Diurnal Pattern of Translocation and Carbohydrate Metabolism in Source Leaves of Beta vulgaris L

Transitions in carbohydrate metabolism and translocation rate were studied for evidence of control of export by the sugar beet (Beta vulgaris L. Klein E.) source leaf. Steady-state labeling was carried out for two consecutive 14-hour light periods and various quantities related to translocation were measured throughout two 24-hour periods. Starch accumulation following illumination was delayed. Near the end of the light period, starch stopped accumulating, whereas photosynthesis rate and sucrose level remained unchanged. At the beginning of the dark period there was a 75-minute delay before starch was mobilized. The rate of import to the developing sink leaves at night was similar to that during the day, whereas export decreased considerably at night.

Starch accumulation and degradation seemed to be initiated in response to the level of illumination. Cessation of starch accumulation before the end of the light period was initiated endogenously. Exogenous control appeared to be mediated by the level of sucrose in the source leaf while endogenous control seemed to be keyed to photoperiod or photosynthetic duration.

     

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.     

Trends in carbohydrate depletion, respiratory carbon loss, and assimilate export from soybean leaves at night

To evaluate assimilate export from soybean (Glycine max [L.] Merrill) leaves at night, rates of respiratory CO₂ loss, specific leaf weight loss, starch mobilization, and changes in sucrose concentration were measured during a 10-hour dark period in leaves of pod-bearing `Amsoy 71' and `Wells II' plants in a controlled environment. Lateral leaflets were removed at various times between 2200 hours (beginning dark period) and 0800 hours (ending dark period) for dry weight determination and carbohydrate analyses. Respiratory CO₂ loss was measured throughout the 10-hour dark period. Rate of export was estimated from the rate of loss in specific leaf weight and rate of CO₂ efflux. Rate of assimilate export was not constant. Rate of export was relatively low during the beginning of the dark period, peaked during the middle of the dark period, and then decreased to near zero by the end of darkness. Rate of assimilate export was associated with rate of starch mobilization and amount of starch reserves available for export. Leaves of Amsoy 71 had a higher maximum export rate in conjunction with a greater total change in starch concentration than did leaves of Wells II. Sucrose concentration rapidly declined during the first hour of darkness and then remained constant throughout the rest of the night in leaves of both cultivars. Rate of assimilate export was not associated with leaf sucrose concentration.     

Photosynthetic Responses to Irradiance by the Grey Mangrove, Avicennia marina, Grown under Different Light Regimes

Photosynthetic responses to irradiance by the grey mangrove, Avicennia marina (Forstk.) Vierh var. australasica (Walp.) Moldenke, were studied using seedlings grown under natural understory shade and exposed conditions as well as in the laboratory under high and low light regimes, i.e. 100% and 6% sunlight, respectively. Leaves in exposed locations were subjected to daylight quantum flux densities greater than 1,000 microeinsteins per square meter per second from 0900 to 1700 hours, whereas those in understory shade experienced only 30 to 120 microeinsteins per square meter per second, interrupted for brief periods by sunflecks ranging in quantum flux density from 800 to 1,500 microeinsteins per square meter per second. The low light regime was similar in light intensity to that of the understory environment, but lacked sunflecks.

Leaves from the understory environment showed several properties of `shade' leaves; i.e. they contained more chlorophyll on both a leaf area and fresh weight basis but had a lower specific weight and greater area than exposed leaves, and were enriched in chlorophyll b relative to a. However, there were no significant differences in either the gas exchange or leaf chlorophyll fluorescence characteristics of the two populations, both being typical of `sun' leaves.

Leaves grown in the laboratory under low and high light regimes had similar properties. However, light saturated assimilation rates in the leaves from the low light treatment were 20% less and became light saturated at a lower quantum flux density than those of leaves grown under the high light regime. The ecological significance of these results is discussed.

     

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.     

