Effects of Potassium Deficiency on the Photosynthesis and Respiration of Leaves of Sugar Beet under Conditions of Low Sodium Supply

Sugar beet plants (Beta vulgaris L. cv. F58-554H1) were germinated and cultured under standardized environmental conditions. The effects of K deficiency on photosynthetic and respiratory CO₂ exchange rates of attached leaves were studied under conditions of low Na supply by withholding both Na and K from the culture medium at cut-off (28 days after planting). Potassium and Na concentrations in the leaf blade and petiole decreased rapidly during the 8 days after cut-off, then more slowly.     

Measurement of CO(2) and H(2)O Vapor Exchange in Spinach Leaf Discs : Effects of Orthophosphate

A leaf chamber has been designed which allows the measurement of both CO₂ and water vapor exchange in Spinacia oleracea leaf discs. The center of the disc lies within a cylindrical gas chamber and its margins are enclosed within a cavity through which water or various metabolites can be pumped. In saturating light and normal atmospheres, the leaf discs have a relatively low resistance to H₂O vapor transfer (r_w = 1.87 seconds per centimeter) and can support high rates of photosynthesis for several hours. The abaxial surface of a disc had a higher resistance to water vapor transfer (r_w = 3.22 seconds per centimeter) than the adaxial (r_w = 2.45 seconds per centimeter) despite having a higher stomatal frequency (abaxial, 105/square millimeter; adaxial, 58/square millimeter). In 2% O₂, the discs required an internal concentration of CO₂ of 115 microliters per liter to support one-half of the maximal velocity of apparent photosynthesis (average value, 66 milligrams CO₂ per square decimeter per hour). In 20% O₂, the comparable values are 156 microliters per liter and 56 milligrams CO₂ per square decimeter per hour. In air, apparent photosynthesis saturated at intensities (750 microeinsteins per square meter per second) well below that of daylight but, when the internal CO₂ was raised to 700 to 900 microliters per liter, photosynthesis was not saturated even at daylight intensities (2025 microeinsteins per square meter per second). The distribution of Prussian blue crystals, formed after ferrocyanide feeding, showed that water entered the disc via the vasculature. When 25-minute pulses of orthophosphate were provided in the feeding solution, there were concentration-dependent increases in both r_w and r_m leading to inhibition of photosynthesis. The orthophosphate-dependent inhibitions were reversible.     

Prechilling of Xanthium strumarium L. Reduces Net Photosynthesis and, Independently, Stomatal Conductance, While Sensitizing the Stomata to CO(2)

Greenhouse-grown plants of Xanthium strumarium L. were exposed in a growth cabinet to 10 C during days and 5 C during nights for periods of up to 120 hours. Subsequently, CO₂ exchange, transpiration, and leaf temperature were measured on attached leaves and in leaf sections at 25 or 30 C, 19 C dew point of the air, 61 milliwatts per square centimeter irradiance, and CO₂ concentrations between 0 and 1000 microliters per liter ambient air. Net photosynthesis and stomatal conductance decreased and dark respiration increased with increasing duration of prechilling. The reduction in net photosynthesis was not a consequence of decreased stomatal conductance because the intercellular CO₂ concentration in prechilled leaves was equal to or greater than that in greenhouse-grown controls. The intercellular CO₂ concentration at which one-half maximum net photosynthesis occurred remained the same in prechilled leaves and controls (175 to 190 microliters per liter). Stomata of the control plants responded to changes in the CO₂ concentration of the air only slightly. Prechilling for 24 hours or more sensitized stomata to CO₂; they responded to changes in CO₂ concentration in the range from 100 to 1000 microliters per liter.     

