Phloem unloading in tobacco sink leaves: insensitivity to anoxia indicates a symplastic pathway

Phloem unloading in transition sink leaves of tobacco (Nicotiana tabacum L.) was analyzed by quantitative autoradiography. Detectable levels of labeled photoassimilates entered sink leaves approx. 1 h after source leaves were provided with ¹⁴CO₂. Samples of tissue were removed from sink leaves when label was first detected and further samples were taken at the end of an experimental phloem-unloading period. The amount of label in veins and in surrounding cells was determined by microdensitometry of autoradiographs using a microspectrophotometer. Photoassimilate unloaded from first-, second-and third-order veins but not from smaller veins. Import termination in individual veins was gradual. Import by the sink leaf was completely inhibited by exposing the sink leaf to anaerobic conditions, by placing the entire plant in the cold, or by steam-girdling the sink-leaf petiole. Phloem unloading was completely inhibited by cold; however, phloem unloading continued when the sink-leaf petiole was steam girdled or when the sink leaf was exposed to a N₂ atmosphere. Compartmental efflux-analysis indicated that only a small percentage of labeled nutrients was present in the free space after unloading from sink-leaf veins in a N₂ atmosphere. The results are consistent with passive symplastic transfer of photoassimilates from phloem to surrounding cells.Symbol VI radio of ¹⁴C in veins and interveinal tissue     

Influence of galls of Phanacis taraxaci on carbon partitioning within common dandelion, Taraxacum officinale

We examined how leaf galls, induced by the cynipid wasp Phanacis taraxaci, influence the partitioning of photoassimilates within the host, the common dandelion, Taraxacum officinale. Galled and ungalled plants were exposed to ¹⁴CO₂ and the labelled photoassimilates accumulating within galls and other parts of the host were measured. During the growth phase of the gall they were physiological sinks for photoassimilates, accumulating 9% to 70% of total carbon produced by the host, depending upon the number of galls per plant. High levels of ¹⁴C assimilation in the leaves of galled plants compared to controls, suggest that galls actively redirect carbon resources from unattacked leaves of their host plant. This represents a significant drain on the carbon resources of the host, which increases with the number and size of galls per plant. Active assimilation of ¹⁴C by the gall is greatest in the growth phase and is several orders of magnitude lower in the maturation phase. This finding is consistent with physiological and anatomical changes that occur during the two phases of gall development and represents a key developmental strategy by cynipids to ensure adequate food resources before larval growth begins.     

Photosynthetic capacity of the capsule wall and its contribution to carbon fixation and seed yield in castor (<Emphasis Type="Italic">Ricinus communis</Emphasis> L.)

Defoliation occurs in castor due to several reasons, but the crop has propensity to compensate for the seed yield. Photosynthetic efficiency in terms of functional (gas exchange and chlorophyll fluorescence) and structural characteristics (photosynthetic pigment profiles and anatomical properties) of castor capsule walls under light- and dark-adapted conditions was compared with that of leaves. Capsule wall showed high intrinsic efficiency of photosystem II (F _v/F _m, 0.82) which was comparable to leaves (F _v/F _m, 0.80). With increasing photon flux densities (PFD), actual quantum yields and photochemical quenching coefficients of the capsule walls were similar to that in leaves, while electron transport rates reached a maximum corresponding to about 118 % of the leaves. However, maximum net photosynthetic rate of the capsule walls (2.60 µmol CO₂ m^?2 s^?1) was less than one-fourth of the leaves (15.67 µmol CO₂ m^?2 s^?1) at the CO₂ concentration of 400 µmol mol^?1, and the difference was attributed to about 80 % lower stomatal density and the 75 % lower total chlorophyll content of capsule walls than the leaves. Furthermore, seed weight in dark-adapted capsules was 2.70–12.42 % less as compared to the capsules developed under light. The results indicate that castor capsule walls are photosynthetically active (about 15–30 % of the leaves) and contribute significantly to carbon fixation and seed yield accounting for 10 % photoassimilates towards seed weight.     

