Photosynthetic and stomatal responses of spinach leaves to salt stress

The gas exchange of spinach plants, salt-stressed by adding NaCl to the nutrient solution in increments of 25 millimolar per day to a final concentration of 200 millimolar, was studied 3 weeks after starting NaCl treatment. Photosynthesis became light saturated at 1100 to 1400 micromoles per square meter per second in salt-treated plants and at approximately 2000 micromoles per square meter per second in control plants. Photosynthetic capacity of the mesophyll measured as a function of intercellular partial pressure of CO₂ at the light intensity prevailing during growth and at light saturation were both decreased in the salttreated plants. The CO₂ compensation points and relative enhancements of photosynthesis at low O₂ were not affected by salinity. The lower photosynthetic rates in salt-treated leaves at 450 micromoles per square meter per second were associated with a 70% reduction in stomatal conductance and low intercellular CO₂ (219 microbars; cf. 285 microbars for controls). Increasing photon flux density to light saturation extended the linear portions of the CO₂ response curves, increased stomatal conductances, increased intercellular CO₂ in the salt-treated plants, but lowered it in controls, and accentuated differences in photosynthetic rate (area basis) between the treatments.

Leaves from salt-treated plants were thicker but contained about 73% of the chlorophyll per unit area of control plants. When photosynthetic rates were expressed on a chlorophyll basis there was no difference in initial slope of assimilation versus intercellular CO₂ between treatments. Photosynthetic rates (chlorophyll basis) at light saturation differed only by 20% which was also observed earlier with isolated, intact chloroplasts (Robinson et al. 1983 Plant Physiol 73: 238-242).

Measurement of carbon isotope ratio revealed less discrimination against ¹³C with salt treatment and confirmed the persistence of low intercellular partial pressures of CO₂ during plant growth. The development of a thicker leaf with less chlorophyll per unit area during salt treatment permitted stomatal conductance and intercellular partial pressure of CO₂ to decline without restricting photosynthesis and had the benefit of greatly increasing water use efficiency.

     

Effect of Photon Fluence Rate on Oxygen Evolution and Uptake by Chlamydomonas reinhardtii Suspensions Grown in Ambient and CO(2)-Enriched Air

A closed system consisting of an assimilation chamber furnished with a membrane inlet from the liquid phase connected to a mass spectrometer was used to measure O₂ evolution and uptake by Chlamydomonas reinhardtii cells grown in ambient (0.034% CO₂) or CO₂-enriched (5% CO₂) air. At pH = 6.9, 28°C and concentrations of dissolved inorganic carbon (DIC) saturating for photosynthesis, O₂ uptake in the light (U_o) equaled O₂ production (E_o) at the light compensation point (15 micromoles photons per square meter per second). E_o and U_o increased with increasing photon fluence rate (PFR) but were not rate saturated at 600 micromoles photons per square meter per second, while net O₂ exchange reached a saturation level near 500 micromoles photons per square meter per second which was nearly the same for both, CO₂-grown and air-grown cells. Comparison of the U_o/E_o ratios between air-grown and CO₂-grown C. reinhardtii showed higher values for air-grown cells at light intensities higher than light compensation. For both, air-grown and CO₂-grown algae the rates of mitochondrial O₂ uptake in the dark measured immediately before and 5 minutes after illumination were much lower than U_o at PFR saturating for net photosynthesis. We conclude that noncyclic electron flow from water to NADP⁺ and pseudocyclic electron flow via photosystem I to O₂ both significantly contribute to O₂ exchange in the light. In contrast, mitochondrial respiration and photosynthetic carbon oxidation cycle are regarded as minor O₂ consuming reactions in the light in both, air-grown and CO₂-grown cells. It is suggested that the “extra” O₂ uptake by air-grown algae provides ATP required for the energy dependent CO₂/HCO₃⁻ concentrating mechanism known to be present in these cells.     