Influence of Seismic Stress on Photosynthetic Productivity, Gas Exchange, and Leaf Diffusive Resistance of Glycine max (L.) Merrill cv Wells II

Relative growth rate (RGR), leaf water potential (Ψw), transpiration rate (Tr), photosynthetic rate (Pn), and stomatal and mesophyll resistances to CO₂ exchange were measured or calculated to determine how periodic seismic (shaking) stress decreased dry weight accumulation by soybean (Glycine max [L.] Merrill cv Wells II). Seismic stress was applied with a gyratory shaker at 240 to 280 revolutions per minute for 5 minutes three times daily at 0930, 1430, and 1930 hours. Fifteen days of treatment decreased stem length 21%, leaf area 17%, and plant dry weight 18% relative to undisturbed plants. Seismic stress also decreased RGR 4%, which was due entirely to decreased net assimilation rate. Transpiration decreased 17% and leaf Ψw increased 39% 30 minutes after treatment. A reduction in Pn began within seconds after the onset of treatment and had declined 16% after 20 minutes, at which time gradual recovery began. Assimilation rate recovered fully before the next seismic treatment 5 hours later. Resistance analysis and calculation of leaf internal CO₂ levels indicated that the transitory decrease in Pn caused by periodic seismic stress was due to increased stomatal resistance on the abaxial leaf surface.     

Assimilatory Power (Postillumination CO(2) Uptake) in Leaves: Measurement, Environmental Dependencies, and Kinetic Properties

Assimilatory power was measured in ten C₃ species by means of a rapid-response gas exchange device as the total amount of CO₂ fixed in N₂-CO₂ atmosphere after switching the light off. Different steady-state levels of the assimilatory power were obtained by varying light intensity and O₂ and CO₂ concentrations during the preexposition periods in the leaf chamber.

Within the limits of the linear part of the CO₂ curve of photosynthesis in N₂, the assimilatory power is constant, being sufficient for the assimilation of about 20 nanomoles CO₂ per square centimeter leaf. The pool starts to decrease with the onset of the CO₂ saturation of photosynthesis. Increase in O₂ concentration from 0 to 100% at 350 microliters CO₂ per liter produces a considerable decrease in the assimilatory power.

The mesophyll conductance (M) was found to be proportional to the assimilatory power (A): M = mA. The most frequently occurring values of the proportionality constant (m) (called the specific efficiency of carboxylation) were concentrated between 0.03 and 0.04 centimeter per second per nanomole A per square centimeter but the measured extreme values were 0.01 and 0.06 centimeter per second per nanomole A per square centimeter. The specific rate of carboxylation (the rate per unit A) showed a hyperbolic dependence on CO₂ conentration with the most frequent values of K_m (CO₂) ranging from 25 to 35 micromolar in the liquid phase of mesophyll cells (extremes 23 and 100 micromolar).

It is concluded that the CO_2⁻ and light-saturated rate of photosynthesis is limited by the reactions of the formation of the assimilatory power and not by ribulose-1,5-bisphosphate carboxylase. O₂ is a competitive consumer of the assimilatory power, and the inhibitory effect of O₂ on photosynthesis is caused mainly by a decrease in the pool of the assimilatory power at high O₂ concentrations. In intact leaves, the kinetic properties of ribulose-1,5-bisphosphate carboxylase seem to be variable.

     

Increases in Phosphorus Requirements for CO(2)-Enriched Pine Species

Pinus radiata D. Don (half-sib families 20010 and 20062) and Pinus caribaea var hondurensis (an open-pollinated family) were grown for 49 weeks at seven levels of phosphorus and at CO₂ concentrations of either 340 or 660 microliters per liter, to establish if the phosphorus requirements differed between the CO₂ concentrations and if mycorrhizal associations were affected. When soil phosphorus availability was low, phosphorus uptake was increased by elevated CO₂. This may have been related to changes in mycorrhizal competition. When the phosphorus concentration in the youngest fully expanded needles was above 600 milligrams per kilogram the shoot weight of all pine families was greater at high CO₂ due to increases in rates of photosynthesis. More dry weight was partitioned to the stems of P. radiata family 20010 and P. caribaea. At foliar phosphorus concentrations above 1000 milligrams per kilogram (P. radiata) and 700 milligrams per kilogram (P. caribaea), growth did not increase at 340 microliters of CO₂ per liter. Soluble sugar levels in the same needles mirrored the growth response, but the starch concentration declined with increasing phosphorus. At 660 microliters of CO₂ per liter, shoot weight and soluble sugar concentrations were still increasing up to a foliar P concentration of 1800 milligrams per kilogram for P. radiata and 1600 milligrams per kilogram for P. caribaea. The starch concentrations did not decline. These results indicate that higher foliar phosphorus concentrations are required to realize the maximum growth potential of pines at elevated CO₂.     

