Amino Acid Synthesis in Photosynthesizing Spinach Cells : EFFECTS OF AMMONIA ON POOL SIZES AND RATES OF LABELING FROM CO(2)

Isolated cells from leaves of Spinacia oleracea have been maintained in a state capable of high rates of photosynthetic CO₂ fixation for more than 60 hours. The incorporation of ¹⁴CO₂ under saturating CO₂ conditions into carbohydrates, carboxylic acids, and amino acids, and the effect of ammonia on this incorporation have been studied. Total incorporation, specific radioactivity, and pool size have been determined as a function of time for most of the protein amino acids and for γ-aminobutyric acid. The measurements of specific radio-activities and of the approaches to ¹⁴C “saturation” of some amino acids indicate the presence and relative sizes of metabolically active and passive pools of these amino acids.     

Adaptation to CO(2) Level and Changes in the Phosphorylation of Thylakoid Proteins during the Cell Cycle of Chlamydomonas reinhardtii

The photosynthetic performance of synchronously grown Chlamydomonas reinhardtii alternated rhythmically during the cell cycle. The activity of the “CO₂ concentrating mechanism” including the ability to accumulate CO₂ internally and the activity of carbonic anhydrase peaked after 6 to 9 hours of light and reached minimum after 6 to 9 hours of dark. Consequently, the apparent photosynthetic affinity to extracellular CO₂ alternated rhythmically. At the end of the dark period the cells behaved as if they were adapted to high CO₂ even though they were continuously aerated with air. Results from experiments in which the light or dark periods were extended bear on the interaction between the internal (cell cycle or biological clock) and the external (light) signal. The observed rhythmical alterations in photosynthetic V_max may result from changes in PSII activity. The latter may be partly explained by the capacity for phosphorylation of thylakoid proteins, which reached maximum after 9 hours of light and decreased toward the dark period.     

Nucleic Acid Metabolism in Germinating Onion: I. Changes in Root Tip Nucleic Acid during Germination

Nucleic acid synthesis in the G₁ cell population of the 1-millimeter apex of the Allium cepa embryo was studied during the initial 73 hours of germination. Quantitative data indicate that the total amount of RNA per cell began to increase after 18 hours of germination while the initial DNA per cell increase did not occur until some 20 hours later. Polyacrylamide gel electrophoresis patterns of ³H-uridine-labeled total nucleic acid samples indicated that synthesis of all detectable RNA fractions present in the pre-emergent 1-millimeter apex (i.e., cytoplasmic and “chloroplast-like” RNA) began at approximately the same time (18 hours). Synthesis of the various cytoplasmic RNA fractions continued throughout the germination period. Data indicating synthesis of the “chloroplast-like” RNA were obtained only for the initial 36 hours of germination. Specific radioactivity of ³H-uridine-labeled total nucleic acid increased during the first 41.5 hours of germination but then decreased while the accumulation of RNA per cell continued to increase throughout the 73-hour period. In addition, a method is described which reduced the bacterial contamination of Allium seed to a level not detectable by incorporation of radioactive precursors into bacterial ribosomal RNA.     

Photoinhibition and P700 in the Marine Diatom Amphora sp

The marine diatom Amphora sp. was grown at a light intensity of 7.0 × 10¹⁵ quanta centimeter⁻² second⁻¹. Light saturation of photosynthesis for these cells was between 6.0 and 7.0 × 10¹⁶ quanta centimeter⁻² second⁻¹. At light intensities greater than saturation, photosynthetic ¹⁴CO₂ fixation was depressed, while P700 unit size (chlorophyll a concentration/P700 activity) increased and number of P700 units per cell decreased. After a 1-hour exposure of Amphora sp. to a photoinhibitory light intensity of 2.45 × 10¹⁷ quanta centimeter⁻² second⁻¹, there was a 45 to 50% decrease in the rate of ¹⁴CO₂ fixation relative to the rate at the culture light intensity. There also was a 25% increase in P700 unit size and a 30% reduction in the number of P700 units per cell but no change in total chlorophyll a concentration. Following this period of photoinhibition, the cells were returned to a light regime similar to that in the original culture conditions. Within 1 hour, both number of P700 units per cell and P700 unit size returned to levels similar to those of cells which were kept at the culture light intensity. The rates of photosynthesis did not recover as rapidly, requiring 2 to 3 hours to return to the rate for the nonphotoinhibited cells. Our results indicate that a decrease in P700 activity (with a resultant increase in P700 unit size) may be partially responsible for the photoinhibition of algal photosynthetic carbon dioxide fixation.     