Photosynthetic dynamics in chrysanthemum in response to single step increases and decreases in photon flux density

The time-course of CO₂ assimilation rate and stomatal conductance to step changes in photosynthetic photon flux density (PPFD) was observed in Chrysanthemum × morifolium Ramat. `Fiesta'. When PPFD was increased from 200 to 600 micromoles per square meter per second, the rate of photosynthetic CO<sub>2</sub> assimilation showed an initial rapid increase over the first minute followed by a slower increase over the next 12 to 38 minutes, with a faster response in low-light-grown plants. Leaves exposed to small step increases (100 micromoles per square meter per second) reached the new steady-state assimilation rate within a minute. Both stomatal and biochemical limitations played a role during photosynthetic induction, but carboxylation limitations seemed to predominate during the first 5 to 10 minutes. Stomatal control during the slow phase of induction was less important in low-light compared to high-light-grown plants. In response to step decreases in PPFD, photosynthetic rate decreased rapidly and a depression in CO<sub>2</sub> assimilation prior to steady-state was observed. This CO<sub>2</sub> assimilation `dip' was considerably larger for the large step (400 micromoles per square meter per second) than for the small step. The rapid photosynthetic response seems to be controlled by biochemical processes. High- and low-light-grown plants did not differ in their photosynthetic response to PPFD step decreases.     

Kinetic characterization of reduced pyridine nucleotide dehydrogenases (duroquinone-dependent) in cucurbita microsomes

Stomatal conductances of normally oriented and inverted leaves were measured as light levels (photosynthetic photon flux densities) were increased to determine whether abaxial stomata of Vicia faba leaves were more sensitive to light than adaxial stomata. Light levels were increased over uniform populations of leaves of plants grown in an environmental chamber. Adaxial stomata of inverted leaves reached maximum water vapor conductances at a light level of 60 micromoles per square meter per second, the same light level at which abaxial stomata of normally oriented leaves reached maximum conductances. Abaxial stomata of inverted leaves reached maximum conductances at a light level of 500 micromoles per square meter per second, the same light level at which adaxial stomata of normally oriented leaves reached maximum conductances. Maximum conductances in both normally oriented and inverted leaves were about 200 millimoles per square meter per second for adaxial stomata and 330 millimoles per square meter per second for abaxial stomata. Regardless of whether leaves were normally oriented or inverted, when light levels were increased to values high enough that upper leaf surfaces reached maximum conductances (about 500 micromoles per square meter per second), light levels incident on lower, shaded leaf surfaces were just sufficient (about 60 micromoles per square meter per second) for stomata of those surfaces to reach maximum conductances. This `coordinated' stomatal opening on the separate epidermes resulted in total leaf conductances for normally oriented and inverted leaves that were the same at any given light level. We conclude that stomata in abaxial epidermes of intact Vicia leaves are not more sensitive to light than those in adaxial epidermes, and that stomata in leaves of this plant do not respond to light alone. Additional factors in bulk leaf tissue probably produce coordinated stomatal opening on upper and lower leaf epidermes to optimally meet photosynthetic requirements of the whole leaf for CO₂.     

Biphasic Activation of Ribulose Bisphosphate Carboxylase in Spinach Leaves as Determined from Nonsteady-State CO(2) Exchange

The activation kinetics of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) following an increase in photon flux density (PFD) were studied by analyzing CO₂ assimilation time courses in spinach leaves (Spinacia oleracea). When leaves were exposed to 45 minutes of darkness before illumination at 690 micromoles per square meter per second, Rubisco activation followed apparent first-order kinetics with a relaxation time of about 3.8 minutes. But when leaves were illuminated for 45 minutes at 160 micromoles per square meter per second prior to illumination at 690 micromoles per square meter per second the relaxation time for Rubisco activation was only 2.1 minutes. The kinetics of this change in relaxation times were investigated by exposing dark-adapted leaves to 160 micromoles per square meter per second for different periods before increasing the PFD to 690 micromoles per square meter per second. It was found that the apparent relaxation time for Rubisco activation changed from 3.8 to 2.1 minutes slowly, requiring at least 8 minutes for completion. This result indicates that at least two sequential, slow processes are involved in light-mediated activation of Rubisco in spinach leaves and that the relaxation times characterizing these two processes are about 4 and 2 minutes, respectively. The kinetics of the first process in the reverse direction and the dependence of the relaxation time for the second process on the magnitude of the increase in PFD were also determined. Evidence that the first slow process is activation of the enzyme Rubisco activase and that the second slow process is the catalytic activation of Rubisco by activase is discussed.     

