit and the CO₂ concentration at the leaf surface, respectively; g₀ is the conductance as A → 0 when leaf irradiance → 0; and D₀ and a₁ are empirical coefficients. This model is a modified version of g_sc=a₁A h_s/c_s first proposed by Ball, Woodrow & Berry (1987, in Progress in Photosynthesis Research, Martinus Mijhoff, Publ., pp. 221–224), in which h_s is relative humidity. Inclusion of the CO₂ compensation point, τ, improved the behaviour of the model at low values of c_s, while a hyperbolic function of D_s for humidity response correctly accounted for the observed hyperbolic and linear variation of g_sc and c_i/c_s as a function of D_s, where C_i is the intercellular CO₂ concentration. In contrast, use of relative humidity as the humidity variable led to predictions of a linear decrease in g_sc and a hyperbolic variation in c_i/c_s as a function of D_s, contrary to data from E. grandis leaves. The revised model also successfully described the response of stomata to variations in A, D_s and c_s for published responses of the leaves of several other species. Coupling of the revised stomatal model with a biochemical model for photosynthesis of C₃ plants synthesizes many of the observed responses of leaves to light, humidity deficit, leaf temperature and CO₂ concentration. Best results are obtained for well-watered plants.     

Macroscopic Features of Wind Damage to Leaves of Acer pseudoplatanus L. and Its Relationship with Season, Leaf Age, and Windspeed

Exposure to wind produces permanent abrasive damage to Acerpseudoplatanus L. leaves in the form of brown lesions, holes,distortions and tearing of the lamina. The zones of leaf thatare readily damaged change as the leaves expand and their topographyis altered. In the field, most damage was sustained early in the seasondespite the persistence of wind throughout the year. Wind tunnelstudies demonstrated that young expanding leaves were far moresusceptible to wind than mature leaves and that the percentagearea damaged increased linearly with windspeed. Acer pseudoplatanus L., sycamore, wind, leaf damage     

Humidity response of cuticular conductance of beech (Fagus sylvatica L.) leaf discs maintained at high relative water content

Cuticular conductance of adaxial (astomatous) and abaxial (stomatous)surfaces ofFagus sylvatica L. leaves was measured under varyingvapour pressure deficits (D). Conductance was determined fromgravimetric measurements of water flux made using a leaf discenvelope specially designed to maintain leaf relative watercontent and minimize reduction in cuticular hydration. The adaxialsurface provided a determination of ‘true’ cuticularconductance (g_c) and transpiration (E_c). The abaxial surfacewas used to estimate minimum leaf surface conductance (g^MIN_sur)and transpiration (E^MIN_sur). In experiment I, leaf discs wereplaced under one of a range of water vapour pressure deficits(0.4-2.0 kPa). Both g_c and g^MIN_sur decreased approximately 2-foldwith an increase in D between 0.4-2.0 kPa. The decrease in g_cwas linear, but g^MIN_sur declined more steeply at D between 0.4-0.95kPa than at D between 0.95-2.0 kPa. In experiment II, leaf discswere exposed to a stepwise change in D. After a period of acclimationto D of 0.95 kPa, responses of g_c and g^MIN_sur to an increaseor decrease in D were recorded. The response time of g_c to increasingor decreasing D were similar (<60 min). By contrast, g^MIN_surresponded more slowly to increasing than to decreasing D. Thesesignificant responses of g_c and g^MIN_sur to increasing and decreasingD are discussed in relation to hydration state of the cuticleand current knowledge of cuticle structure. Key words: Cuticle, cuticular conductance, cuticular membrane, Fagus sylvatica, humidity, vapour pressure deficit     

Partitioning of mesophyll conductance for CO2 into intercellular and cellular components using carbon isotope composition of cuticles from opposite leaf sides