Maintenance of High Photosynthetic Rates during the Accumulation of High Leaf Starch Levels in Sunflower and Soybean

Sunflower (cv. “Mammoth Greystripe”) and soybean (Merr. cv. “Amsoy 71”) leaves were exposed to continuous light for at least 52 hours in an attempt to determine the relationship between leaf starch levels and photosynthetic rates. Immature rapidly expanding and relatively mature slowly expanding sunflower leaves were studied. After 52 hours continuous light, the rapidly expanding leaves accumulated high starch levels (3.3 milligrams per square centimeter, 43% of dry weight) with only about a 10% decline from the initial photosynthetic rate of 42 milligrams CO₂ per square decimeter per hour. Under the same conditions, the slowly expanding leaves accumulated less starch, but the photosynthetic rate declined 30%. Soybean leaves, which were slowly expanding, accumulated less starch than sunflower leaves (2.1 milligrams per square centimeter, 34% of dry weight), and their photosynthetic rates declined only about 10% after 54 hours continuous light.     

Stomatal and Nonstomatal Components to Inhibition of Photosynthesis in Leaves of Capsicum annuum during Progressive Exposure to NaCl Salinity

Young bell pepper (Capsicum annuum L.) plants grown in nutrient solution were gradually acclimated to 50, 100, or 150 moles per cubic meter NaCl, and photosynthetic rates of individual attached leaves were measured on several occasions during the salinization period at external CO₂ concentrations ranging from approximately 70 to 1900 micromoles per mole air. Net CO₂ assimilation (A) was plotted against computed leaf internal CO₂ concentration (C_i), and the initial slope of this A-C_i curve was used as a measure of photosynthetic ability. During the 10 to 14 days after salinization began, leaves from plants exposed to 50 moles per cubic meter NaCl showed little change in photosynthetic ability, whereas those treated to 100 or 150 moles per cubic meter NaCl had up to 85% inhibition, with increase in CO₂ compensation point. Leaves appeared healthy, and leaf chlorophyll content showed only a 14% reduction at the highest salinity levels. Partial stomatal closure occurred with salinization, but reductions in photosynthesis were primarily nonstomatal in origin. Photosynthetic ability was inversely related to the concentration of either Na⁺ or Cl⁻ in the leaf laminas sampled at the end of the experimental period. However, the concentration of Cl⁻ expressed on a tissue water basis was greater, exceeding 300 moles per cubic meter, and Cl⁻ was more closely associated (R² = 0.926) with the inhibition of photosynthetic ability. Leaf turgor was not reduced by salinization and leaf osmotic potential decreased to a slightly greater extent than the osmotic potential decreases of the nutrient solutions. Concentration of accumulated Na⁺ and Cl⁻ (on a tissue water basis) accounted quantitatively for maintenance of leaf osmotic balance, assuming that these ions were sequestered in the vacuoles.     

Role of Carbon Dioxide in Catabolism of Propane by “Nocardia paraffinicum” (Rhodococcus rhodochrous)

The catabolism of propane by “Nocardia paraffinicum” (Rhodococcus rhodochrous) has been shown to involve CO₂ fixation after its oxidation to propionic acid. “N. paraffinicum” failed to grow on either propane or 1-propanol in the absence of CO₂. The rate of propane utilization was directly related to the initial CO₂ concentration, and Warburg respirometry suggested that CO₂ was required for the catabolism of 1-propanol, propionaldehyde, and propionate but not for 2-propanol. These data also suggested that the predominant pathway for the utilization of propane by “N. paraffinicum” was through 1-propanol. The use of [2-¹⁴C]propane and ¹⁴CO₂ confirmed the catabolism of propane and the fixation of CO₂. Through the use of these isotopes and the pyruvate carboxylase inhibitor sodium arsenite, the labeled 2,4-dinitrophenylhydrazine derivative of pyruvate was trapped and isolated via thin-layer chromatography. The trapping of [¹⁴C]pyruvate in this manner was considered to be indicative of the presence of the methylmalonyl coenzyme A pathway for CO₂ fixation.     