Influence of elevated carbon dioxide on water relations of soybeans

Soybean (Glycine max L. Merrill cv `Bragg') plants were grown in pots at six elevated atmospheric CO₂ concentrations and two watering regimes in open top field chambers to characterize leaf xylem potential, stomatal resistance and conductance, transpiration, and carbohydrate contents of the leaves in response to CO₂ enrichment and water stress conditions. Groups of plants at each CO₂ concentration were subjected to water stress by withholding irrigation for 4 days during the pod-filling stage.

Under well watered conditions, the stomatal conductance of the plants decreased with increasing CO₂ concentration. Therefore, although leaf area per plant was greater in the high CO₂ treatments, the rate of water loss per plant decreased with CO₂ enrichment. After 4 days without irrigation, plants in lower CO₂ treatments showed greater leaf tissue damage, lower leaf water potential, and higher stomatal resistance than high CO₂ plants. Stomatal closure occurred at lower leaf water potentials for the low CO₂ grown plants than the high CO₂ grown plants. Significantly greater starch concentrations were found in leaves of high CO₂ plants, and the reductions in leaf starch and increases in soluble sugars due to water stress were greater for low CO₂ plants. The results showed that even though greater growth was observed at high atmospheric CO₂ concentrations, lower rates of water use delayed and, thereby, prevented the onset of severe water stress under conditions of low moisture availability.

     

Influence of Applied NaCl on Crassulacean Acid Metabolism and Ionic Levels in a Cactus, Cereus validus

To determine possible physiological responses to salinity, seedlings of Cereus validus Haworth, a cactus from Salinas Grandes, Argentina, were treated with up to 600 millimolar NaCl for up to 16 days when they were about 9 months old and 100 millimeters tall. Salt stress decreased stem biomass, e.g. it was 19.7 grams for controls and 11.4 grams for plants treated with 400 millimolar NaCl for 14 days. Nocturnal CO₂ uptake in these obligate Crassulacean acid metabolism (CAM) plants was inhibited 67% upon treatment with 400 millimolar NaCl for 14 days (controls, 181 millimoles CO₂ per square meter), while nocturnal accumulation of malate was inhibited 49% (controls, 230 millimoles malate per square meter). The larger accumulation of malate as compared to uptake of atmospheric CO₂ suggests that internal CO₂ recycling occurred during the dark period. Such recycling was lower in the controls (~20%) than in the NaCl-treated plants (~50%). The nocturnal increase in malate and titratable acidity depended on the total daily photosynthetically active radiation available; measurements suggest a quantum requirment of 26 photons per malate. As NaCl in the medium was increased to 600 millimolar in daily increments of 50 millimolar, Na and Cl concentrations in the roots increased from about 7 to 100 millimolar, but K concentration in the cell sap remained near 26 millimolar. Concomitantly, concentrations of Na and Cl in the shoots increased from 8 to 17 millimolar and from 1 to 7 millimolar, respectively, while the K concentration increased about 16 to 60 millimolar. In plants maintained for 14 days at 500 millimolar NaCl, the root levels of Na and Cl increased to 260 millimolar, the shoot levels were about 60 millimolar, and the stem bases began to become necrotic. Such Na retention in the roots together with the special possibilities of carbon reutilization given by CAM are apparently survival mechanisms for the temporarily saline conditions experienced in its natural habitat.     

The effect of glyoxylate on photosynthesis and photorespiration by isolated soybean mesophyll cells

Incubating isolated soybean leaf mesophyll cells with glyoxylate increased the rates of CO₂ fixation by as much as 150%. In order to cause this stimulation, the glyoxylate must be presented to the cells before the NaHCO₃. Significant stimulation was observed 15 seconds after beginning the glyoxylate treatment. The glyoxylate-dependent stimulation was increased by high O₂ concentrations and decreased by high CO₂ concentrations. Glyoxylate treatment resulted in a 71% inhibition in the rate of CO₂ incorporation into glycolate and glycine. Glyoxylate may be stimulating net photosynthesis solely by decreasing photorespiration or it may be increasing the amount of CO₂ fixed by both decreasing photorespiration and increasing gross photosynthesis. Ribulose bisphosphate carboxylase, when preactivated and assayed in situ, was unaffected by the glyoxylate treatment.     