Induced carbon reallocation and compensatory growth as root herbivore tolerance mechanisms

Upon attack by leaf herbivores, many plants reallocate photoassimilates below ground. However, little is known about how plants respond when the roots themselves come under attack. We investigated induced resource allocation in maize plants that are infested by the larvae Western corn rootworm Diabrotica virgifera virgifera. Using radioactive ¹¹CO₂, we demonstrate that root‐attacked maize plants allocate more new ¹¹C carbon from source leaves to stems, but not to roots. Reduced meristematic activity and reduced invertase activity in attacked maize root systems are identified as possible drivers of this shoot reallocation response. The increased allocation of photoassimilates to stems is shown to be associated with a marked thickening of these tissues and increased growth of stem‐borne crown roots. A strong quantitative correlation between stem thickness and root regrowth across different watering levels suggests that retaining photoassimilates in the shoots may help root‐attacked plants to compensate for the loss of belowground tissues. Taken together, our results indicate that induced tolerance may be an important strategy of plants to withstand belowground attack. Furthermore, root herbivore‐induced carbon reallocation needs to be taken into account when studying plant‐mediated interactions between herbivores.     

Photosynthesis and Carbon Partitioning in Transgenic Tobacco Plants Deficient in Leaf Cytosolic Pyruvate Kinase

Whole-plant diurnal C exchange analysis provided a noninvasive estimation of daily net C gain in transgenic tobacco (Nicotiana tabacum L.) plants deficient in leaf cytosolic pyruvate kinase (PK_c−). PK_c− plants cultivated under a low light intensity (100 μmol m⁻² s⁻¹) were previously shown to exhibit markedly reduced root growth, as well as delayed shoot and flower development when compared with plants having wild-type levels of PK_c (PK_c+). PK_c− and PK_c+ source leaves showed a similar net C gain, photosynthesis over a range of light intensities, and a capacity to export newly fixed ¹⁴CO₂ during photosynthesis. However, during growth under low light the nighttime, export of previously fixed ¹⁴CO₂ by fully expanded PK_c− leaves was 40% lower, whereas concurrent respiratory ¹⁴CO₂ evolution was 40% higher than that of PK_c+ leaves. This provides a rationale for the reduced root growth of the PK_c− plants grown at low irradiance. Leaf photosynthetic and export characteristics in PK_c− and PK_c+ plants raised in a greenhouse during winter months resembled those of plants grown in chambers at low irradiance. The data suggest that PK_c in source leaves has a critical role in regulating nighttime respiration particularly when the available pool of photoassimilates for export and leaf respiratory processes are low.     

Transport processes in stimulated and non-stimulated leaves ofMimosa pudica

Summary Mature leaves ofMimosa pudica L. or parts of them were exposed to¹⁴CO₂, and translocation was recorded by macroautoradiography. It was observed that considerable amounts of labelled photoassimilates were accumulated in pulvini when the leaf was stimulated. In non-stimulated leaves, no such accumulation of label was observed.Microautoradiographs of pulvinar regions of the non-stimulated leaf showed¹⁴C- label restricted to the phloem. When stimulated, the¹⁴C- label was unloaded from the phloem of the pulvini. Labelled photoassimilates appeared most concentrated in the walls of the collenchymatous cells and beyond in the extensor region of the motor cortex. There, label was accumulated in the apoplastic compartments. Stimulation causes a sudden phloem unloading of sucrose, and its accumulation in the apoplast lowers the water potential which eventually exceeds the osmotic potential of the extensor cells of the motor cortex. By removal of cytoplasmic water the motor cells lose turgidity which results in the closing movement of the leaflets, and — some seconds later — in the bending down of the petiole. In late afternoon night-stimulation triggers sucrose unloading in secondary pulvini. During phases of relaxation, labelled material is taken up by motor cells of the extensor, which concomitantly gain turgor.Part of the doctoral dissertation of Jörg Fromm supported by the Deutsche Forschungsgemeinschaft     

ABA and GA3 increase carbon allocation in different organs of grapevine plants by inducing accumulation of non‐structural carbohydrates in leaves,enhancement of phloem area and expression of sugar transporters