Correlation of Stomatal Conductance with Photosynthetic Capacity of Cotton Only in a CO2-Enriched Atmosphere: Mediation by Abscisic Acid?

Some evidence indicates that photosynthetic rate (A) and stomatal conductance (g) of leaves are correlated across diverse environments. The correlation between A and g has led to the postulation of a “messenger” from the mesophyll that directs stomatal behavior. Because A is a function of intercellular CO₂ concentration (c_i), which is in turn a function of g, such a correlation may be partially mediated by c_i if g is to some degree an independent variable. Among individual sunlit leaves in a cotton (Gossypium hirsutum L.) canopy in the field, A was significantly correlated with g (r² = 0.41, n = 63). The relative photosynthetic capacity of each leaf was calculated as a measure of mesophyll properties independent of c_i. This approach revealed that, in the absence of c_i effects, mesophyll photosynthetic capacity was unrelated to g (r² = 0.06). When plants were grown in an atmosphere enriched to about 650 microliters per liter of CO₂, however, photosynthetic capacity remained strongly correlated with g even though the procedure discounted any effect of variable c_i. This “residual” correlation implies the existence of a messenger in CO₂-enriched plants. Enriched CO₂ also greatly increased stomatal response to abscisic acid (ABA) injected into intact leaves. The data provide no evidence for a messenger to coordinate g with A at ambient levels of CO₂. In a CO₂-enriched atmosphere, though, ABA may function as such a messenger because the sensitivity of the system to ABA is enhanced.     

Photosynthetic activity of spinach chloroplasts after isopycnic centrifugation in gradients of silica

Chloroplast suspensions from spinach (Spinacia oleracea L.) were clearly resolved into intact and stripped chloroplasts by isopycnic centrifugation in density gradients of silica sol (“Ludox”) and polyethlene glycol. The intact chloroplasts fixed CO₂ and evolved O₂ more rapidly than the crude suspensions; the stripped chloroplasts were inactive. During the photosynthetic fixation of ¹⁴CO₂ in the intact chloroplasts recovered from the gradient, the ¹⁴C label was observed to spread through the photosynthetic intermediate pools, as well as into starch, which indicates that the purified chloroplasts are metabolically competent. This appears to be the first report of the retention of photosynthetic activity following the purification of chloroplasts in density gradients.     

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.     

Effects of glycine hydroxamate, carbon dioxide, and oxygen on photorespiratory carbon and nitrogen metabolism in spinach mesophyll cells

The effects of added glycine hydroxamate on the photosynthetic incorporation of ¹⁴CO₂ into metabolites by isolated mesophyll cells of spinach (Spinacia oleracea L.) was investigated under conditions favorable to photorespiratory (PR) metabolism (0.04% CO₂ and 20% O₂) and under conditions leading to nonphotorespiratory (NPR) metabolism (0.2% CO₂ and 2.7% O₂). Glycine hydroxamate (GH) is a competitive inhibitor of the photorespiratory conversion of glycine to serine, CO₂ and NH₄⁺. During PR fixation, addition of the inhibitor increased glycine and decreased glutamine labeling. In contrast, labeling of glycine decreased under NPR conditions. This suggests that when the rate of glycolate synthesis is slow, the primary route of glycine synthesis is through serine rather than from glycolate. GH addition increased serine labeling under PR conditions but not under NPR conditions. This increase in serine labeling at a time when glycine to serine conversion is partially blocked by the inhibitor may be due to serine accumulation via the “reverse” flow of photorespiration from 3-P-glycerate to hydroxypyruvate when glycine levels are high. GH increased glyoxylate and decreased glycolate labeling. These observations are discussed with respect to possible glyoxylate feedback inhibition of photorespiration.     