Inorganic Carbon Diffusion between C(4) Mesophyll and Bundle Sheath Cells: Direct Bundle Sheath CO(2) Assimilation in Intact Leaves in the Presence of an Inhibitor of the C(4) Pathway

Photosynthesis rates of detached Panicum miliaceum leaves were measured, by either CO₂ assimilation or oxygen evolution, over a wide range of CO₂ concentrations before and after supplying the phosphoenolpyruvate (PEP) carboxylase inhibitor, 3,3-dichloro-2-(dihydroxyphosphinoyl-methyl)-propenoate (DCDP). At a concentration of CO₂ near ambient, net photosynthesis was completely inhibited by DCDP, but could be largely restored by elevating the CO₂ concentration to about 0.8% (v/v) and above. Inhibition of isolated PEP carboxylase by DCDP was not competitive with respect to HCO₃⁻, indicating that the recovery was not due to reversal of enzyme inhibition. The kinetics of ¹⁴C-incorporation from ¹⁴CO₂ into early labeled products indicated that photosynthesis in DCDP-treated P. miliaceum leaves at 1% (v/v) CO₂ occurs predominantly by direct CO₂ fixation by ribulose 1,5-bisphosphate carboxylase. From the photosynthesis rates of DCDP-treated leaves at elevated CO₂ concentrations, permeability coefficients for CO₂ flux into bundle sheath cells were determined for a range of C₄ species. These values (6-21 micromoles per minute per milligram chlorophyll per millimolar, or 0.0016-0.0056 centimeter per second) were found to be about 100-fold lower than published values for mesophyll cells of C₃ plants. These results support the concept that a CO₂ permeability barrier exists to allow the development of high CO₂ concentrations in bundle sheath cells during C₄ photosynthesis.     

Environmental Effects on Photorespiration of C(3)-C(4) Species : I. Influence of CO(2) and O(2) during Growth on Photorespiratory Characteristics and Leaf Anatomy

The possibility of altering CO₂ exchange of C₃-C₄ species by growing them under various CO₂ and O₂ concentrations was examined. Growth under CO₂ concentrations of 100, 350, and 750 micromoles per mole had no significant effect on CO₂ exchange characteristics or leaf anatomy of Flaveria pringlei (C₃), Flaveria floridana (C₃-C₄), or Flaveria trinervia (C₄). Carboxylation efficiency and CO₂ compensation concentrations in leaves of F. floridana developed under the different CO₂ concentrations were intermediate to F. pringlei and F. trinervia. When grown for 12 days at an O₂ concentration of 20 millimoles per mole, apparent photosynthesis was strongly inhibited in Panicum milioides (C₃-C₄) and to a lesser degree in Panicum laxum (C₃). In P. milioides, acute starch buildup was observed microscopically in both mesophyll and bundle sheath cells. Even after only 4 days exposure to 20 millimoles per mole O₂, the presence of starch was more pronounced in leaf cross-sections of P. milioides compared to those at 100 and 210 millimoles per mole. Even though this observation suggests that P. milioides has a different response to low O₂ with respect to translocation of photosynthate or sink activity than C₃ species, the concentration of total available carbohydrate increased in shoots of all species by 33% or more when grown at low O₂. This accumulation occurred even though relative growth rates of Festuca arundinacea (C₃) and P. milioides grown for 4 days at 210 millimoles per mole O₂, were inhibited 83 and 37%, respectively, when compared to plants grown at 20 millimoles per mole O₂.     