Regulation of Ribulose-1,5-Bisphosphate Carboxylase Activity in Response to Light Intensity and CO(2) in the C(3) Annuals Chenopodium album L. and Phaseolus vulgaris L

The light and CO₂ response of (a) photosynthesis, (b) the activation state and total catalytic efficiency (k_cat) of ribulose-1,5-bisphosphate carboxylase (rubisco), and (c) the pool sizes of ribulose 1,5-bisphosphate, (RuBP), ATP, and ADP were studied in the C₃ annuals Chenopodium album and Phaseolus vulgaris at 25°C. The initial slope of the photosynthetic CO₂ response curve was dependent on light intensity at reduced light levels only (less than 450 micromoles per square meter per second in C. album and below 200 micromoles per square meter per second in P. vulgaris). Modeled simulations indicated that the initial slope of the CO₂ response of photosynthesis exhibited light dependency when the rate of RuBP regeneration limited photosynthesis, but not when rubisco capacity limited photosynthesis. Measured observations closely matched modeled simulations. The activation state of rubisco was measured at three light intensities in C. album (1750, 550, and 150 micromoles per square meter per second) and at intercellular CO₂ partial pressures (C₁) between the CO₂ compensation point and 500 microbars. Above a C₁ of 120 microbars, the activation state of rubisco was light dependent. At light intensities of 550 and 1750 micromoles per square meter per second, it was also dependent on C₁, decreasing as the C₁ was elevated above 120 microbars at 550 micromoles per square meter per second and above 300 microbars at 1750 micromoles per square meter per second. The pool size of RuBP was independent of C₁ only under conditions when the activation state of rubisco was dependent on C₁. Otherwise, RuBP pool sizes increased as C₁ was reduced. ATP pools in C. album tended to increase as C₁ was reduced. In P. vulgaris, decreasing C₁ at a subsaturating light intensity of 190 micromoles per square meter per second increased the activation state of rubisco but had little effect on the k_cat. These results support modelled simulations of the rubisco response to light and CO₂, where rubisco is assumed to be down-regulated when photosynthesis is limited by the rate of RuBP regeneration.     

Growth Kinetics, Carbohydrate, and Leaf Phosphate Content of Clover (Trifolium subterraneum L.) after Transfer to a High CO(2) Atmosphere or to High Light and Ambient Air

Intact air-grown (photosynthetic photon flux density, 400 microeinsteins per square meter per second) clover plants (Trifolium subterraneum L.) were transfered to high CO₂ (4000 microliters CO₂ per liter; photosynthetic photon flux density, 400 microeinsteins per square meter per second) or to high light (340 microliters CO₂ per liter; photosynthetic photon flux density, 800 microeinsteins per square meter per second) to similarly stimulate photosynthetic net CO₂ uptake. The daily increment of net CO₂ uptake declined transiently in high CO₂, but not in high light, below the values in air/standard light. After about 3 days in high CO₂, the daily increment of net CO₂ uptake increased but did not reach the high light values. Nightly CO₂ release increased immediately in high light, whereas there was a 3-day lag phase in high CO₂. During this time, starch accumulated to a high level, and leaf deterioration was observed only in high CO₂. After 12 days, starch was two- to threefold higher in high CO₂ than in high light, whereas sucrose was similar. Leaf carbohydrates were determined during the first and fourth day in high CO₂. Starch increased rapidly throughout the day. Early in the day, sucrose was low and similar in high CO₂ and ambient air (same light). Later, sucrose increased considerably in high CO₂. The findings that (a) much more photosynthetic carbon was partitioned into the leaf starch pool in high CO₂ than in high light, although net CO₂ uptake was similar, and that (b) rapid starch formation occurred in high CO₂ even when leaf sucrose was only slightly elevated suggest that low sink capacity was not the main constraint in high CO₂. It is proposed that carbon partitioning between starch (chloroplast) and sucrose (cytosol) was perturbed by high CO₂ because of the lack of photorespiration. Total phosphate pools were determined in leaves. Concentrations based on fresh weight of orthophosphate, soluble esterified phosphate, and total phosphate markedly declined during 13 days of exposure of the plants to high CO₂ but changed little in high light/ambient air. During this time, the ratio of orthophosphate to soluble esterified phosphate decreased considerably in high CO₂ and increased slightly in high light/ambient air. It appears that phosphate uptake and growth were similarly stimulated by high light, whereas the coordination was weak in high CO₂.     