Chemical inhibition of the glycolate pathway in soybean leaf cells

Isolated soybean (Glycine max [L.] Merr.) leaf cells were treated with three inhibitors of the glycolate pathway in order to evaluate the potential of such inhibitors for increasing photosynthetic efficiency. Preincubation of cells under acid conditions in α-hydroxypyridinemethanesulfonic acid increased ¹⁴CO₂ incorporation into glycolate, but severely inhibited photosynthesis. Isonicotinic acid hydrazide (INH) increased the incorporation of ¹⁴CO₂ into glycine and reduced label in serine, glycerate, and starch. Butyl 2-hydroxy-3-butynoate (BHB) completely and irreversibly inhibited glycolate oxidase and increased the accumulation of ¹⁴C into glycolate. Concomitant with glycolate accumulation was the reduction of label in serine, glycerate, and starch, and the elimination of label in glycine. The inhibitors INH and BHB did not eliminate serine synthesis, suggesting that some serine is synthesized by an alternate pathway. The per cent incorporation of ¹⁴CO₂ into glycolate by BHB-treated cells or glycine by INH-treated cells was determined by the O₂/CO₂ ratio present during assay. Photosynthesis rate was not affected by INH or BHB in the absence of O₂, but these compounds increased the O₂ inhibition of photosynthesis. This finding suggests that the function of the photorespiratory pathway is to recycle glycolate carbon back into the Calvin cycle, so if glycolate metabolism is inhibited, Calvin cycle intermediates become depleted and photosynthesis is decreased. Thus, chemicals which inhibit glycolate metabolism do not reduce photorespiration and increase photosynthetic efficiency, but rather exacerbate the problem of photorespiration.     

Photosynthetic carbon metabolism in isolated maize bundle sheath strands

Photosynthetically active bundle sheath strands capable of assimilating up to 8 micromoles CO₂ per milligram chlorophyll per hour have been isolated from fully expanded leaves of Zea mays L. Mesophyll cell contamination of the preparations was negligible, as evidenced by light and electron microscopy and by a high ratio of chlorophyll a to chlorophyll b in the strands. Ribose 5-phosphate markedly stimulated the rate of photosynthetic ¹⁴CO₂ fixation by the isolated strands. In contrast, both pyruvate and phosphoenolpyruvate had a comparatively small stimulatory effect on bundle sheath ¹⁴CO₂ fixation. After 5 minutes of photosynthesis in ¹⁴C-bicarbonate, 95% of the incorporated ¹⁴C was found in compounds other than C₄-dicarboxylic acids, most notably in 3-phosphoglycerate and sugar phosphates. A similar distribution of ¹⁴C was observed in the presence of exogenous ribose 5-phosphate. Extracts of bundle sheath strands contained high specific activities of “malic” enzyme, phosphoglycolate phosphatase, hydroxypyruvate reductase, and ribulose 1,5-diphosphate carboxylase, whereas the specific activities of NADP⁺-malate dehydrogenase and phosphopyruvate carboxylase were extremely low. These results indicate that the Calvin cycle occurs in the bundle sheath cells of maize.     

Inorganic Carbon Uptake during Photosynthesis : I. A Theoretical Analysis Using the Isotopic Disequilibrium Technique

Equations have been developed which quantitatively predict the theoretical time-course of photosynthetic ¹⁴C incorporation when CO₂ or HCO₃⁻ serves as the sole source of exogenous inorganic carbon taken up for fixation by cells during steady state photosynthesis. Comparison between the shape of theoretical (CO₂ or HCO₃⁻) and experimentally derived time-courses of ¹⁴C incorporation permits the identification of the major species of inorganic carbon which crosses the plasmalemma of photosynthetic cells and facilitates the detection of any combined contribution of CO₂ and HCO₃⁻ transport to the supply of intracellular inorganic carbon. The ability to discriminate between CO₂ or HCO₃⁻ uptake relies upon monitoring changes in the intracellular specific activity (by ¹⁴C fixation) which occur when the inorganic carbon, present in the suspending medium, is in a state of isotopic disequilibrium (JT Lehman 1978 J Phycol 14: 33-42). The presence of intracellular carbonic anhydrase or some other catalyst of the CO₂-HCO₃⁻ interconversion reaction is required for quantitatively accurate predictions. Analysis of equations describing the rate of ¹⁴C incorporation provides two methods by which any contribution of HCO₃⁻ ions to net photosynthetic carbon uptake can be estimated.     