Changes in Leaf Morphology and Composition with Future Increases in CO<Subscript>2</Subscript> and Temperature Revisited: Wheat in Field Chambers

Whether leaf morphology is altered by future increases in atmospheric CO₂ and temperature has been reexamined over 3 years in wheat grown in field chambers at two levels of nitrogen supply. Flag leaf fresh and dry mass, area, volume, and ratios of these parameters, as well as the contents of water, chlorophyll, nonstructural carbohydrates, and nitrogen compounds have been determined at anthesis and 14 days later. High CO₂ decreased rather than increased, as reported in the literature, leaf mass per area and leaf density, and increased water content per area and per volume and water percentage. Warmer temperatures also decreased leaf mass per area, but did not affect density or water per area or per volume, whereas they increased water percentage. Nitrogen supply did not change CO₂ and temperature effects on leaf morphology. Nonstructural carbohydrates increased and nitrogen compounds decreased in elevated CO₂, and the sum of these compounds decreased with warmer temperatures. These changes in composition did not account for modifications of leaf morphology. We conclude that increases in atmospheric CO₂ and temperature after leaf initiation can decrease leaf mass per area, and elevated CO₂ can also decrease leaf density, due to decreases in leaf structural compounds. The functional significance of these changes is probably a decrease in photosynthetic capacity per unit leaf area.     

Photosynthetic Characteristics of the C(3)-C(4) Intermediate Parthenium hysterophorus

The weedy species Parthenium hysterophorus (Asteraceae) possesses a Kranz-like leaf anatomy. The bundle sheath cells are thick-walled and contain numerous granal chloroplasts, prominent mitochondria, and peroxisomes, all largely arranged in a centripetal position. Both mesophyll and bundle sheath chloroplasts accumulate starch. P. hysterophorus exhibits reduced photorespiration as indicated by a moderately low CO₂ compensation concentration (20-25 microliters per liter at 30°C and 21% O₂) and by a reduced sensitivity of net photosynthesis to 21% O₂. In contrast, the related C₃ species P. incanum and P. argentatum (guayule) lack Kranz anatomy, have higher CO₂ compensation concentrations (about 55 microliters per liter), and show a greater inhibition of photosynthesis by 21% O₂. Furthermore, in P. hysterophorus the CO₂ compensation concentration is relatively less sensitive to changes in O₂ concentrations and shows a biphasic response to changing O₂, with a transition point at about 11% O₂. Based on these results, P. hysterophorus is classified as a C₃-C₄ intermediate. The activities of diagnostic enzymes of C₄ photosynthesis in P. hysterophorus were very low, comparable to those observed in the C₃ species P. incanum (e.g. phosphoenolpyruvate carboxylase activity of 10-29 micromoles per milligram of chlorophyll per hour). Exposures of leaves of each species to ¹⁴CO₂ (for 8 seconds) in the light resulted in 3-phosphoglycerate and sugar phosphates being the predominant initial ¹⁴C products (77-84%), with ≤4% of the ¹⁴C-label in malate plus aspartate. These results indicate that in the C₃-C₄ intermediate P. hysterophorus, the reduction in leaf photorespiration cannot be attributed to C₄ photosynthesis.     

Saturation Kinetics of the Velocity of Stomatal Closing in Response to CO(2)

Stomatal closing movements in response to changes from CO₂-free to CO₂-containing air were recorded in leaf sections of Zea mays using air flow porometers. The response to CO₂ was fast; the shortest lag between the application of 300 microliters CO₂ per liter of air and the beginning of a stomatal response was 3 seconds. The velocity of stomatal closing increased with CO₂ concentration and approached its maximal value between 10³ and 10⁴ microliters CO₂ per liter of air. The CO₂ concentration at which the closing velocity reached half its maximal value was approximately 200 microliters CO₂ per liter of air, both in the light and in darkness. This indicates that the mechanism of stomatal responses to CO₂ is the same in both light regimes and that the range of stomatal sensitivity to changes in CO₂ concentration coincides with the range of CO₂ concentrations known to occur in the intercellular spaces of illuminated leaves.     

