Salinity effects on leaf anatomy: consequences for photosynthesis

Increasing salinity led to substantially higher ratios of mesophyll surface area to leaf area (A^mes/A) for Phaseolus vulgaris and Gossypium hirsutum and a smaller increase for Atriplex patula, a salt-tolerant species. The increase in internal surface for CO₂ absorption did not lead to higher CO₂ uptake rates, since the CO₂ resistance expressed on the basis of mesophyll cell wall area (r_cell) increased even more with salinity. The differences among species in the sensitivity of photosynthesis to salinity in part reflect the different A^mes/A and r_cell responses.     

Changes in Morphology,Anatomy, and Photosynthetic Capacity of Needles of Japanese Larch (Larix kaempferi) Seedlings Grown in High CO2 Concentrations

Photosynthetic traits of two-year-old Japanese larch seedlings (Larix kaempferi Carr.) grown at elevated CO₂ concentrations were studied in relation to structural changes in the needles. Seedlings were grown at two CO₂ concentrations, 360 (AC) and 720 (EC) mol mol^–1 at high and low nutrient supply rates, high N (HN) and low N (LN). The photosynthetic capacity fell significantly in EC+LN, but increased significantly in EC+HN. Since the mesophyll surface area exposed to intercellular space per unit leaf area (A^mes/A) is correlated with the photosynthetic rate, we measured A^mes/A for larch needles growing in EC. Changes of A^mes/A in both EC+HN and EC+LN were very similar to the changes in photosynthetic capacity. This suggests that the changes of A^mes/A in EC probably caused the changes in the photosynthetic capacity. The changes of A^mes/A in EC were attributed to changes in the mesophyll cell size and mesophyll cell number. The photosynthetic capacity in EC can be explained by taking morphological and structural adaptations into account as well as biochemical factors.     

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.     

Phospholipid synthesis and exchange in castor bean endosperm homogenates

Leaves on a bush of Hyptis emoryi Torr. varied in length from less than 1 cm when development occurred in full sunlight (e.g. 40 Mjoules m⁻²) to over 7 cm when the total daily solar irradiance was less than 3 Mjoules m⁻². The 1-cm sun leaves were 3-fold higher than the 7-cm shade leaves in chlorophyll per unit area, mesophyll thickness, and the internal to external leaf area ratio (A^mes/A). The higher A^mes/A caused a 1.2-cm leaf to have a 3-fold lower CO₂ liquid phase resistance than did a 7.1-cm leaf. Large thin shade leaves captured photosynthetically active radiation effectively (less than 7% passed through), but were not adapted to full sunlight. Specifically, when a 6.9-cm leaf was placed at 910 w m⁻² for 30 min, its temperature exceeded that of the air by nearly 8 C. For the common daytime air temperatures above 30 C for this desert shrub, large shade leaves would have temperatures far in excess of that optimum for photosynthesis for H. emoryi, 29 to 32 C.     

Internal Leaf Area and Cellular CO2 Resistance: Photosynthetic Implications of Variations with Growth Conditions and Plant Species

The influences of illumination, temperature, and soil water potential during development on leaf thickness, mesophyll cell wall area per unit leaf area (^Ames/A), and the cellular CO₂, resistance expressed on a mesophyll cell wall area basis (r_CO2^cell,) were examined for Plectranthus parviflorus Henckel. Although the ranges of all three growth conditions caused at least 9-fold variations in the leaf biomass produced in 4 weeks, only the illumination had a major effect on internal leaf morphology, e.g. the thickness went from 279 to 831 μm and A^mes/A from 10.5 to 34.8 as the photosynthetically active radiation was raised from 3 to 53 nEinsteins cm^?2 s^?1, while r_CO2^cell remained close to 154 s cm^?1. Variations in the growth temperature, soil water potential, and the nutritional status of the plant, affected photosynthesis mainly by changes in r_CO2^cell. To compare the influence of internal leaf area on photosynthesis for other plants, especially those with low A^mes/A values, the maximum rates of CO₂ uptake at light saturation and photosynthetically optimal temperatures were also determined for a moss, Mnium ciliare (C. Muell.) Schimp., and two ferns, Adiantum decorum Moore and Alsophila australe R. Br. As Ames/A went from 2.00 for the moss to 3.8, 7.5, 11.7, and 20.8 for the fens, the illumination at light saturation and the maximum rate of photosynthesis both progressively increased. The cellular CO₂ resistance, which theoretically might have a lower limit of 20 s cm^?1, ranged from 85 to 190 s cm^?1.     