Comparison of ethylenediaminetetraacetate-enhanced exudation from detached and translocation from attached bean leaves

A technique for collection of phloem exudate from detached leaves using 20 millimolar EDTA (pH 7.0) has previously been developed (King, Zeevaart 1974 Plant Physiol 53: 96-103). It was the aim of the present study to determine the efficiency of this technique in relation to undisturbed export from attached leaves. Paired primary leaves of bean seedlings (Phaseolus vulgaris L. cv Montcalm) were used to minimize variations in plant material. Attached leaves, exposed to ¹⁴CO₂ for 10 minutes with subsequent excision of one of the leaves and collection of the exudate over a 12-hour period, showed a 25% export of total assimilated ¹⁴C from the attached versus 15% of total assimilated ¹⁴C in the form of exudation from the detached ones. Leaf excision changed the labeling pattern within the leaf, increasing% total leaf ¹⁴C-activity in the ethanolic fraction, while decreasing activity in the starch fraction, as compared to attached leaves. This was presumably caused by a lack of translocation from the detached leaves. Excision did not affect dark respiration. However, measurements of total nonstructural carbohydrates in leaf starch and neutral fractions indicated no significant differences between attached and leaves detached in EDTA. Thus, in terms of actual carbon export, and accompanying distribution of nonexported carbohydrate within the leaf, EDTA-enhanced exudation compares favorably with translocation from attached leaves.     

The geometry of light interception by shoots of Heteromeles arbutifolia: morphological and physiological consequences for individual leaves

The influence of leaf orientation and position within shoots on individual leaf light environments, carbon gain, and susceptibility to photoinhibition was studied in the California chaparral shrub Heteromeles arbutifolia with measurements of gas exchange and chlorophyll fluorescence, and by application of a three-dimensional canopy architecture model. Simulations of light absorption and photosynthesis revealed a complex pattern of leaf light environments and resulting leaf carbon gain within the shoots. Upper, south-facing leaves were potentially the most productive because they intercepted greater daily photon flux density (PFD) than leaves of any other orientation. North-facing leaves intercepted less PFD but of this, more was received on the abaxial surface because of the steep leaf angles. Leaves differed in their response to abaxial versus adaxial illumination depending on their orientation. While most had lower photosynthetic rates when illuminated on their abaxial as compared to adaxial surface, the photosynthetic rates of north-facing leaves were independent of the surface of illumination. Because of the increasing self-shading, there were strong decreases in absorbed PFD and daily carbon gain in the basipetal direction. Leaf nitrogen per unit mass also decreased in the basipetal direction but on a per unit area basis was nearly constant along the shoot. The decrease in leaf N per unit mass was accounted for by an increase in leaf mass per unit area (LMA) rather than by movement of N from older to younger leaves during shoot growth. The increased LMA of older lower leaves may have contributed directly to their lower photosynthetic capacities by increasing the limitations to diffusion of CO₂ within the leaf to the sites of carboxylation. There was no evidence for sun/shade acclimation along the shoot. Upper leaves and especially south-facing upper leaves had a potential risk for photoinhibition as demonstrated by the high PFDs received and the diurnal decreases in the fluorescence ratio F _v/F _m. Predawn F _v/F _m ratios remained high (>0.8) indicating that when in their normal orientations leaves sustained no photoinhibition. Reorientation of the leaves to horizontal induced a strong sustained decrease in F _v/F _m and CO₂ exchange that slowly recovered over the next 10–15?days. If leaves were also inverted so that the abaxial surface received the increased PFDs, then the reduction in F _v/F _m and CO₂ assimilation was much greater with no evidence for recovery. The heterogeneity of responses was due to a combination of differences between leaves of different orientation, differences between responses on their abaxial versus adaxial surfaces, and differences along the shoot due to leaf age and self-shading effects.     

Daytime and nighttime carbon balance and assimilate export in soybean leaves at different photon flux densities

To evaluate daytime and nighttime carbon balance and assimilate export in soybean (Glycine max [L.] Merrill) leaves at different photon flux densities, rates of CO₂ exchange, specific leaf weights, and concentrations of sucrose and starch were measured at intervals in leaves of pod-bearing `Amsoy 71' and `Wells II' plants grown in a controlled environment room. Assimilate export was estimated from CO₂ exchange and change in specific leaf weight. Total diurnal assimilate export was similar for both cultivars. Large cultivar differences existed, however, in the partitioning of carbon into starch reserves and the relative amounts of assimilate exported during the day and the night. Total amounts of both daytime and nighttime export increased with increasing photon flux density, as did sucrose and starch concentrations, specific leaf weight, and rate of respiratory carbohydrate loss at night. Cultivar differences in nighttime rate of export were more closely related to the differences in amount of assimilate available at the end of the day than to differences in daytime rate of net CO₂ assimilation. Daytime rates of export, however, were closely related to daytime rates of net CO₂ assimilation within each cultivar. The total amount of starch depleted during the 10-hour night increased as starch concentration at the beginning of the night increased.     