Salinity effects on photosynthesis in isolated mesophyll cells of cowpea leaves

Mesophyll cells from leaves of cowpea (Vigna unquiculata [L.] Walp.) plants grown under saline conditions were isolated and used for the determination of photosynthetic CO₂ fixation. Maximal CO₂ fixation rate was obtained when the osmotic potential of both cell isolation and CO₂ fixation assay media were close to leaf osmotic potential, yielding a zero turgor pressure. Hypotonic and hypertonic media decreased the rate of photosynthesis regardless of the salinity level during plant growth. No decrease in photosynthesis was obtained for NaCl concentrations up to 87 moles per cubic meter in the plant growing media and only a 30% decrease was found at 130 moles per cubic meter when the osmotic potential of cell isolation and CO₂ fixation media were optimal. The inhibition was reversible when stress was relieved. At 173 moles per cubic meter NaCl, photosynthesis was severely and irreversibly inhibited. This inhibition was attributed to toxic effects caused by high Cl⁻ and Na⁺ accumulation in the leaves. Uptake of sorbitol by intact cells was insignificant, and therefore not associated with cell volume changes. The light response curve of cells from low salinity grown plants was similar to the controls. Cells from plants grown at 173 moles per cubic meter NaCl were light saturated at a lower radiant flux density than were cells from lower salinity levels.     

Two Systems for Concentrating CO(2) and Bicarbonate during Photosynthesis by Scenedesmus

Scenedesmus cells grown on high CO₂, when adapted to air levels of CO₂ for 4 to 6 hours in the light, formed two concentrating processes for dissolved inorganic carbon: one for utilizing CO₂ from medium of pH 5 to 8 and one for bicarbonate accumulation from medium of pH 7 to 11. Similar results were obtained with assays by photosynthetic O₂ evolution or by accumulation of dissolved inorganic carbon inside the cells. The CO₂ pump with K_0.5 for O₂ evolution of less than 5 micromolar CO₂ was similar to that previously studied with other green algae such as Chlamydomonas and was accompanied by plasmalemma carbonic anhydrase formation. The HCO₃⁻ concentrating process between pH 8 to 10 lowered the K_0.5 (DIC) from 7300 micromolar HCO₃⁻ in high CO₂ grown Scenedesmus to 10 micromolar in air-adapted cells. The HCO₃⁻ pump was inhibited by vanadate (K_i of 150 micromolar), as if it involved an ATPase linked HCO₃⁻ transporter. The CO₂ pump was formed on low CO₂ by high-CO₂ grown cells in growth medium within 4 to 6 hours in the light. The alkaline HCO₃⁻ pump was partially activated on low CO₂ within 2 hours in the light or after 8 hours in the dark. Full activation of the HCO₃⁻ pump at pH 9 had requirements similar to the activation of the CO₂ pump. Air-grown or air-adapted cells at pH 7.2 or 9 accumulated in one minute 1 to 2 millimolar inorganic carbon in the light or 0.44 millimolar in the dark from 150 micromolar in the media, whereas CO₂-grown cells did not accumulate inorganic carbon. A general scheme for concentrating dissolved inorganic carbon by unicellular green algae utilizes a vanadate-sensitive transporter at the chloroplast envelope for the CO₂ pump and in some algae an additional vanadate-sensitive plasmalemma HCO₃⁻ transporter for a HCO₃⁻ pump.     