Photosynthesis and Growth of Water Hyacinth under CO(2) Enrichment

Water hyacinth (Eichhornia crassipes [Mart.] Solms) plants were grown in environmental chambers at ambient and enriched CO₂ levels (330 and 600 microliters CO₂ per liter). Daughter plants (ramets) produced in the enriched CO₂ gained 39% greater dry weight than those at ambient CO₂, but the original mother plants did not. The CO₂ enrichment increased the number of leaves per ramet and leaf area index, but did not significantly increase leaf size or the number of ramets formed. Flower production was increased 147%. The elevated CO₂ increased the net photosynthetic rate of the mother plants by 40%, but this was not maintained as the plants acclimated to the higher CO₂ level. After 14 days at the elevated CO₂, leaf resistance increased and transpiration decreased, especially from the adaxial leaf surface. After 4 weeks in elevated as compared to ambient CO₂, ribulose bisphosphate carboxylase activity was 40% less, soluble protein content 49% less, and chlorophyll content 26% less; whereas starch content was 40% greater. Although at a given CO₂ level the enriched CO₂ plants had only half the net photosynthetic rate of their counterparts grown at ambient CO₂, they showed similar internal CO₂ concentrations. This suggested that the decreased supply of CO₂ to the mesophyll, as a result of the increased stomatal resistance, was counterbalanced by a decreased utilization of CO₂. Photorespiration and dark respiration were lower, such that the CO₂ compensation point was not altered. The photosynthetic light and CO₂ saturation points were not greatly changed, nor was the O₂ inhibition of photosynthesis (measured at 330 microliters CO₂ per liter). It appears that with CO₂ enrichment the temporary increase in net photosynthesis produced larger ramets. After acclimation, the greater total ramet leaf area more than compensated for the lower net photosynthetic rate on a unit leaf area basis, and resulted in a sustained improvement in dry weight gain.     

Acclimation to High CO(2) in Monoecious Cucumbers : I. Vegetative and Reproductive Growth

CO₂ concentrations of 1000 compared to 350 microliters per liter in controlled environment chambers did not increase total fruit weight or number in a monoecious cucumber (Cucumis sativus L. cv Chipper) nor did it increase biomass, leaf area, or relative growth rates beyond the first 16 days after seeding. Average fruit weight was slightly, but not significantly greater in the 1000 microliters per liter CO₂ treatment because fruit numbers were changed more than total weight. Plants grown at 1000 and 350 microliters per liter CO₂ were similar in distribution of dry matter and leaf area between mainstem, axillary, and subaxillary branches. Early flower production was greater in 1000 microliters per liter plants. Subsequent flower numbers were either lower in enriched plants or similar in the two treatments, except for the harvest at fruiting when enriched plants produced many more male flowers than the 350 microliters per liter treatments.     

Daily Changes in CO(2) and Water Vapor Exchange, Chlorophyll Fluorescence, and Leaf Water Relations in the Halophyte Mesembryanthemum crystallinum during the Induction of Crassulacean Acid Metabolism in Response to High NaCl Salinity

Simultaneous measurements of net CO₂ exchange, water vapor exchange, and leaf water relations were performed in Mesembryanthemum crystallinum during the development of crassulacean acid metabolism (CAM) in response to high NaCl salinity in the rooting medium. Determinations of chlorophyll a fluorescence were used to estimate relative changes in electron transport rate. Alterations in leaf mass per unit area, which—on a short-term basis—largely reflect changes in water content, were recorded continuously with a beta-gauge. Turgor pressure of mesophyll cells was determined with a pressure probe. As reported previously (K Winter, DJ von Willert [1972] Z Pflanzenphysiol 67: 166-170), recently expanded leaves of plants grown under nonsaline conditions showed gas-exchange characteristics of a C₃ plant. Although these plants were not exposed to any particular stress treatment, water content and turgor pressure regularly decreased toward the end of the 12 hour light periods and recovered during the following 12 hours of darkness. When the NaCl concentration of the rooting medium was raised to 400 millimolar, in increments of 100 millimolar given at the onset of the photoperiods for 4 consecutive days, leaf water content and turgor pressure decreased by as much as 30 and 60%, respectively, during the course of the photoperiods. These transient decreases probably triggered the induction of the biochemical machinery which is required for CAM to operate. After several days at 400 millimolar NaCl, when leaves showed features typical of CAM, overall turgor pressure and leaf mass per unit area had increased above the levels before onset of the salt treatment, and diurnal alterations in leaf water content were reduced. Net carbon gain during photoperiods and average intercellular CO₂ partial pressures at which net CO₂ uptake occurred, progressively decreased upon salinization. Reversible diurnal depressions in leaf conductance and net CO₂ uptake, with minima recorded in the middle of the photoperiods, preceded the occurrence of nocturnal net CO₂ uptake. During these reductions, intercellular CO₂ partial pressure and rates of photosynthetic electron transport decreased. With advancing age, leaves of plants grown under nonsaline conditions exhibited progressively greater diurnal reductions in turgor pressure and developed a low degree of CAM activity.     