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.     

Carbon Partitioning and Export in Mature Leaves of Pepper (Capsicum annuum)

The partitioning of recently fixed carbon by mature pepper leaveshas been examined over a 10 h photoperiod using a constant specificradioactivity ¹⁴CO₂ labelling technique. Changes in the ratesof carbon partitioning into export, starch, sucrose and hexoseswere examined following changes in irradiance during the photoperiod.Leaves grown under 80 W m^–2 PAR were exposed to this irradiancefor the first 4 h of the photoperiod then the iiradiance wasdecreased. Leaves accumulated sufficient reserves in the first4 h to maintain export at the initial rate (approximately 20µg carbon cm^–2 leaf h^–1) over the following6 h of the photoperiod when the net photosynthesis rate (Pn)was decreased to 10% of the initial rate by the decreased irradiance.Export was initially maintained by the depletion of sucroseand hexose and then by carbon from the degradation of starchin the light. If leaves were exposed to low irradiance at the beginning ofthe photoperiod, then the export rate was linearly related tothe Pn during that period. When Pn exceeded that required tomaintain an export rate of approximately 20 µg carboncm^–2 h^–1, then more carbon was partitioned intostarch. At low initial irradiance, a greater proportion of photosynthatewas partitioned into export rather than starch and at high initialirradiancc the reverse occurred. There was a linear relationship between starch accumulationrate and Pn for all leaves but the relationship between Pn andexport rate was only significant for leaves with low levelsof reserve carbon. The results show that mature pepper leaves subjected to differentirradiances maintain constant export rates through alterationsof carbon partitioning. Export at low Pn is maintained at theexpense of sugar and starch reserves, with partitioning in highirradiance being predominantly to starch. Key words: Carbon partitioning, Starch, Export, Pepper (Capsicum annuum L.)     

Diurnal changes in allocation of newly fixed carbon in exporting sugar beet leaves

Storage of newly fixed carbon as starch and sucrose follows a regular daily pattern in exporting sugar beet leaves under constant day length and level of illumination. Up to the final two hours of the light period, when starch storage declines, a nearly constant proportion of newly fixed carbon was allocated to carbohydrate storage, principally starch. Sucrose is stored only early in the light period, when there is little accumulation of starch. Pulse labeling with ¹⁴CO₂ revealed that considerable starch synthesis was taking place at this time. Starch made the previous day was not mobilized during this period but breakdown of newly synthesized starch may occur when carbon flow into sucrose synthesis increases early in the day. At the end of the day, starch storage declined from the constant level observed during most of the day, but no diversion of label into export of specific alternative compounds could be detected. Lowered storage of starch persisted when the 14-hour light period was lengthened. Changed allocation of recently fixed carbon to sucrose and starch at the beginning and end of the light period was not the result of outright inactivation of pathways but of regulation of carbon flow.     

Leaf Phosphate Status, Photosynthesis, and Carbon Partitioning in Sugar Beet: III. Diurnal Changes in Carbon Partitioning and Carbon Export

The effect of low phosphate supply (low P) was determined on the diurnal changes in the rate of carbon export, and on the contents of starch, sucrose, glucose, and fructose 2,6-bisphosphate (F2,6BP) in leaves. Low-P effects on the activities of a number of enzymes involved in starch and sucrose metabolism were also measured. Sugar beets (Beta vulgaris L. cv. F58-554H1) were cultured hydroponically in growth chambers and the low-P treatment induced nutritionally. Low-P treatment decreased carbon export from the leaf much more than it decreased photosynthesis. At growth chamber photon flux density, low P decreased carbon export by 34% in light; in darkness, export rates fell but more so in the control so that the average rate in darkness was higher in low-P leaves. Low P increased starch, sucrose, and glucose contents per leaf area, and decreased F2, 6BP. The total extractable activities of enzymes involved in starch and sucrose synthesis were increased markedly by low P, e.g. adenosine 5-diphosphoglucose pyrophosphorylase, cytoplasmic fructose-1,6-bisphosphatase, uridine 5-diphosphoglucose pyrophosphorylase, and sucrose-phosphate synthase. The activities of some enzymes involved in starch and sucrose breakdown were also increased by low P. We propose that plants adapt to low-P environments by increasing the total activities of several phosphatases and by increasing the concentrations of phosphate-free carbon compounds at the expense of sugar phosphates, thereby conserving Pi. The partitioning of carbon among the various carbon pools in low-P adapted leaves appears to be determined in part by the relative capacities of the enzymes for starch and sucrose metabolism.     