Effects of Irradiance on Crassulacean Acid Metabolism in the Epiphyte Tillandsia usneoides L. (Bromeliaceae)

Spanish moss (Tillandsia usneoides L.) was collected in South Carolina, maintained in a greenhouse, then exposed to five levels of photosynthetic photon flux density (PPFD) for 3 weeks. Following this treatment, plants were sampled for chlorophyll concentrations, nocturnal acid accumulations, and photosynthetic responses to subsequent exposure at a range of PPFD. No acclimation to PPFD was observed; all plants exhibited similar patterns of nocturnal CO₂ uptake and acid accumulation regardless of initial PPFD treatment. These patterns revealed that at a PPFD level of approximately 200 micromoles per square meter per second (daytime integrated PPFD of 10 moles per square meter per day), CAM saturated or, in low-PPFD plants, was optimal. The results of this study indicate that adaptation to high PPFD is not necessarily a requirement of CAM.     

Carbon Dioxide Fixation in the Carbon Economy of Developing Seeds of Lupinus albus (L.)

The effects of CO₂ concentration and illumination on net gas exchange and the pathway of ¹⁴CO₂ fixation in detached seeds from developing fruits of Lupinus albus (L.) have been studied.

Increasing the CO₂ concentration in the surrounding atmosphere (from 0.03 to 3.0% [v/v] in air) decreased CO₂ efflux by detached seeds either exposed to the light flux equivalent to that transmitted by the pod wall (500 to 600 micro-Einsteins per square meter per second) in full sunlight or held in darkness. Above 1% CO₂ detached seeds made a net gain of CO₂ in the light (up to 0.4 milligrams of CO₂ fixed per gram fresh weight per hour) but ¹⁴CO₂ injected into the gas space of intact fruits (containing around 1.5% CO₂ naturally) was fixed mainly by the pod and little by the seeds.

Throughout development seeds contained ribulose-1,5-bisphosphate carboxylase activity (EC 4.1.1.39), especially in the embryo (up to 99 micromoles of CO₂ fixed per gram fresh weight per hour) and phosphoenolpyruvate carboxylase (EC 4.1.1.31) in both testa (up to 280 micromoles of CO₂ fixed per gram fresh weight per hour) and embryo (up to 355 micromoles of CO₂ fixed per gram fresh weight per hour).

In kinetic experiments the most significant early formed product of ¹⁴CO₂ fixation in both light and dark was malate but in the light phosphoglyceric acid and sugar phosphates were also rapidly labeled. ¹⁴CO₂ fixation in the light was linked to the synthesis of sugars and amino acids but in the dark labeled sugars were not formed.

     

Crassulacean Acid Metabolism and Photochemical Efficiency of Photosystem II in the Adaxial and Abaxial Parts of the Succulent Leaves of Kalanchoë daigremontiana Grown at Four Photon Flux Densities

Kalanchoë daigremontiana, a species possessing crassulacean acid metabolism, was grown at four photon flux densities (1300, 400, 60, and 25 micromole photons per square meter per second). In leaves which had developed at 1300 and 400 micromole photons per square meter per second, CO₂ was mainly incorporated through the lower, shaded leaf surfaces, and the chlorenchyma adjacent to the lower surfaces showed a higher degree of nocturnal acid synthesis than the chlorenchyma adjacent to the upper surfaces. In leaves acclimated to 60 and 25 micromole photons per square meter per second, the gradient in CAM activity was reversed, i.e. more CO₂ was taken up through the upper than through the lower surfaces and nocturnal acidification was higher in the tissue next to the upper surfaces. Total net carbon gain and total nocturnal acid synthesis were highest in leaves which had developed at 400 micromole photons per square meter per second. Chlorophyll content was markedly reduced in leaves which had developed at 1300 micromole photons per square meter per second, especially in the exposed adaxial parts. There was also a sustained reduction in photosystem II photochemical efficiency as indicated by measurements of the ratio of variable over maximum chlorophyll a fluorescence. These findings suggest that, at high growth photon flux densities, the reduced activity of the exposed portions of these succulent leaves is caused by (a) the adverse effects of excess light, (b) together with a genotypic component which favors CO₂ uptake and acid synthesis in the abaxial (lower) leaf parts even when light is not or only marginally excessive. This latter component is predominant at medium photon flux densities, e.g. at 400 micromole photons per square meter per second. It becomes overridden, however, under conditions of deep shade when strongly reduced light levels in the abaxial parts of the leaf chlorenchyma severely limit photosynthesis.     

