Chlorophyll, Ribulose-1,5-diphosphate Carboxylase, and Hill Reaction Activity in Developing Leaves of Populus deltoides

Chlorophyll a fluorescence and carbon assimilation in developing leaves of light-grown cucumber

The development of photochemical activity and carbon assimilation in light-grown cucumber (Cucumis sativus L. cv Natsusairaku) leaves was studied to determine the pattern of acquisition and its relationship to leaf growth and expansion. Measurements of chlorophyll a fluorescence showed that leaves acquire photochemical function over a period of 6 or more days, and gas exchange studies showed increases in carbon assimilation over a parallel time period. As leaves expand and mature, they undergo a sequential, three-step series of changes in fluorescence response. The initial kinetics show the absence of wholly functional quenching mechanisms. Dynamic imaging of fluorescence kinetics showed that a temporal series of changes occurred within defined areas of individual developing leaves. The spatial acquisition of photochemical activity in leaves was basipetal as is their directional expansion, development of air spaces and stomata, and the cessation of imported carbon.

Quantitative Measurements of Hexokinase Activity in the Shoot Apical Meristem, Leaf Primordia, and Leaf Tissues of Dianthus chinensis L

Hexokinase was measured by quantitative histochemical techniques in the apical meristem, primordia, and leaves of Dianthus chinensis L. The structural stages of development in the leaves sampled were determined by light and scanning electron microscopy. The results showed that activity decreased from the youngest primordia (1500 millimoles per kilogram dry weight per hour) to the mature leaves (200 millimoles per kilogram dry weight per hour) and that an intermediate leaf, the fourth youngest, showed the same declining pattern from its base to its tip. Surface views and measurements of these leaves revealed their basipetal maturation as seen by cell size, stomatal development, trichome differentiation, cuticular appearance, and leaf thickness. The intermediate leaf showed features representative of several stages in structural differentiation. It was concluded that the changes in hexokinase activity among the leaves of a shoot and within an individual leaf are similar and correlate with the degree of structural differentiation of the leaves.

Changes in photosynthetic characteristics during leaf development in apple

A comprehensive developmental survey of leaf area, chlorophyll, photosynthetic rate, leaf resistance, transpiration ratio, CO2 compensation point and photorespiration was conducted in apple. The largest changes in each of the photosynthetic characteristics studied took place during the earliest stages of leaf development, coinciding with the period of greatest leaf expansion and chlorophyll synthesis. During early development, photosynthesis increased 5-fold, reaching a maximum rate of 40 mg CO2 dm(-2) hr(-1) at a leaf plastochron index (LPI) of 10. During this same period, leaf resistance, transpiration ratio, CO2 compensation point and mesophyll resistance decreased, while carboxylation efficiency increased. Two especially interesting aspects of the data discussed are simultaneous changes that occur at a LPI of 10 and 12 in all of the photosynthetic characteristics examined and an apparent decrease in photorespiration as leaves age. From our results it is clear that stage of leaf development is an important factor affecting the rate of photosynthesis and photorespiration.

(14)C fixation, metabolic labeling patterns, and translocation profiles during leaf development in Populus deltoides

The incorporation of photosynthetically fixed (14)CO2 and the distribution of (14)C among the main chemical constituents of laminae and petioles were examined in cottonwood (Populus deltoides Bartr. ex Marsh.) leaves ranging in age from Leaf Plastochron Index (LPI) 3 (about one-quarter to one-third expanded) to LPI 30 (beginning of senescence). In addition, carbon flow among chemical fractions and translocation from leaves of LPI 7 and 14 were examined periodically up to 24 h after labeling. Specific activity of (14)C (on dry-weight basis) increased in developing laminae to full leaf expansion, decreased in the mature leaves to LPI 16, then remained constant to LPI 30. In developing leaves (LPI 3-5), after 2 h, most of the (14)C was found in protein, pigments, lipids, and other structural and metabolic components necessary for cell development; only 28% was in the sugar fraction of the lamina. In fully expanded leaves (LPI 6-8), after 2 h, the sugar fraction contained 50-60% and about 90% of fixed (14)C in the lamina and the petiole, respectively. In a pulsechase "kinetic series" with recently mature leaves, 60% of the (14)C was found in the sugar fraction after 15 min of (14)CO2 fixation. Over the 24-h translocation period, (14)C decreased in sugars to 23% and increased in the combined residue fraction (protein, starch, and structural carbohydrates) to about 60% of the total activity left in the lamina. Within 24 h after labeling, the turnover of (14)C-organic acids,-sugar, and-amino acids (either metabolzed or translocated from the leaf) was 30, 70 and 80%, respectively, of that initially incorporated into these fractions by a leaf at LPI 7 (turnover was 55% of (14)C-organic acids, 80% of (14)C-sugar, and 95% of (14)C-amino acids at LPI 14). Anatomical maturity in cottonwood leaves is closely correlated with physiological maturity and with production of translocatable sugar.