Grape quality for winemaking depends on sugar accumulation and metabolism in berries. Abscisic acid (ABA) and gibberellins (GAs) have been reported to control sugar allocation in economically important crops, although the mechanisms involved are still unknown. The present study tested if ABA and gibberellin A₃ (GA₃) enhance carbon allocation in fruits of grapevines by modifying phloem loading, phloem area and expression of sugar transporters in leaves and berries. Pot‐grown Vitis vinifera cv. Malbec plants were sprayed with ABA and GA₃ solutions. The amount of soluble sugars in leaves and berries related to photosynthesis were examined at three points of berry growth: pre‐veraison, full veraison and post‐veraison. Starch levels and amylase activity in leaves, gene expression of sugar transporters in leaves and berries and phloem anatomy were examined at full veraison. Accumulation of glucose and fructose in berries was hastened in ABA‐treated plants at the stage of full veraison, which was correlated with enhancement of Vitis vinifera HEXOSE TRANSPORTER 2 (VvHT2) and Vitis vinifera HEXOSE TRANSPORTER 6 (VvHT6) gene expression, increases of phloem area and sucrose content in leaves. On the other hand, GA₃ increased the quantity of photoassimilates delivered to the stem thus increasing xylem growth. In conclusion, stimulation of sugar transport by ABA and GA₃ to berries and stems, respectively, was due to build‐up of non‐structural carbohydrates in leaves, modifications in phloem tissue and modulation in gene expression of sugar transporters.     

Changes in photosynthetic capability and carbohydrate production in an epiphytic CAM orchid plantlet exposed to super-elevated CO2

The effects on growth in super-elevated (1%) CO₂ in terms of photosynthetic capability and carbohydrate production were studied in an epiphytic CAM (Crassulacean acid metabolism) orchid plantlet, Mokara Yellow (Arachnis hookeriana×Ascocenda Madame Kenny). The growth of the plantlets was greatly enhanced after growing for 3 months at 1% CO₂ compared with the control plantlets (0.035% CO₂). CO₂ enrichment produced more than a 2-fold increase in dry matter production. The enhanced root growth at 1% CO₂ led to a higher root:shoot ratio. Plantlets grown at super-elevated CO₂ had higher F_v/F_m values, a higher photochemical quenching (q_P) and a relatively lower non-photochemical quenching (q_N). CO₂ at 1% appeared to enhance the utilization of captured light energy in the orchid plantlets. CO₂ enrichment also increased contents of soluble sugars (glucose and sucrose) and starch in the orchid plantlets. The extra starch formed under 1% CO₂ did not cause a disruption of the chloroplasts. Chlorophyll content was higher and a clear granal stacking was evident in young leaves and roots of plantlets grown at 1% CO₂. An extensive thylakoid system was observed in the young leaf chloroplasts of the CO₂-enriched plantlets indicating an improved development of the photosynthetic apparatus when compared to that of the control plantlets. The increased photosynthetic capacity and enhanced growth of the epiphytic roots under CO₂ enrichment would facilitate the generation of more photoassimilates and acquisition of essential resources, thereby increasing the survival rate of orchid plantlets under stressful field conditions.     

Fruit production in three masting tree species does not rely on stored carbon reserves

Fruiting is typically considered to massively burden the seasonal carbon budget of trees. The cost of reproduction has therefore been suggested as a proximate factor explaining observed mast-fruiting patterns. Here, we used a large-scale, continuous ¹³C labeling of mature, deciduous trees in a temperate Swiss forest to investigate to what extent fruit formation in three species with masting reproduction behavior (Carpinus betulus, Fagus sylvatica, Quercus petraea) relies on the import of stored carbon reserves. Using a free-air CO₂ enrichment system, we exposed trees to ¹³C-depleted CO₂ during 8 consecutive years. By the end of this experiment, carbon reserve pools had significantly lower δ¹³C values compared to control trees. δ¹³C analysis of new biomass during the first season after termination of the CO₂ enrichment allowed us to distinguish the sources of built-in carbon (old carbon reserves vs. current assimilates). Flowers and expanding leaves carried a significant ¹³C label from old carbon stores. In contrast, fruits and vegetative infructescence tissues were exclusively produced from current, unlabeled photoassimilates in all three species, including F. sylvatica, which had a strong masting season. Analyses of δ¹³C in purified starch from xylem of fruit-bearing shoots revealed a complete turn-over of starch during the season, likely due to its usage for bud break. This study is the first to directly demonstrate that fruiting is independent from old carbon reserves in masting trees, with significant implications for mechanistic models that explain mast seeding.     