Relationships between Photosynthetically Active Radiation, Nocturnal Acid Accumulation, and CO(2) Uptake for a Crassulacean Acid Metabolism Plant, Opuntia ficus-indica

The influences of photosynthetically active radiation (PAR) and water status on nocturnal Crassulacean acid metabolism (CAM) were quantitatively examined for a widely cultivated cactus, Opuntia ficus-indica (L.) Miller. When the total daily PAR was maintained at 10 moles photons per square meter per day but the instantaneous PAR level varied, the rate of nocturnal H⁺ accumulation (tissue acidification) became 90% saturated near 700 micromoles per square meter per second, a PAR level typical for similar light saturation of C₃ photosynthesis. The total nocturnal H⁺ accumulation and CO₂ uptake reached 90% of maximum for a total daily PAR of about 22 moles per square meter per day. Light compensation occurred near 0 moles per square meter per day for nocturnal H⁺ accumulation and 4 moles per square meter per day for CO₂ uptake. Above a total daily PAR of 36 moles per square meter per day or for an instantaneous PAR of 1150 micromoles per square meter per second for more than 6 hours, the nocturnal H⁺ accumulation actually decreased. This inhibition, which occurred at PAR levels just above those occurring in the field, was accompanied by a substantial decrease in chlorophyll content over a 1-week period.

A minimum ratio of H⁺ accumulated to CO₂ taken up of 2.5 averaged over the night occurred for a total daily PAR of 31 moles per square meter per day under wet conditions. About 2 to 6 hours into the night under such conditions, a minimum H⁺-to-CO₂ ratio of 2.0 was observed. Under progressively drier conditions, both nocturnal H⁺ accumulation and CO₂ uptake decreased, but the H⁺-to-CO₂ ratio increased. A ratio of two H⁺ per CO₂ is consistent with the H⁺ production accompanying the conversion of starch to malic acid, and it apparently occurs for O. ficus-indica when CAM CO₂ uptake is strongly favored over respiratory activity.

     

Growth and Tuberization of Potato (Solanum tuberosum L.) under Continuous Light

The growth and tuberization of potatoes (Solanum tuberosum L.) maintained for 6 weeks under four different regimes of continuous irradiance were compared to plants given 12 hours light and 12 hours dark. Treatments included: (a) continuous photosynthetic photon flux of 200 micromoles per square meter per second cool-white fluorescent (CWF); (b) continuous 400 micromoles per square meter per second CWF; (c) 12 hours 400 micromoles per square meter per second CWF plus 12 hours dim CWF at 5 micromoles per square meter per second; (d) 12 hours micromoles per square meter per second CWF plus 12 hours dim incandescent (INC) at 5 micromoles per square meter per second and a control treatment of 12 hours light at 400 micromoles per square meter per second CWF and 12 hours dark. The study included five cultivars ranging from early- to late-season types: `Norland,' `Superior,' `Norchip,' `Russet Burbank,' and `Kennebec.' Tuber development progressed well under continuous irradiation at 400 micromoles per square meter per second and under 12 hours irradiance and 12 hours dark, while tuber development was suppressed in all other light treatments. Continuous irradiation at 200 or 400 micromoles per square meter per second resulted in severe stunting and leaf malformation on `Superior' and `Kennebec' plants, but little or no injury and vigorous shoot growth in the other cultivars. No injury or stunting were apparent under 12-dim light or 12-dark treatments. Plants given 12 hours dim INC showed significantly greater stem elongation but less total biomass than plants in other treatments. The continuous light encouraged shoot growth over tuber growth but this trend was overridden by providing a high irradiance level. The variation among cultivars for tolerance to continuous lighting indicates that potato may be a useful species for photoinhibition studies.     