Glyphosate inhibits photosynthesis and allocation of carbon to starch in sugar beet leaves

Application of glyphosate (N-[phosphonomethyl] glycine) to exporting leaves of sugar beet (Beta vulgaris, L.) during the day lowered stomatal conductance and carbon fixation. Allocation of newly fixed carbon to foliar starch accumulation was nearly completely inhibited, being decreased by the same amount as net carbon fixation. In contrast, decreasing net carbon fixation in untreated leaves by lowering CO₂ concentration caused starch accumulation to decrease, but only in the same proportion as net carbon fixation. Shikimate level increased 50-fold in treated leaves but the elevated rate of carbon accumulation in shikimate was only 4% of the decrease in the rate of starch accumulation. Application of steady state labeling with ¹⁴CO₂ to exporting leaves confirmed the above changes in carbon metabolism, but revealed no other major daytime differences in the ¹⁴C-content of amino acids or other compounds between treated and control leaves. Less ¹⁴C accumulated in treated leaves because of decreased fixation, not increased export. The proportion of newly fixed carbon allocated to sucrose increased, maintaining export at the level in control leaves. Returning net carbon exchange to the rate before treatment restored starch accumulation fully and prevented a decrease in export during the subsequent dark period.     

Relationships between Carbon Assimilation, Partitioning, and Export in Leaves of Two Soybean Cultivars

To evaluate leaf carbon balance during rapid pod-fill in soybean (Glycine max [L.] Merrill), measurements were made of CO₂ assimilation at mid-day and changes in specific leaf weight, starch, and sucrose concentrations over a 9-hour interval. Assimilate export was estimated from CO₂ assimilation and leaf dry matter accumulation. Chamber-grown `Amsoy 71' and `Wells' plants were subjected on the day of the measurements to one of six photosynthetic photon flux densities in order to vary CO₂ assimilation rates.

Rate of accumulation of leaf dry matter and rate of export both increased as CO₂ assimilation rate increased in each cultivar.

Starch concentrations were greater in Amsoy 71 than in Wells at all CO₂ assimilation rates. At low CO₂ assimilation rates, export rates in Amsoy 71 were maintained in excess of 1.0 milligram CH₂O per square decimeter leaf area per hour at the expense of leaf reserves. In Wells, however, export rate continued to decline with decreasing CO₂ assimilation rate. The low leaf starch concentration in Wells at low CO₂ assimilation rates may have limited export by limiting carbon from starch remobilization.

Both cultivars exhibited positive correlations between CO₂ assimilation rate and sucrose concentration, and between sucrose concentration and export rate. Carbon fixation and carbon partitioning both influenced export rate via effects on sucrose concentration.

     

Diurnal fluctuations in cotton leaf carbon export, carbohydrate content, and sucrose synthesizing enzymes

In fully expanded leaves of greenhouse-grown cotton (Gossypium hirsutum L., cv Coker 100) plants, carbon export, starch accumulation rate, and carbon exchange rate exhibited different behavior during the light period. Starch accumulation rates were relatively constant during the light period, whereas carbon export rate was greater in the afternoon than in the morning even though the carbon exchange rate peaked about noon. Sucrose levels increased throughout the light period and dropped sharply with the onset of darkness; hexose levels were relatively constant except for a slight peak in the early morning. Sucrose synthase, usually thought to be a degradative enzyme, was found in unusually high activities in cotton leaf. Both sucrose synthase and sucrose phosphate synthetase activities were found to fluctuate diurnally in cotton leaves but with different rhythms. Diurnal fluctuations in the rate of sucrose export were generally aligned with sucrose phosphate synthase activity during the light period but not with sucrose synthase activity; neither enzyme activity correlated with carbon export during the dark. Cotton leaf sucrose phosphate synthase activity was sufficient to account for the observed carbon export rates; there is no need to invoke sucrose synthase as a synthetic enzyme in mature cotton leaves. During the dark a significant correlation was found between starch degradation rate and leaf carbon export. These results indicate that carbon partitioning in cotton leaf is somewhat independent of the carbon exchange rate and that leaf carbon export rate may be linked to sucrose formation and content during the light period and to starch breakdown in the dark.     