Elevated CO2 may impair the beneficial effect of arbuscular mycorrhizal fungi on the mineral and phytochemical quality of lettuce

Arbuscular mycorrhizal fungi (AMF) can improve growth and nutritional quality of greenhouse‐grown lettuces cultivated at ambient CO₂. Moreover, mycorrhizal symbiosis is predicted to be important in defining plant responses to elevated atmospheric CO₂ concentrations. Our main objective was to assess the effects of elevated CO₂ on growth and nutritional quality of greenhouse‐grown lettuces inoculated or not with AMF. Results showed that the accumulation of mineral nutrients (e.g. P, Cu, Fe) and antioxidant compounds (carotenoids, phenolics, anthocyanins, ascorbate) induced by AMF in leaves of lettuces cultivated at ambient CO₂ may diminish or disappear under elevated CO₂. It is hypothesized that a relevant quantity of photoassimilates could be used for improving shoot growth and spreading mycorrhizal colonization in detriment to the secondary metabolism. However, important differences can be found among different cultivars of lettuces.     

Chlorophenoxyacetic acid and chloropyridylphenylurea accelerate translocation of photoassimilates to parthenocarpic and seeded fruits of muskmelon (Cucumis melo)

We compared the effect of p-chlorophenoxyacetic acid (p-CPA) and 1-(2-chloro-4-pyridyl)-3-phenylurea (CPPU) on parthenocarpic and seeded muskmelon (Cucumis melo) fruits in regards to fruit development and the transport of photoassimilates from leaves exposed to ¹⁴CO₂ to the developing fruits. Ten days after anthesis (DAA), the fresh weight, total ¹⁴C-radioactivity and contents of ¹⁴C-sucrose and ¹⁴C-fructose were higher in the CPPU-induced parthenocarpic fruits than in seeded fruits. However, at 35 DAA, fresh weight and sucrose content in mesocarp, placenta and empty seeds of the parthenocarpic fruits were lower than in seeded fruits. Also, total ¹⁴C-radioactivity and ¹⁴C-sugar content of the parthenocarpic fruits were lower as well as the translocation rate of ¹⁴C-photoassimilates into these fruits. Application of p-CPA to the parthenocarpic fruits at 10 and 25 DAA increased fresh weight and sugar content. Moreover, these treatments elevated the total ¹⁴C-radioactivity, ¹⁴C-sucrose content and the translocation rate of ¹⁴C-photoassimilates. The ¹⁴C-radioactivity along the translocation pathway from leaf to petiole, stem, lateral shoot and peduncle showed a declining pattern but dramatically increased again in the fruits. These results suggest that the fruit's sink strength was regulated by the seed and enhanced by the application of p-CPA.     

Pathway of phloem loading in the C3 tropical orchid hybrid Oncidium Goldiana

The apoplast of mature leaves of the tropical orchid OncidiumGoldiana was perfused with 0.5 mM p-chloromercuribenzenesulphonicacid (PCMBS) via the transpiration stream in order to test themode of phloem loading. The efficacy of introducing PCMBS byperfusion was shown by saffranin O dye movement in the veinsand leaf apoplast in control experiments. Photoassimilate exportas the result of phloem loading was measured by collection of¹⁴CO₂-derived photoassimilates from the basal cut-ends of intactleaves. Phloem loading and translocation of photoassimilates was inhibitedby 89% in leaves perfused with PCMBS for 1 h. The effect ofPCMBS on leaf photosynthesis was minimal. The amount of radiocarbonfixed by PCMBS-treated leaves averaged 89% of control leavesperfused with distilled water. A negative correlation betweenthe total amount of photoassimilate exuded and the calculatedconcentration of PCMBS in the leaf apoplast was also observed.The results indicate that phloem loading in Oncidium Goldianaoccurs via the apoplastic pathway. Key words: Phloem loading, apoplast, PCMBS, tropical orchid     