Photosynthetic and anatomical responses of Eucalyptus grandis leaves to potassium and sodium supply in a field experiment

Although vast areas in tropical regions have weathered soils with low potassium (K) levels, little is known about the effects of K supply on the photosynthetic physiology of trees. This study assessed the effects of K and sodium (Na) supply on the diffusional and biochemical limitations to photosynthesis in Eucalyptus grandis leaves. A field experiment comparing treatments receiving K (+K) or Na (+Na) with a control treatment (C) was set up in a K‐deficient soil. The net CO₂ assimilation rates were twice as high in +K and 1.6 times higher in +Na than in the C as a result of lower stomatal and mesophyll resistance to CO₂ diffusion and higher photosynthetic capacity. The starch content was higher and soluble sugar was lower in +K than in C and +Na, suggesting that K starvation disturbed carbon storage and transport. The specific leaf area, leaf thickness, parenchyma thickness, stomatal size and intercellular air spaces increased in +K and +Na compared to C. Nitrogen and chlorophyll concentrations were also higher in +K and +Na than in C. These results suggest a strong relationship between the K and Na supply to E. grandis trees and the functional and structural limitations to CO₂ assimilation rates.     

Net CO(2) Assimilation Rate of Grapevine Leaves in Response to Trunk Girdling and Gibberellic Acid Application

Net CO₂ assimilation rate (A), stomatal conductance (g_s), and weight per unit leaf area (W) were determined on Thompson Seedless grapevines grown in the field. Treatments included fruit set applications of gibberellic acid (40 milligrams gibberellic acid (GA₃) per liter) to vines, shoots and clusters, alone and in combination with trunk girdling. Leaf A and g_s were measured prior to and 3, 6, and 13 days after fruit set. Weight per unit leaf area was determined on leaves collected subsequent to gas exchange measurements. Leaf A of girdled vines was reduced approximately 30% when compared to the control 13 days after treatment. The reduction in A due to girdling was not as great when vines were sprayed with GA₃. GA₃ sprays alone had no significant effect on A. Stomatal conductance was reduced by girdling 13 days after treatment. Weight per unit leaf area was 17% greater for trunk girdled vines when compared to the controls. Results indicate GA₃ affected net CO₂ assimilation rate only on girdled vines, a treatment which increased weight per unit leaf area.     

Limiting Factors in Photosynthesis: IV. Iron Stress-Mediated Changes in Light-Harvesting and Electron Transport Capacity and its Effects on Photosynthesis in Vivo

Using iron stress to reduce the total amount of light-harvesting and electron transport components per unit leaf area, the influence of light-harvesting and electron transport capacity on photosynthesis in sugar beet (Beta vulgaris L. cv F58-554H1) leaves was explored by monitoring net CO₂ exchange rate (P) in relation to changes in the content of Chl.

In most light/CO₂ environments, and especially those with high light (≥1000 microeinsteins photosynthetically active radiation per square meter per second) and high CO₂ (≥300 microliters CO₂ per liter air), P per area was positively correlated with changes in Chl (a + b) content (used here as an index of the total amount of light-harvesting and electron transport components). This positive correlation of P per area with Chl per area was obtained not only with Fe-deficient plants, but also over the normal range of variation in Chl contents found in healthy, Fe-sufficient plants. For example, light-saturated P per area at an ambient CO₂ concentration close to normal atmospheric levels (300 microliters CO₂ per liter air) increased by 36% with increase in Chl over the normal range, i.e. from 40 to 65 micrograms Chl per square centimeter. Iron deficiency-mediated changes in Chl content did not affect dark respiration rate or the CO₂ compensation point. The results suggest that P per area of sugar beet may be colimited by light-harvesting and electron transport capacity (per leaf area) even when CO₂ is limiting photosynthesis as occurs under field conditions.