We suggest a new technique for estimating the relative drawdown of CO₂ concentration (c) in the intercellular air space (IAS) across hypostomatous leaves (expressed as the ratio c_d/c_b, where the indexes d and b denote the adaxial and abaxial edges, respectively, of IAS), based on the carbon isotope composition (δ¹³C) of leaf cuticular membranes (CMs), cuticular waxes (WXs) or epicuticular waxes (EWXs) isolated from opposite leaf sides. The relative drawdown in the intracellular liquid phase (i.e., the ratio c_c/c_bd, where c_c and c_bd stand for mean CO₂ concentrations in chloroplasts and in the IAS), the fraction of intercellular resistance in the total mesophyll resistance (r_IAS/r_m), leaf thickness, and leaf mass per area (LMA) were also assessed. We show in a conceptual model that the upper (adaxial) side of a hypostomatous leaf should be enriched in ¹³C compared to the lower (abaxial) side. CM, WX, and/or EWX isolated from 40 hypostomatous C₃ species were ¹³C depleted relative to bulk leaf tissue by 2.01–2.85‰. The difference in δ¹³C between the abaxial and adaxial leaf sides (δ¹³C_AB ? ¹³C_AD, Δ_b–d), ranged from ??2.22 to +?0.71‰ (??0.09 ± 0.54‰, mean ± SD) in CM and from ??7.95 to 0.89‰ (??1.17 ± 1.40‰) in WX. In contrast, two tested amphistomatous species showed no significant Δ_b–d difference in WX. Δ_b–d correlated negatively with LMA and leaf thickness of hypostomatous leaves, which indicates that the mesophyll air space imposes a non-negligible resistance to CO₂ diffusion. δ¹³C of EWX and 30-C aldehyde in WX reveal a stronger CO₂ drawdown than bulk WX or CM. Mean values of c_d/c_b and c_c/c_bd were 0.90 ± 0.12 and 0.66 ± 0.11, respectively, across 14 investigated species in which wax was isolated and analyzed. The diffusion resistance of IAS contributed 20 ± 14% to total mesophyll resistance and reflects species-specific and environmentally-induced differences in leaf functional anatomy.

     

Negative effects of hydroxyl radical-generating mists (simulated dew water) on the photosynthesis and growth of Japanese apricot seedlings (Prunus mume)

The hydroxyl (OH) radical, which is generated in polluted dew water on leaf surfaces of the Japanese apricot (Prunus mume), is known to be a potent oxidant. In order to investigate the effects of the OH radical formed in polluted dew water on the photosynthesis and growth of 3-year-old seedlings of P. mume, OH radical-generating solutions simulating polluted dew water were sprayed in the early morning as a mist throughout a growing season onto the leaf surfaces of seedlings growing in experimental greenhouses. Four OH radical-generating solutions (0, 6, 18 and 54 M H₂O₂ with Fe(III) and an oxalate ion) were used in the mist treatment. Five months after the beginning of treatment, the leaves exposed to the mist containing 54 M H₂O₂ showed a significantly smaller maximum CO₂ assimilation rate (A_max) and stomatal conductance (g_s) as compared to the leaves exposed to the mist containing 0 M H₂O₂. Exposure of P. mume seedlings to the OH radical-generating mist had caused a reduction in the dry weight and relative growth rate (RGR) of the above-ground parts (stem + branch) at the end of the growing season. A significant positive correlation was shown between RGR and A_max. Thus, the effects of oxidants generated in polluted dew water on leaf surfaces can be considered to be a cause of the decrease in leaf photosynthesis and growth of P. mume.     

Effects of the interaction between ozone and carbon dioxide on gas exchange,ascorbic acid content,and visible leaf symptoms in rice leaves

Tropospheric ozone (O₃) decreases photosynthesis, growth, and yield of crop plants, while elevated carbon dioxide (CO₂) has the opposite effect. The net photosynthetic rate (P _N), dark respiration rate (R _D), and ascorbic acid content of rice leaves were examined under combinations of O₃ (0, 0.1, or 0.3 cm³ m⁻³, expressed as O⁰, O^0.1, O^0.3, respectively) and CO₂ (400 or 800 cm³ m⁻³, expressed as C⁴⁰⁰ or C⁸⁰⁰, respectively). The P _N declined immediately after O₃ fumigation, and was larger under O^0.3 than under O^0.1. When C⁸⁰⁰ was combined with the O₃, P _N was unaffected by O^0.1 and there was an approximately 20 % decrease when the rice leaves were exposed to O^0.3 for 3 h. The depression of stomatal conductance (g _s) observed under O^0.1 was accelerated by C⁸⁰⁰, and that under O^0.3 did not change because the decline under O^0.3 was too large. Excluding the stomatal effect, the mesophyll P _N was suppressed only by O^0.3, but was substantially ameliorated when C⁸⁰⁰ was combined. Ozone fumigation boosted the R _D value, whereas C⁸⁰⁰ suppressed it. An appreciable reduction of ascorbic acid occurred when the leaves were fumigated with O^0.3, but the reduction was partially ameliorated by C⁸⁰⁰. The degree of visible leaf symptoms coincided with the effect of the interaction between O₃ and CO₂ on P _N. The amelioration of O₃ injury by elevated CO₂ was largely attributed to the restriction of O₃ intake by the leaves with stomatal closure, and partly to the maintenance of the scavenge system for reactive oxygen species that entered the leaf mesophyll, as well as the promotion of the P _N.