Chlorophyll fluorescence kinetics, photosynthetic activity, and pigment composition of blue-shade and half-shade leaves as compared to sun and shade leaves of different trees

The chlorophyll (Chl) fluorescence induction kinetics, net photosynthetic CO₂ fixation rates P _N, and composition of photosynthetic pigments of differently light exposed leaves of several trees were comparatively measured to determine the differences in photosynthetic activity and pigment adaptation of leaves. The functional measurements were carried out with sun, half-shade and shade leaves of seven different trees species. These were: Acer platanoides L., Ginkgo biloba L., Fagus sylvatica L., Platanus x acerifolia Willd., Populus nigra L., Quercus robur L., Tilia cordata Mill. In three cases (beech, ginkgo, and oak), we compared the Chl fluorescence kinetics and photosynthetic rates of blue-shade leaves of the north tree crown receiving only blue sky light but no direct sunlight with that of sun leaves. In these cases, we also determined in detail the pigment composition of all four leaf types. In addition, we determined the quantum irradiance and spectral irradiance of direct sunlight, blue skylight as well as the irradiance in half shade and full shade. The results indicate that sun leaves possess significantly higher mean values for the net CO₂ fixation rates P _N (7.8–10.7 μmol CO₂ m^?2 s^?1 leaf area) and the Chl fluorescence ratio R _Fd (3.85–4.46) as compared to shade leaves (mean P _N of 2.6–3.8 μmol CO₂ m^?2 s^?1 leaf area.; mean R _Fd of 1.94–2.56). Sun leaves also exhibit higher mean values for the pigment ratio Chl a/b (3.14–3.31) and considerably lower values for the weight ratio total chlorophylls to total carotenoids, (a + b)/(x + c), (4.07–4.25) as compared to shade leaves (Chl a/b 2.62–2.72) and (a + b)/(x + c) of 5.18–5.54. Blue-shade and half-shade leaves have an intermediate position between sun and shade leaves in all investigated parameters including the ratio F _v/F _o (maximum quantum yield of PS2 photochemistry) and are significantly different from sun and shade leaves but could not be differentiated from each other. The mean values of the Chl fluorescence decrease ratio R _Fd of blue-shade and half-shade leaves fit well into the strong linear correlation with the net photosynthetic rates P _N of sun and shade leaves, thus unequivocally indicating that the determination of the Chl fluorescence decrease ratio R _Fd is a fast and indirect measurement of the photosynthetic activity of leaves. The investigations clearly demonstrate that the photosynthetic capacity and pigment composition of leaves and chloroplasts strongly depend on the amounts and quality of light received by the leaves.     

Changes in growth and oxidative response of leaves and nodules in <Emphasis Type="Italic">Medicago ciliaris</Emphasis> during salt stress recovery

In the present study, we exposed Medicago ciliaris isolated from a saline sebhka and growing symbiotically with Sinorhizobium medicae to 100 mM of NaCl and followed this stress by a recovery period (complete NaCl withdrawal from the root medium for two or one weeks). This experiment was conducted in order i) to check whether a reduction in growth and nitrogen fixation could be reversed by alleviating salt stress and ii) to determine specific changes related to salt-induced growth and nitrogen fixation decline. Salt stress reduced growth of all organs (particularly nodules), leaf area, photosynthetic activity and nitrogen fixation. The depressive effect of salt was not linked to osmotic stress. The removal of saline conditions restored growth of all organs after 2 weeks of recovery, with aerial organs recovering quickly after 1 week. Both photosynthetic and nitrogen fixation recovered only after a 2 week period of recovery. Salt stress in these saline-tolerant plants was accompanied by oxidative damage (electrolyte leakage), which was more accentuated in nodules in comparison with leaves. The observed recovery in growth, nitrogen fixation and photosynthesis was mainly linked to a preferential preservation of shoots growth owing to the maintenance of SOD activity (namely the isoforms A₂ and A₃); such maintenance would allow higher photosynthetic activity permitting an adequate supply of nodules with photoassimilates and thus facilitating stress withstand.     