Mesophyll cell properties for some C3 and C4 species with high photosynthetic rates

Leaves of twelve C₃ species and six C₄ species were examined to understand better the relationship between mesophyll cell properties and the generally high photosynthetic rates of these plants. The CO₂ diffusion conductance expressed per unit mesophyll cell surface area (g_CO2^cell) cell was determined using measurements of the net rate of CO₂ uptake, water vapor conductance, and the ratio of mesophyll cell surface area to leaf surface area (A^mes/A). A^mes/A averaged 31 for the C₃ species and 16 for the C₄ species. For the C₃ species g_CO2^cell ranged from 0.12 to 0.32 mm s^-1, and for the C₄ species it ranged from 0.55 to 1.5 mm s^-1, exceeding a previously predicted maximum of 0.5 mm s^-1. Although the C₃ species Cammissonia claviformis did not have the highest g_CO2^cell, the combination of the highest A^mes and highest stomatal conductance resulted in this species having the greatest maximum rate of CO₂ uptake in low oxygen, 93 μmol m^-2 s^-1 (147 mg dm^-2 h^-1). The high g_CO2^cell of the C₄ species Amaranthus retroflexus (1.5 mm s^-1) was in part attributable to its thin cell wall (72 nm thick).     

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.     

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.     

Temporal expression of effects of varying nitrogen supply on canopy growth, photosynthesis and fruit production for Actinidia deliciosa vines in the field

The effects of varying nitrogen supply on canopy leaf area, response of leaf net photosynthesis (A_n) to quantum flux density (Q), and fruit yields of kiwifruit vines (Actinidia deliciosa var. deliciosa) were examined in a two-year field experiment. Vines were grown with 0, 250 or 750 kg N ha^?1 year^?1. The responses to nitrogen supply were compared with responses to shade, to examine the impact of reduced carbon assimilation on canopy leaf area and fruit yields. Nitrogen supply did not affect significantly any of the measured variables during the first season of the experiment. In the second season, canopy leaf area was reduced significantly where nitrogen supply was limited. The quantum efficiency of photosynthesis (φ_q) increased from 0. 03 mol CO₂ mol^?1 Q soon after leaf emergence to more than 0. 05 mol CO₂ mol^?1 Q during the middle of the growing season. The quantum saturated rate of A_n (A_sat) also increased during the season, from 7–10 μmol CO₂ m^?2 s^?1 soon after leaf emergence, to 15–20 (μmol CO₂ m^?2 s^?1 during the middle of the growing season. φ_q and A_sat increased significantly with nitrogen supply at all measurement times during the second season. For vines with high nitrogen, fruit yields in both seasons were similar, averaging 3. 05 kg m^?2. Fruit yields in the second season were reduced significantly where nitrogen supply was limited, due to reduced fruit numbers. The relative effects of reduced leaf area and reduced leaf photosynthesis for carbon assimilation by nitrogen deficient vines were examined using a mathematical model of canopy photosynthesis for kiwifruit vines. Simulations of canopy photosynthesis indicated that effects on leaf area and on leaf photosynthesis were of similar importance in the overall effects of nitrogen deficiency on carbon assimilation. The effects of nitrogen supply on fruit numbers (i. e. flower development) preceded the measured effects on carbon assimilation, indicating that the nitrogen supply affected carbon partitioning to reserves in the first season.     