Photosynthetic carbon metabolism during leaf ontogeny in Zea mays L.: Enzyme studies

The activities of several enzymes, including ribulose-1,5-diphosphate (RuDP) carboxylase (EC 4.1.1.39) and phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) were measured as a function of leaf age in Z. mays. Mature leaf tissue had a RuDP-carboxylase activity of 296.7 μmol CO2 g(-1) fresh weight h(-1) and a PEP-carboxylase activity of 660.6 μmol CO2 g(-1) fresh weight h(-1). In young corn leaves the activity of the two enzymes was 11 and 29%, respectively, of the mature leaves. In senescent leaf tissue, RuDP carboxylase activity declined more rapidly than that of any of the other enzymes assayed. On a relative basis the activities of NADP malic enzyme (EC 1.1.1.40), aspartate (EC 2.6.1.1) and alanine aminotransferase (EC 2.6.1.2), and NAD malate dehydrogenase (EC 1.1.1.37) exceeded those of both PEP and RuDP carboxylase in young and senescent leaf tissue. Pulse-chase labeling experiments with mature and senescent leaf tissue show that the predominant C4 acid differs between the two leaf ages. Labeling of alanine in senescent tissue never exceeded 4% of the total (14)C remaining during the chase period, while in mature leaf tissue alanine accounted for 20% of the total after 60 s in (12)CO2. The activity of RuDP carboxylase during leaf ontogeny in Z. mays parallels the development of the activity of this enzyme in C3 plants.

Development of Photosystem I and Photosystem II Activities in Leaves of Light-grown Maize (Zea mays)

To compare chloroplast development in a normally grown plant with etiochloroplast development, green maize plants (Zea mays), grown under a diurnal light regime (16-hour day) were harvested 7 days after sowing and chloroplast biogenesis within the leaf tissue was examined. Determination of total chlorophyll content, ratio of chlorophyll a to chlorophyll b, and O(2)-evolving capacity were made for intact leaf tissue. Plastids at different stages of development were isolated and the electron-transporting capacities of photosystem I and photosystem II measured. Light saturation curves were produced for O(2)-evolving capacity of intact leaf tissue and for photosystem I and photosystem II activities of isolated plastids. Structural studies were also made on the developing plastids. The results indicate that the light-harvesting apparatus becomes increasingly efficient during plastid development due to an increase in the photosynthetic unit size. Photosystem I development is completed before that of photosystem II. Increases in O(2)-evolving capacity during plastid development can be correlated with increased thylakoid fusion. The pattern of photosynthetic membrane development in the light-grown maize plastids is similar to that found in greening etiochloroplasts.

Metabolic Activity of Isolated Leaf Cells of Phaseolus vulgaris in Relation to Leaf Development

Mesophyll cells were isolated from primary leaves of 5- to 21-day Phaseolus vulgaris plants. The rate of photosynthesis and respiration, and RNA, protein, and lipid synthesis was determined for these cells. Appropriate (14)C substrates and product purification procedures were used for each process prior to liquid scintillation counting. The size of the leaves increased about 5-fold between days 5 and 11, and then remained relatively constant. The greatest increase in size occurred between days 5 and 6. The age of the leaf from which the cells were isolated had a pronounced effect on the rate of all of these processes. The largest changes occurred during the period of leaf expansion (days 5-11). Initially the rate of RNA, protein, and lipid synthesis increased rapidly, maintained a maximum rate for only 1 day (day 6 or day 7), and then declined. The rate of photosynthesis increased more slowly reaching a maximum at day 9, remained relatively constant until day 15, and then declined. The rate of respiration decreased during the first 4 days to low level which was maintained throughout the experiment. The time course patterns of these biochemical processes in isolated cells were similar to those which have been reported for intact leaves. It seems that isolation of leaf cells does not modify their metabolic activity.