Properties of ribulose-1,5-bisphosphate carboxylase from dark- and light-exposed soybean leaves

The low activity of ribulose bisphosphate carboxylase from darkened soybean (Glycine max [L.] Merr. cv. Bragg) leaves was not raised to the level of that from leaves in the light by CO₂ and Mg²⁺, even after a 4-h incubation. The extract of darkened leaves, unlike the extract from illuminated leaves, was not fully CO₂/Mg²⁺-activatable after Sephadex gel filtration in the absence of Mg²⁺. (NH₄)₂SO₄ fractionation eliminated the inhibition effect found in the dark extracts resulting in similar rates for the extracts obtained from leaves in the dark and light. Although the V_max values of the gel-filtered extracts from dark and light leaves differed by 3-fold, the K_m(CO₂)-values were the same (12.7 μM), as were the K_m(RuBP)-values (250 μM). These data support the hypothesis that for soybean leaves in the dark a tightly-binding inhibitor renders much of the ribulose bisphosphate carboxylase enzyme catalytically non-functional.     

Water potential,water saturation deficit and their relationship in leaves of different insertion levels

The water potential (Ψ _w ) and the water saturation deficit (δW _sat) in leaves of different insertion levels of potted kale plants were simultaneously measured. In non-wilting plantsδW _sat gradually decreased andΨ _w slightly increased from the upper to the lower leaves. During the wilting of the plants induced by decreasing of soil moistureΨw practically decreased paralelly in all the leaves but the same decrease ofΨ _w was connected with the lowest increase ofδW _sat in upper leaves and the highest increase ofδW _sat in lower leaves. Not only the values ofΨ _w andδW _sat but also their relationship varied considerably with the leaf insertion levels.     

Leaf orientation,photorespiration and xanthophyll cycle protect young soybean leaves against high irradiance in field

In order to fully understand the adaptive strategies of young leaves in performing photosynthesis under high irradiance, leaf orientation, chloroplast pigments, gas exchange, as well as chlorophyll a fluorescence kinetics were explored in soybean plants. The chlorophyll content and photosynthesis in young leaves were much lower than that in fully expanded leaves. Both young and fully expanded leaves exhibited down-regulation of the maximum quantum yield (F_V/F_M) at noon in their natural position, no more serious down-regulation being observed in young leaves. However, when restraining leaf movement and vertically exposing the leaves to 1200 μmol m⁻² s⁻¹ irradiance, more pronounced down-regulation of F_V/F_M was observed in young leaves; and the actual photosystem II (PS II) efficiency (Ф_PSII) drastically decreased with the significant enhancement of non-photochemical quenching (NPQ) and ‘High energy’ quenching (q_E) in young leaves. Under irradiance of 1200 μmol m⁻² s⁻¹, photorespiration (P_r) in young leaves measured by gas exchange were obviously lower, whereas the ratio of photorespiration/gross photosynthetic rate (P_r/P_g) were higher than that in fully expanded leaves. Compared with fully expanded leaves, young leaves exhibited higher xanthophyll pool and a much higher level of de-epoxidation components when exposure to high irradiance. During leaf development, the petiole angle gradually increased all the way. Especially, the midrib angle decreased with the increasing of irradiance in young leaves; however, no distinct changes were observed in mature leaves. The changes of leaf orientation greatly reduced the irradiance on young leaf surface under natural positions. In this study, we suggested that the co-operation of leaf angle, photorespiration and thermal dissipation depending on xanthophyll cycle could successfully prevent young leaves against high irradiance in field.     

Characteristics of a Diurnal Rhythm of Assimilate Translocation in Grapevine Plants

Characteristics of photoassimilate efflux from the leaves of grapevine (Vitis vinifera L., var. Rkatsiteli) plants under natural conditions and upon darkening, defoliation, and ABA treatment at short exposures (2, 6, and 24 h) as well as diurnal pattern of starch content in the leaves were investigated. Translocation of assimilates from the leaves of grapevine usually occurred at night and started after 20:00 or 21:00. However, such pattern of translocation was not strictly determinated and could be changed upon the action of some factors. Darkening, defoliation, and treatment with ABA initiated the translocation of photoassimilates from the leaves in the daytime. Darkening and ABA treatment turned out to be more efficient than defoliation. Darkening and defoliation significantly changed the character of partitioning of photoassimilates in the stem. We corroborated the opinion that in grapevine, the efflux of assimilates from source leaves is also governed by the feedback mechanism and probably much depends on the level of carbohydrates.     