Altitudinal Change in the Photosynthetic Capacity of Tropical Trees: A Case Study from Ecuador and a Pantropical Literature Analysis

In tropical mountains, trees are the dominant life form from sea level to above 4,000-m altitude under highly variable thermal conditions (range of mean annual temperatures: <8 to >28°C). How light-saturated net photosynthesis of tropical trees adapts to variation in temperature, atmospheric CO₂ concentration, and further environmental factors, that change along elevation gradients, is not precisely known. With gas exchange measurements in mature trees, we determined light-saturated net photosynthesis at ambient temperature (T) and [CO₂] (A _sat) of 40 tree species from 21 families in tropical mountain forests at 1000-, 2000-, and 3000-m elevation in southern Ecuador. We tested the hypothesis that stand-level averages of A _sat and leaf dark respiration (R _D) per leaf area remain constant with elevation. Stand-level means of A _sat were 8.8, 11.3, and 7.2?μmol?CO₂?m^?2?s^?1; those of R _D 0.8, 0.6, and 0.7?μmol?CO₂?m^?2?s^?1 at 1000-, 2000-, and 3000-m elevation, respectively, with no significant altitudinal trend. We obtained coefficients of among-species variation in A _sat and R _D of 20–53% (n?=?10–16 tree species per stand). Examining our data in the context of a pan-tropical A _sat data base for mature tropical trees (c. 170 species from 18 sites at variable elevation) revealed that area-based A _sat decreases in tropical mountains by, on average, 1.3?μmol?CO₂?m^?2?s^?1?per?km altitude increase (or by 0.2?μmol?CO₂?m^?2?s^?1 per K temperature decrease). The A _sat decrease occurred despite an increase in leaf mass per area with altitude. Local geological and soil fertility conditions and related foliar N and P concentrations considerably influenced the altitudinal A _sat patterns. We conclude that elevation is an important influencing factor of the photosynthetic activity of tropical trees. Lowered A _sat together with a reduced stand leaf area decrease canopy C gain with elevation in tropical mountains.     

Variation of leaf traits and pigment content in three species of steppe plants depending on the climate aridity

Mesophyll structure and content of photosynthetic pigments in the leaves of three species of steppe plants, Centaurea scabiosa L., Euphorbia virgata Waldst. et Kit., Helichrysum arenarium (L.) Moench, were investigated in four geographical sites of the Volga region and the Urals located in the forest-steppe and steppe zones. Variations of the studied parameters between geographical points depended both on the species and on the structural organization of the leaf. The highest level of variation was observed for leaf area and pigment content per unit leaf area, the size and the number of chloroplasts in the cell changed to a lesser extent. The leaf thickness, leaf area and mesophyll cell sizes mostly depended on the plant species. C. scabiosa had large leaves (40–50 cm²) with large thickness (280–290 μm) and large mesophyll cells (up to 15000 μm³). The leaves of H. arenarium and E. virgata were ten times smaller and characterized by 1.5 times smaller thickness and 2?3 times smaller cell size. Geographical location and climate of the region affected leaf density, proportion of partial tissue volume, and the ratio of the photosynthetic pigments. In the southern point of Volga region with the highest climate aridity, all studied species were characterized by maximum values of volumetric leaf density (LD), due to the high proportion of sclerenchyma and vascular bundles, and specificity of the mesophyll structure. With the decline in latitude, chlorophyll (Chl) and carotenoid (Car) contents in leaf area were reduced, the ratio Chl/Car was increased, and the ratio Chl a/b was declined. The reduction of the pigment content in the leaf in all species was associated with a reduction in the amount of Chl per chloroplast, and for C. scabiosa and H. arenarium it was associated also with the reduction of chloroplast amount in the leaf area. In turn, chloroplast number per leaf area and the total cell area (A_mes/A) depended on the ratio of the number and size of mesophyll cells inherent to this plant species. At the same time, we found a similar mechanism of spatial organization of leaf restructuring for all studied species—decrease in A_mes/A was accompanied by increasing in the proportion of intercellular air spaces in the leaf. It is concluded that variations in structural and functional parameters of the photosynthetic apparatus of steppe plants were associated with plant adaptation to climate features. General direction of the changes of leaf parameters of the studied species with aridity was the increase of LD and the decrease of pigment content per leaf area however the cellular mechanisms of changes in the pigment content and integral parameters of mesophyll were determined by the plant species properties.     