Phytochrome control of two low-irradiance responses in etiolated oat seedlings

Light-induced coleoptile stimulation and mesocotyl suppression in etiolated Avena sativa (cv. Lodi) has been quantitated. Etiolated seedlings showed the greatest response to light when they were illuminated 48 to 56 hours after imbibition. Two low-irradiance photoresponses for each tissue have been described. Red light was 10 times more effective than green and 1,000 times more effective than far red light in evoking these responses. The first response, which resulted in a 45% mesocotyl suppression and 30% coleoptile stimulation, had a threshold at 10⁻¹⁴ einsteins per square centimeter and was saturated at 3.0 × 10⁻¹² einsteins per square centimeter of red light. This very low-irradiance response could be induced by red, green, or far red light and was not photoreversible. Reciprocity failed if the duration of the red illumination exceeded 10 minutes. The low-irradiance response which resulted in 80% mesocotyl suppression and 60% coleoptile stimulation, had a threshold at 10⁻¹⁰ einsteins per square centimeter and was saturated at 3.0 × 10⁻⁸ einsteins per square centimeter of red light. A complete low-irradiance response could be induced by either red or green light but not by far red light. This response could be reversed by a far red dose 30 times greater than that of the initial red dose for both coleoptiles and mesocotyls. Reciprocity failed if the duration of the red illumination exceeded 170 minutes. Both of these responses can be explained by the action of phytochrome.     

The Dynamics of Carbon Supply from Leaves of Barley Plants Grown in Long or Short Days

The role of the mature leaf in supplying carbon for growth inother parts of the plant was examined using a steady-rate ¹⁴CO₂labelling technique. The pattern of events occurring in theleaf during one complete 24 h cycle was compared in plants grownin, and adapted to long and short photoperiods. The rates ofleaf photosynthesis, night respiration and daytime loss of carbonfrom the growing regions of the plant Were similar in long orshort photoperiods. As a percentage of the total carbon fixedduring the photoperiod, total respiration was c. 50% for shortday plants but only 25% for long day plants. Thirty to forty per cent of the carbon fixed during the photoperiodwas retained in the leaf for export during darkness—therest was exported immediately. In leaves of short day plantssucrose and starch were the main form of the stored carbon.By the end of the dark period these compounds had been almostcompletely depleted. In leaves of long day plants there weremuch larger basal levels of sucrose and starch, upon which thediurnal variations were superimposed. These leaves also accumulatedfructosans. The delay in starch remobilization previously foundin leaves of short day plants was also evident in leaves oflong day plants even though large concentrations of sucroseand fructosans were present This suggests the presence of distinctpools of sucrose in the leaf.     

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.     

Kinetics of C-14 translocation in soybean: I. Kinetics in the stem

A kinetic study was made of the translocation of ¹⁴C-photosynthate through soybean stems following pulse labeling and during steady state labeling of the first trifoliolate leaf. The translocation profile proceeded down the stem with little or no change in shape. Following pulse labeling, sucrose accounted for 90 to 95% of the radioactivity in the stem at all times up to 2 hours, at which time less than 3% of the activity was in an insoluble form. Kinetic data on the relative specific activities of sucrose in the leaf and petiole indicated that two-thirds of the petiolar sucrose was in the translocation stream and the remaining one-third was in a stationary pool which slowly accumulated sucrose from the translocation stream. With this assumption, the rate of sucrose efflux from the leaf was calculated to be 22 micrograms per minute, which was equivalent to a sucrose mass flux in the sieve tubes of 20 grams per square centimeter per hour.