Root-zone temperature effects on photosynthesis, <Superscript>14</Superscript>C-photoassimilate partitioning and growth of temperate lettuce (<Emphasis Type="Italic">Lactuca sativa</Emphasis> cv. ‘Panama’) in the tropics

The effect of root growth temperature on maximal photosynthetic CO₂ assimilation (P _max), carbohydrate content, ¹⁴C-photoassimilate partitioning, growth, and root morphology of lettuce was studied after transfer of the root system from cool root-zone temperature (C-RZT) of 20 °C to hot ambient-RZT (A-RZT) and vice versa. Four days after RZT transfer, P _max and leaf total soluble sugar content were highest and lowest, respectively, in C-RZT and A-RZT plants. P _max and total leaf soluble sugar content were much lower in plants transferred from C-to A-RZT (C→A-RZT) than in C-RZT plants. However, these two parameters were much higher in plants transferred from A-to C-RZT (A→C-RZT) than in A-RZT plants. A-RZT and C→A-RZT plants had higher root total soluble sugar content than A→C-RZT and C-RZT plants. Leaf total insoluble sugar content was similar in leaves of all plants while it was the highest in the roots of C-RZT plants. Developing leaves of C-RZT plants had higher ¹⁴C-photoassimilate content than A-RZT plants. The A→C-RZT plants also had higher ¹⁴C-photoassimilate content in their developing leaves than A-RZT plants. However, more ¹⁴C-photoassimilates were translocated to the roots of A-RZT and C→A-RZT plants, but they were mainly used for root thickening than for its elongation. Increases in leaf area, shoot and root fresh mass were slower in C→A-RZT than in C-RZT plants. Conversely, A→C-RZT plants had higher increases in these parameters than A-RZT plants. Lower root/shoot ratio (R/S) in C-RZT than in A-RZT plants confirmed that more photoassimilates were channelled to the shoots than to the roots of C-RZT plants. Roots of C-RZT plants had greater total length with a greater number of tips and surface area, and smaller average diameter as compared to A-RZT plants. In C→A-RZT plants, there was root thickening but the increases in its length, tip number and surface area decreased. The reverse was observed for A→C-RZT plants. These results further supported the idea that newly fixed photoassimilates contributed more to root thickening than to root elongation in A-RZT and C→A-RZT plants.     

Pyruvate orthophosphate dikinase of c(3) seeds and leaves as compared to the enzyme from maize

Pyruvate orthophosphate dikinase (PPDK) was found in various immature seeds of C₃ plants (wheat, pea, green bean, plum, and castor bean), in some C₃ leaves (tobacco, spinach, sunflower, and wheat), and in C₄ (maize) kernels. The enzyme in the C₃ plants cross-reacts with rabbit antiserum against maize PPDK. Based on protein blot analysis, the apparent subunit size of PPDK from wheat seeds and leaves and from sunflower leaves is about 94 kdaltons, the same as that of the enzyme from maize, but is slightly less (about 90 kdaltons) for the enzyme from spinach and tobacco leaves. The amount of this enzyme per mg of soluble protein in C₃ seeds and leaves is much less than in C₄ leaves. PPDK is present in kernels of the C₄ plant, Zea mays in amounts comparable to those in C₄ leaves.

Regulatory properties of the enzyme from C₃ tissues (wheat) are similar to those of the enzyme from C₄ leaves with respect to in vivo light activation and dark inactivation (in leaves) and in vivo cold lability (seeds and leaves).

Following incorporation of ¹⁴CO₂ by illuminated wheat pericarp and adjoining tissue for a few seconds, the labeled metabolites were predominantly products resulting from carboxylation of phosphoenolpyruvate, with lesser labeling of compounds formed by carboxylation of ribulose 1,5-bisphosphate and operation of the reductive pentose phosphate cycle of photosynthesis. PPDK may be involved in mechanisms of amino acid interconversions during seed development.