A comparison of photosynthesis in two thalloid liverworts

Standard infra-red gas analysis techniques were used to compare the photosynthesis of the liverworts Marchantia foliacea Mitt. and Monoclea forsteri Hook. Parameters measured include net photosynthetic rates, light response curves, quantum efficiencies, diffusive resistances to CO₂ and water, apparent photorespiration and chlorophyll content. A series of morphological measurements were also made to determine resistance of pores and the mesophyll to dorsal surface ratio, A _mes/A. Marchantia has a cuticularised thallus with the photosynthetic tissues arranged in air chambers giving an A _mes/A of 9 whilst Monoclea has a solid thallus, A _mes/A of 1. Both species are shade adapted and it was found that whilst the air chambers were advantageous for water relations they increased maximum photosynthesis only slightly. Calculations showed that the solid thallus would be photosynthetically superior in very moist environments. The results are discussed with reference to existing ideas on the evolution of the structure of land plants.     

RELATIONSHIPS BETWEEN PHOTOSYNTHETIC CAPACITY AND LEAF STRUCTURE IN SEVERAL SHADE PLANTS

Leaf structure, photosynthetic characteristics and related physiological parameters have been studied in three ornamental shade species: Fatsia japonica, Cissus rhombifolia (relatively light-tolerant plants), and Philodendron scandens (obligate shade plant). Species were grown in a shadehouse. Maximum photosynthetic photon flux density was 470 μmol m^-2 s^-1. Net rate of CO₂ uptake at light saturation (maximum Pn) in Fatsia was 6.90 ± 1.27 μmol m^-2 s^-1. In Cissus and Philodendron values were about 30% and 63% less respectively, than those measured in Fatsia. The nitrogen content, relative dry wt, specific leaf dry wt (SLDW), chlorophyll a/b ratio, and nitrogen to chlorophyll ratio were lower in Philodendron. However, leaf thickness in Philodendron (296 ± 17 μm) was about 54% and 160% higher, respectively, than in Fatsia and Cissus, and the ratio between mesophyll cell area and leaf surface area (A^mes/A) was nearly similar in the three species. However Philodendron exhibited a percentage of palisade parenchyma about three times lower than that observed in the two other species. The chloroplast number per mm of cell wall in transverse sections (chloroplast density) in the palisade parenchyma was fairly constant (about 65), irrespective of species. The “chloroplast density” in the spongy parenchyma of Philodendron was about 53% and 63%, respectively, of Fatsia and Cissus values. In Fatsia and Cissus chloroplast ultrastructure seems to change gradually and continuously from sun to shade type with the depth from the adaxial to abaxial surface. Special emphasis was given in order to determine the structural parameters best correlated with maximum Pn between the different species. In this way chloroplast number in transverse sections (chloroplast number) and the ratio between chloroplast area and leaf surface area (A^chl/A) were the parameters best correlated with maximum Pn, and stomatal frequency was also a good determinant of maximum Pn. However, leaf thickness, SLDW, and even A^mes/A ratio were weakly correlated with maximum Pn.     

Nutrient Influences on Leaf Photosynthesis: EFFECTS OF NITROGEN, PHOSPHORUS, AND POTASSIUM FOR GOSSYPIUM HIRSUTUM L

The net rate of CO₂ uptake for leaves of Gossypium hirsutum L. was reduced when the plants were grown at low concentrations of NO₃^-, PO₄^2-, or K⁺. The water vapor conductance was relatively constant for all nutrient levels, indicating little effect on stomatal response. Although leaves under nutrient stress tended to be lower in chlorophyll and thinner, the ratio of mesophyll surface area to leaf area did not change appreciably. Thus, the reduction in CO₂ uptake rate at low nutrient levels was due to a decrease in the CO₂ conductance expressed per unit mesophyll cell wall area (g^cell_CO2). The use of g^cell_CO2 and nutrient levels expressed per unit of mesophyll cell wall provides a new means of assessing nutrient effects on CO₂ uptake of leaves.     