Interspecific Variation in SO(2) Flux : Leaf Surface versus Internal Flux, and Components of Leaf Conductance

The objective of this study was to clarify the relationships among stomatal, residual, and epidermal conductances in determining the flux of SO₂ air pollution to leaves. Variations in leaf SO₂ and H₂O vapor fluxes were determined using four plant species: Pisum sativum L. (garden pea), Lycopersicon esculentum Mill. flacca (mutant of tomato), Geranium carolinianum L. (wild geranium), and Diplacus aurantiacus (Curtis) Jeps. (a native California shrub). Fluxes were measured using the mass-balance approach during exposure to 4.56 micromoles per cubic meter (0.11 microliters per liter) SO₂ for 2 hours in a controlled environmental chamber. Flux through adaxial and abaxial leaf surfaces with closed stomata ranged from 1.9 to 9.4 nanomoles per square meter per second for SO₂, and 0.3 to 1.3 millimoles per square meter per second for H₂O vapor. Flux of SO₂ into leaves through stomata ranged from ~0 to 8.5 (dark) and 3.8 to 16.0 (light) millimoles per square meter per second. Flux of H₂O vapor from leaves through stomata ranged from ~0 to 0.6 (dark) to 0.4 to 0.9 (light) millimole per square meter per second. Lycopersicon had internal flux rates for both SO₂ and H₂O vapor over twice as high as for the other species. Stomatal conductance based on H₂O vapor flux averaged from 0.07 to 0.13 mole per square meter per second among the four species. Internal conductance of SO₂ as calculated from SO₂ flux was from 0.04 mole per square meter per second lower to 0.06 mole per square meter per second higher than stomatal conductance. For Pisum, Geranium, and Diplacus stomatal conductance was the same or slightly higher than internal conductance, indicating that, in general, SO₂ flux could be predicted from stomatal conductance for H₂O vapor. However, for the Lycopersicon mutant, internal leaf conductance was much higher than stomatal conductance, indicating that factors inside leaves can play a significant role in determining SO₂ flux.     

Photosynthetic and Respiratory Activity of Fruiting Forms within the Cotton Canopy

The supply of photosynthates by leaves for reproductive development in cotton (Gossypium hirsutum L.) has been extensively studied. However, the contribution of assimilates derived from the fruiting forms themselves is inconclusive. Field experiments were conducted to document the photosynthetic and respiratory activity of cotton leaves, bracts, and capsule walls from anthesis to fruit maturity. Bracts achieved peak photosynthetic rates of 2.1 micromoles per square meter per second compared with 16.5 micromoles per square meter per second for the subtending leaf. However, unlike the subtending leaf, the bracts did not show a dramatic decline in photosynthesis with increased age, nor was their photosynthesis as sensitive as leaves to low light and water-deficit stress. The capsule wall was only a minor site of ¹⁴CO₂ fixation from the ambient atmosphere. Dark respiration by the developing fruit averaged −18.7 micromoles per square meter per second for 6 days after anthesis and declined to −2.7 micromoles per square meter per second after 40 days. Respiratory loss of CO₂ was maximal at −158 micromoles CO₂ per fruit per hour at 20 days anthesis. Diurnal patterns of dark respiration for the fruit were age dependent and closely correlated with stomatal conductance of the capsule wall. Stomata on the capsule wall of young fruit were functional, but lost this capacity with increasing age. Labeled ¹⁴CO₂ injected into the fruit interior was rapidly assimilated by the capsule wall in the light but not in the dark, while fiber and seed together fixed significant amounts of ¹⁴CO₂ in both the light and dark. These data suggest that cotton fruiting forms, although sites of significant respiratory CO₂ loss, do serve a vital role in the recycling of internal CO₂ and therein, function as important sources of assimilate for reproductive development.     

Gas Exchange Analysis of the Fast Phase of Photosynthetic Induction in Alocasia macrorrhiza

When leaves of Alocasia macrorrhiza that had been preconditioned in 10 micromoles photons per square meter per second for at least 2 hours were suddenly exposed to 500 micromoles photons per square meter per second, there was an almost instantaneous increase in assimilation rate. After this initial increase, there was a secondary increase over the next minute. This secondary increase was more pronounced in high CO₂ (1400 microbars), where assimilation rate was assumed to be limited by the rate of regeneration of ribulose 1,5-bisphosphate (RuBP). It was absent in low CO₂ (75 microbars), where RuBP carboxylase/oxygenase (Rubisco) was assumed to be limiting. It was therefore concluded that it represented an increase in the capacity to regenerate RuBP. This fast-inducing component not only gained full induction rapidly, but also lost it rapidly in low photon flux density (PFD) with a half time of 150 to 200 seconds. It was concluded that in environments with fluctuating PFD, this fast-inducing component is an important factor in determining a leaf's potential for photosynthetic carbon gain. It is especially important during brief periods (<30 seconds) of high PFD that follow moderately long periods (1 to 10 minutes) of low PFD.     