Relationship between leaf development, carboxylase enzyme activities and photorespiration in the C4-plant Portulaca oleracea L

Ribulose diphosphate (RuDP) and (PEP) phosphoenolpyruvate carboxylase enzyme activities were studied in young, mature, and senescent Portulaca oleracea leaves. While the absolute amount of both the C3 (RuDP) and C4 (PEP) carboxylase is less in senescent leaves than in mature leaves, RuDP carboxylase activity is reduced to a lesser degree. In senescent leaves, PEP carboxylase activity equals 10% of that in mature tissue, but RuDP carboxylase is 27% of that in mature leaves. The same ontogenetic series was also used to determine photorespiration rates and responses to several gas treatments. Young and mature leaves were unaffected by changes in the light regime or oxygen concentrations, and exhibited typical C4-plant light/dark (14)CO2 evolution ratios. Senescent leaves, on the other hand, have photorespiration ratios similar to C3-plants. In addition, senescent leaves were affected by minus CO2, 100% O2 and N2 in a manner expected of C3-plants, but not C4-plants. These results are discussed in terms of a relative increase in activity of the C3 cycle in later developmental stages in this plant.

Incorporation of C-Photosynthate into Protein during Leaf Development in Young Populus Plants

Gas exchange and protein metabolism were studied in expanding, mature, and near-senescent leaves of young clonal Populus x euramericana cv. Wisconsin-5 plants. Dark respiration, CO(2) evolution in the light, and CO(2) compensation concentrations were highest in unexpanded leaves but declined markedly as leaves matured and aged. Net photosynthesis was highest in nearly mature leaves. Fresh weight continued to increase after leaf expansion was complete, whereas soluble protein levels declined. Changes in the distribution of photosynthetically incorporated (14)C indicated that a high level of protein synthesis and rapid formation of structural components occurred only in expanding leaves. Protein turnover was slight in expanding leaves but was substantial after leaves were mature. Expanding leaves synthesized predominantly fraction I protein (ribulose diphosphate carboxylase). However, formation of this protein from photosynthate was slight once leaves matured.

Leaf development and phloem transport in Cucurbita pepo: Transition from import to export

The capacity of a growing leaf blade of Cucurbita pepo L. to import (14)C-labelled photoassimilate is lost in a basipetal direction. Import into the lamina tip stops when the blade is 10% expanded. Development of the leaf progresses linearly with time and the lamina base stops importing when the blade is 45% expanded. Export capacity also develops basipetally and follows immediately the loss of import capacity, at least in the lamina base. The small amount of material initially exported from the leaf tip is redistributed to the still-importing leaf base, delaying export from the lamina until the blade is 35% expanded. Loss of import capacity by the petiole is both basipetal and dorsoventral. The proximal, adaxial portion of the petiole is the last region to cease importing (14)C. Leaves of Beta vulgaris L. and Nicotiana tabacum L. also lose import capacity in a basipetal direction.

Relationship between leaf development and primary photosynthetic products in the C4 plant Portulaca oleracea L

The photosynthetic products of Portulaca oleracea differ greatly depending on leaf age and length of exposure to (14)CO2. Mature leaves of P. oleracea fix (14)CO2 primarily into organic and amino acids during a 10-s exposure period. Less than 2% of the (14)CO2 fixed appears in phosphorylated compounds. In contrast, incorporation into amino acids can account for over 60% of the total (14)CO2 fixed by young leaves in an equal time period, and incorporation into alanine alone can account for up to one half of this amount. Senescent leaves display a quantitative shift of primary products toward phosphorylated compounds with a concomitant reduction of the label residing in malate and asparate. About 8 times more phosphoglyceric acid is produced in senescent leaves than in mature leaves. The aspartate/ malate ratio is not constant and depends on the length of time the leaves are exposed to (14)CO2 and the age of the leaves under study. It appears as if the stage of leaf development is one of the most important factors determining the operation of a particular enzyme system in C4 plants.

Photophosphorylation and Carbon Dioxide Fixation by Chloroplasts Isolated from Populus deltoides

A system has been developed for the isolation of photosynthetically active chloroplasts from leaves of Populus deltoides. A high proportion of the chloroplasts appeared intact. The maximum rates of different photosynthetic processes were as follows: CO(2) fixation 3.5 micromoles per milligram chlorophyll per hour, noncyclic ATP synthesis 10 micromoles per milligram chlorophyll per hour, and cyclic ATP synthesis 300 micromoles per milligram chlorophyll per hour.