Slow development of leaf photosynthesis in an evergreen broad-leaved tree, Castanopsis sieboldii: relationships between leaf anatomical characteristics and photosynthetic rate

Changes in net photosynthetic rate on a leaf area basis and anatomical properties during leaf development were studied in an evergreen broad‐leaved tree, Castanopsis sieboldii and an annual herb, Phaseolus vulgaris. In C. sieboldii, surface area of mesophyll cells facing the intercellular air spaces on a leaf area basis (S_mes) was already considerable at the time of full leaf area expansion (FLE). However, surface area of chloroplasts facing the intercellular air spaces on a leaf area basis (S_c), and chlorophyll and Rubisco contents on a leaf area basis increased to attain their maximal values 15–40 d after FLE. In contrast, in P. vulgaris, chloroplast number on a leaf area basis, S_c and S_mes at 10 d before FLE were two to three times greater than the steady‐state levels attained at around FLE. In C. sieboldii, the internal CO₂ transfer conductance (g_i) slightly increased for 10 d after FLE but then decreased toward the later stages. Limitation of photosynthesis by g_i was only about 10% at FLE, but then increased to about 30% at around 40 d after FLE. The large limitation after FLE by g_i was probably due to the decrease in CO₂ concentration in the chloroplast caused by the increases in thickness of mesophyll cell walls and in Rubisco content per chloroplast surface area. These results clearly showed that: (1) in C. sieboldii, chloroplast development proceeded more slowly than mesophyll cell expansion and continued well after FLE, whereas in P. vulgaris these processes proceeded synchronously and were completed by FLE; (2) after FLE, photosynthesis in leaves of C. sieboldii was markedly limited by g_i. From these results, it is suggested that, in the evergreen broad‐leaved trees, mechanical protection of mesophyll cells has priority over the efficient CO₂ transfer and quick construction of the chloroplasts.     

Leaf Conductance in Relation to Rate of CO(2) Assimilation: I. Influence of Nitrogen Nutrition, Phosphorus Nutrition, Photon Flux Density, and Ambient Partial Pressure of CO(2) during Ontogeny

Plants of Zea mays were grown with different concentrations of nitrate (0.6, 4, 12, and 24 millimolar) and phosphate (0.04, 0.13, 0.53, and 1.33 millimolar) supplied to the roots, photon flux densities (0.12, 0.5, and 2 millimoles per square meter per second), and ambient partial pressures of CO₂ (305 and 610 microbars). Differences in mineral nutrition and irradiance led to a large variation in rate of CO₂ assimilation per unit leaf area (A, 11 to 58 micromoles per square meter per second) when measured under standard conditions. The variation was shown, with the plants that had received different amounts of nitrate, to be related to variations in the nitrogen and chlorophyll contents, and phosphoenolpyruvate and ribulose-1,5-bisphosphate carboxylase activities per unit leaf area. Irrespective of growth treatment, A and leaf conductance to CO₂ transfer (g), measured under standard conditions were in almost constant proportion, implying that intercellular partial pressure of CO₂ (p_i), was almost constant at 95 microbars. The same proportionality was maintained as A and g increased in an initially nitrogen-deficient plant that had been supplied with abundant nitrate. It was shown that p_i measured at a given ambient partial pressure was not affected by the ambient partial pressure at which the plants had been grown, although it was different when measured at different ambient partial pressures. This suggests that the close coupling between A and g in these experiments is not associated with sensitivity of stomata to change in p_i.

Similar, though less comprehensive, experiments were done with Gossypium hirsutum, and yielded similar conclusions, except that the proportionality between A and g at normal ambient partial pressure of CO₂ implied P_i ≈ 200 microbars.

     

Ontogenetic patterns of leaf CO2 exchange, morphology and chemistry in Betula pendula trees

In order to explore ontogenetic variation in leaf-level physiological traits of Betula pendula trees, we measured changes in mass- (A _mass) and area-based (A _area) net photosynthesis under light-saturated conditions, mass- (RS_mass) and area-based (RS_area) leaf respiration, relative growth rate, leaf mass per area (LMA), total nonstructural carbohydrates (TNC), and macro- and micronutrient concentrations. Expanding leaves maintained high rates of A _area, but due to high growth respiration rates, net CO₂ fixation occurred only at irradiances >200 μmol photons m^–2 s^–1. We found that full structural leaf development is not a necessary prerequisite for maintaining positive CO₂ balance in young birch leaves. Maximum rates of A _area were realized in late June and early July, whereas the highest values of A _mass occurred in May and steadily declined thereafter. The maintenance respiration rate averaged ≈8 nmol CO₂ g^–1 s^–1, whereas growth respiration varied between 0 and 65 nmol CO₂ g^–1 s^–1. After reaching its lowest point in mid-June, leaf respiration increased gradually until the end of the growing season. Mass and area-based dark respiration were significantly positively correlated with LMA at stages of leaf maturity, and senescence. Concentrations of P and K decreased during leaf development and stabilized or increased during maturity, and concentrations of immobile elements such as Ca, Mn and B increased throughout the growing season. Identification of interrelations between leaf development, CO₂ exchange, TNC and leaf nutrients allowed us to define factors related to ontogenetic variation in leaf-level physiological traits and can be helpful in establishing periods appropriate for sampling birch leaves for diagnostic purposes such as assessment of plant and site productivity or effects of biotic or abiotic factors. Received: 29 December 1998 / Accepted: 26 July 1999     