Photoautotrophic growth of soybean cells in suspension culture: I. Establishment of photoautotrophic cultures

Highly chlorophyllous photomixotrophic callus was visually selected from callus originating from soybean (Glycine max (L.) Merr. var. Corsoy) cotyledon. Suspension cultures initiated from this callus became photoautotrophic under continuous light with an atmosphere of 5% CO₂ (balance air). Dry weight increases of 1000 to 1400% in the 2-week subculture period have been observed. The cellular Chl content ranged from 4.4 to 5.9 micrograms per milligram dry weight which is about 75 to 90% of the Chl content in soybean leaves under equivalent illumination (300 micro-Einsteins per square meter per second).

No growth can be observed in the dark in sucrose-lacking medium or in the presence of 0.5 micromolar 3-(3,4-dichlorophenyl)-1,1-dimethylurea, a concentration which does not inhibit heterotrophic growth (on sucrose). Photoautotrophic growth has an absolute requirement for elevated CO₂ concentrations (>1%). During the 14-day subculture period, growth (fresh weight and dry weight) is logarithmic. Photosynthesis quickly increases after day 4, reaching a peak of 83 micromoles CO₂ incorporated per milligram Chl per hour while dark respiration decreases 90% from day 2 to day 6. The pH of the growth medium quickly drops from 7.0 to 4.5 before slowly increasing to 5.0 by day 14. At this pH range and light intensity (200-300 microEinsteins per square meter per second), no O₂ evolution could be detected although at high pH and light intensity O₂ evolution was recorded.

     

Distinctive Light and CO(2)-Fixation Requirements of Nitrate and Ammonium Utilization by the Cyanobacterium Anacystis nidulans

The effect of light intensity on the rates of ammonium and nitrate uptake and of CO₂ fixation has been determined in intact Anacystis nidulans cells. Ammonium uptake became saturated at photon flux values of about 60 microeinsteins per square meter per second, whereas both nitrate uptake and CO₂ fixation reached saturation at about 250 microeinsteins per square meter per second, the rates of the two latter processes being tightly correlated at any light intensity assayed. Inhibition of ammonium assimilation resulted in the loss of correlation between CO₂ fixation and nitrate uptake, the latter process exhibiting then a reduced light requirement. The results establish a clear distinction between ammonium utilization and nitrate utilization with regard to their light requirement and to the nature of their dependence upon CO₂ fixation.     

Sugar Concentrations in Guard Cells of Vicia faba Illuminated with Red or Blue Light : Analysis by High Performance Liquid Chromatography

Concentrations of soluble sugars in guard cells in detached, sonicated epidermis from Vicia faba leaves were analyzed quantitatively by high performance liquid chromatography to determine the extent to which sugars could contribute to changes in the osmotic potentials of guard cells during stomatal opening. Stomata were illuminated over a period of 4 hours with saturating levels of red or blue light, or a combination of red and blue light. When stomata were irradiated for 3 hours with red light (50 micromoles per square meter per second) in a solution of 5 millimolar KCl and 0.1 millimolar CaCl₂, stomatal apertures increased a net maximum of 6.7 micrometers and the concentration of total soluble sugar was 289 femtomoles per guard cell (70% sucrose, 30% fructose). In an identical solution, 2.5 hours of irradiation with 25 micromoles per square meter per second of blue light caused a maximum net increase of 7.1 micrometers in stomatal aperture and the total soluble sugar concentration was 550 femtomoles per guard cell (91% sucrose, 9% fructose). Illumination with blue light at 25 micromoles per square meter per second in a solution lacking KCl caused a maximum net increase in stomatal aperture of 3.5 micrometers and the sugar concentration was 382 femtomoles per guard cell (82% sucrose, 18% fructose). In dual beam experiments, stomata irradiated with 50 micromoles per square meter per second of red light opened steadily with a concomitant increase in sugar production. Addition of 25 micromoles per square meter per second of blue light caused a further net gain of 3.7 micrometers in stomatal aperture and, after 2 hours, sugar concentrations had increased by an additional 138 femtomoles per guard cell. Experiments with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) were performed with epidermis illuminated with 50 micromoles per square meter per second of red light or with 25 micromoles per square meter per second of blue light in solutions containing or lacking KCl. DCMU completely inhibited sugar production under red light, had no effect on guard cell sugar production under blue light when KCl was present, and inhibited sugar production by about 50% when guard cells were illuminated with blue light in solutions lacking KCl. We conclude that soluble sugars can contribute significantly to the osmoregulation of guard cells in detached leaf epidermis of V. faba. These results are consistent with the operation of two different sugar-producing pathways in guard cells: a photosynthetic carbon reduction pathway and a pathway of blue light-induced starch degradation.     