Isolation of Mesophyll Cells from Sedum telephium Leaves

A technique is described for mechanically isolating metabolically active individual spongy mesophyll cells from the Crassulacean acid metabolism plant, Sedum telephium. Mature leaves are selected at about 2 PM when acidity is low, and three different media are used to reduce the problem of leaf acidity and to maintain isotonic conditions. The upper and lower epidermis is peeled from chilled leaves and the leaves are suspended in a buffered “soaking medium,” then gently ground with a mortar and pestle. Cells and debris are separated using a “washing medium,” with cells being filtered through a 136 micron net and collected on an 80 micron net. Cells then are suspended in a “cell suspension medium” and concentrated by centrifugation. Approximately 2 hours are required for the isolation procedure, and activity in CO₂ fixation is constant for up to 4 hours after isolation. Microscopic examination shows about 65% of the isolated cells appear intact and unplasmolyzed and are similar to leaf msophyll cells. The yield of cells on a leaf chlorophyll basis is about 1%. A light micrograph of the isolated cells is given.     

High Affinity Transport of CO(2) in the Cyanobacterium Synechococcus UTEX 625

The active transport of CO₂ in Synechococcus UTEX 625 was measured by mass spectrometry under conditions that preclude HCO₃⁻ transport. The substrate concentration required to give one half the maximum rate for whole cell CO₂ transport was determined to be 0.4 ± 0.2 micromolar (mean ± standard deviation; n = 7) with a range between 0.2 and 0.66 micromolar. The maximum rates of CO₂ transport ranged between 400 and 735 micromoles per milligram of chlorophyll per hour with an average rate of 522 for seven experiments. This rate of transport was about three times greater than the dissolved inorganic carbon saturated rate of photosynthetic O₂ evolution observed under these conditions. The initial rate of chlorophyll a fluorescence quenching was highly correlated with the initial rate of CO₂ transport (correlation coefficient = 0.98) and could be used as an indirect method to detect CO₂ transport and calculate the substrate concentration required to give one half the maximum rate of transport. Little, if any, inhibition of CO₂ transport was caused by HCO₃⁻ or by Na⁺-dependent HCO₃⁻ transport. However, ¹²CO₂ readily interfered with ¹³CO₂ transport. CO₂ transport and Na⁺-dependent HCO₃⁻ transport are separate, independent processes and the high affinity CO₂ transporter is not only responsible for the initial transport of CO₂ into the cell but also for scavenging any CO₂ that may leak from the cell during ongoing photosynthesis.     

Effects of Irradiance during Growth on Adaptive Photosynthetic Characteristics of Velvetleaf and Cotton

We grew velvetleaf (Abutilon theophrasti Medic.) and cotton (Gossypium hirsutum L. var. Stoneville 213) at three irradiances and determined the photosynthetic responses of single leaves to a range of six irradiances from 90 to 2000 μeinsteins m⁻²sec⁻¹. In air containing 21% O₂, velvetleaf and cotton grown at 750 μeinsteins m⁻²sec⁻¹ had maximum photosynthetic rates of 18.4 and 21.9 mg of CO₂ dm⁻²hr⁻¹, respectively. Maximum rates for leaves grown at 320 and 90 μeinsteins m⁻²sec⁻¹ were 15.3 and 10.3 mg of CO₂ dm⁻²hr⁻¹ in velvetleaf and 12 and 6.7 mg of CO₂ dm⁻²hr⁻¹ in cotton, respectively. In 1 O₂, maximum photosynthetic rates were 1.5 to 2.3 times the rates in air containing 21% O₂, and plants grown at medium and high irradiance did not differ in rate. In both species, stomatal conductance was not significantly affected by growth irradiance. The differences in maximum photosynthetic rates were associated with differences in mesophyll conductance. Mesophyll conductance increased with growth irradiance and correlated positively with mesophyll thickness or volume per unit leaf area, chlorophyll content per unit area, and photosynthetic unit density per unit area. Thus, quantitative changes in the photosynthetic apparatus help account for photosynthetic adaptation to irradiance in both species. Net assimilation rates calculated for whole plants by mathematical growth analysis were closely correlated with single-leaf photosynthetic rates.     

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

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