Effects of Water-Deficit Stress on Photosynthesis, Its Components and Component Limitations, and on Water Use Efficiency in Wheat (Triticum aestivum L.)

It is of theoretical as well as practical interest to identify the components of the photosynthetic machinery that govern variability in photosynthesis rate (A) and water-use efficiency (WUE), and to define the extent by which the component processes limit A and WUE during developing water-deficit stress. For that purpose, leaf exchange of CO₂ and H₂O was determined in two growth-chamber-grown wheat cultivars (Triticum aestivum L. cv TAM W-101 and cv Sturdy), and the capacity of A was determined and broken down into carboxylation efficiency (c.e.), light- and CO₂-saturated A, and stomatal conductance (g_s) components. The limitations on A measured at ambient CO₂ concentration (A₃₅₀) were estimated. No cultivar difference was observed when A₃₅₀ was plotted versus leaf water potential (Ψ_w). Light- and CO₂-saturated A, c.e., and g_s decreased with decreasing leaf Ψ_w, but of the corresponding photosynthesis limitations only those caused by insufficient c.e. and g_s increased. Thus, reduced stomatal aperture and Calvin cycle activity, but not electron transport/photophosphorylation, appeared to be major reasons for drought stress-induced inhibition of A₃₅₀. WUE measured as A₃₅₀/g_s first increased with stomatal closure down to a g_s of about 0.25 mol H₂O m⁻² s⁻¹ (Ψ_w = −1.6 MPa). However, it was predicted that A₃₅₀/g_s would decrease with more severe stress due to inhibition of c.e.     

Resistance Analysis of Nocturnal Carbon Dioxide Uptake by a Crassulacean Acid Metabolism Succulent, Agave deserti

Nocturnal CO₂ uptake by a Crassulacean acid metabolism succulent, Agave deserti Engelm. (Agavaceae), was measured so that the resistance properties of the mesophyll chlorenchyma cells and their CO₂ concentrations could be determined. Two equivalents of acidity were produced at night per mole of CO₂ taken up. The nocturnal CO₂ uptake became light-saturated at 3.5 mEinsteins cm⁻² of photosynthetically active radiation (400-700 nm) incident during the preceding day; at least 46 Einsteins were required per mole of CO₂ fixed. Variations in the daytime leaf temperature between 20 and 37 C had little effect on nocturnal CO₂ uptake. After the first few hours in the dark, the leaf liquid phase CO₂ resistance (r^liq_CO2) and the CO₂ concentration in the chlorenchyma cells (cⁱ_CO2) both increased, the latter usually reaching the ambient external CO₂ level at the end of the dark period. Increasing the leaf surface temperature above 15 C at night markedly increased the stomatal resistance, r^liq_CO2, and cⁱ_CO2.

The minimum r^liq_CO2 at night was about 1.6 seconds cm⁻¹. Based on the ratio of chlorenchyma surface area to total leaf surface area of 82, this r^liq_CO2 corresponded to a minimum cellular resistance of approximately 130 seconds cm⁻¹, comparable to values for mesophyll cells of C₃ plants. The contribution of the carboxylation reaction and/or other biochemical steps to r^liq_CO2 may increase appreciably as the nighttime temperature shifts a few degrees from the optimum or after a few hours in the dark, both of which caused large increases in r^liq_CO2. This necessitates a large internal leaf area for CO₂ diffusion into the chlorenchyma to support moderate nocturnal CO₂ uptake rates by these succulent leaves.