Whole Leaf Carbon Exchange Characteristics of Phosphate Deficient Soybeans (Glycine max L.)

Low phosphate nutrition results in increased chlorophyll fluorescence, reduced photosynthetic rate, accumulation of starch and sucrose in leaves, and low crop yields. This study investigated physiological responses of soybean (Glycine max [L.] Merr.) leaves to low inorganic phosphate (Pi) conditions. Responses of photosynthesis to light and CO₂ were examined for leaves of soybean grown at high (0.50 millimolar) or low (0.05 millimolar) Pi. Leaves of low Pi plants exhibited paraheliotropic orientation on bright sunny days rather than the normal diaheliotropic orientation exhibited by leaves of high Pi soybeans. Leaves of plants grown at high Pi had significantly higher light saturation points (1000 versus 630 micromole photons [400-700 nanometers] per square meter per second) and higher apparent quantum efficiency (0.062 versus 0.044 mole CO₂ per mole photons) at ambient (34 pascals) CO₂ than did low Pi leaves, yet stomatal conductances were similar. High Pi leaves also had significantly higher carboxylation efficiency (2.90 versus 0.49 micromole CO₂ per square meter per second per pascal), a lower CO₂ compensation point (6.9 versus 11.9 pascals), and a higher photosynthetic rate at 34 pascals CO₂ (19.5 versus 6.7 micromoles CO₂ per square meter per second) than did low Pi leaves. Soluble protein (0.94 versus 0.73 milligram per square centimeter), ribulose-1,5-bisphosphate carboxylase/oxygenase content (0.33 versus 0.25 milligram per square centimeter), and ribulose-1,5-bisphosphate carboxylase/oxygenase specific activity (25.0 versus 16.7 micromoles per square meter per second) were significantly greater in leaves of plants in the high Pi treatment. The data indicate that Pi stress alters the plant's CO₂ reduction characteristics, which may in turn affect the plant's capacity to accommodate normal radiation loads.     

Combined low temperature-high light effects on gas exchange properties of jojoba leaves

Jojoba (Simmondsia chinensis [Link] Schneider) is an important crop in desert climates. A relatively high frequency of periods of chilling and high photon flux density (PFD) in this environment makes photoinhibition likely, resulting in a reduction of assimilation capacity in overwintering leaves. This could explain the low net photosynthesis found in shoots from the field (4-6 micromoles per square meter per second) when compared to greenhouse grown plants (12-15 micromoles per square meter per second). The responses of photosynthesis and stomatal conductance to changes in absorbed PFD and in substomatal partial pressure of CO₂ were measured on jojoba leaves recovering from chilling temperature (4°C) in high or low PFD. No measurable gas exchange was found immediately after chilling in either high or low PFD. For leaves chilled in low PFD, the original quantum yield was restored after 24 hours. The time course of recovery from chilling in high PFD was much longer. Quantum yield recovered to 60% of its original value in 72 hours but failed to recover fully after 1 week. Measurements of PSII chlorophyll fluorescence at 77 K showed that the reduced quantum yield was caused by photoinhibition. The ratio of variable to maximal fluorescence fell from a control level of 0.82 to 0.41 after the photoinhibitory treatment and recovery was slow. We also found a large increase in net assimilation rate and little closure of stomata as CO₂ was increased from ambient partial pressure of 35 to 85 pascals. For plants grown in full light, the increase in net assimilation rate was 100%. The photosynthetic response at high CO₂ concentration may constitute an ecological advantage of jojoba as a crop in the future.