Chloroembryos: A unique photosynthesis system

The embryos of some angiosperm taxa contain chlorophyll and this chlorophyllous stage is persisting until the embryo matures (further referred as chloroembryos). Besides being chlorophyllous, these embryos seem to have the ability to photosynthesize. This suggests that the chlorophyllous state of the embryo has an important role in seed development. The photosynthesis of chloroembryos is highly shade adaptive in nature as it is embedded within the supporting tissues (several layers of pod wall, seed coat and endosperm). Moreover, these chloroembryos are developing in a highly osmotic environment, and contain various components of the photosynthetic machinery. Detailed studies were performed in these chloroembryos in order to elucidate the structure of the chloroplasts, pigment composition, the photochemical activities, the rate of carbon assimilation and also the shade adaptive features. It has been shown that the respired CO₂ within these chloroembryos is recycled by the efficient photosynthetic components of the chloroembryos and thus potentially influences the seed's carbon economy. Thus, the major role of embryonic photosynthesis is to produce both energy-rich molecules and oxygen, of which the former can be directly used for biosynthesis. During embryogenesis oxygen production is especially important, in a situation wherein the oxygen is limited within the enclosed seed. As these chloroembryos grow in an environment of a sugar rich endosperm, it requires some adaptive mechanisms in this high osmotic environment. The additional polypeptides found in the thylakoids of chloroembryo chloroplasts in comparison to the thylakoids of leaf chloroplast have been suggested to have a role in protecting the photosynthetic components in the chloroembryos in an environment of high osmotic strength. An attempt to understand osmotic stress tolerance existing in these chloroembryos may lead to a better understanding of tolerance of photosynthesis to osmotic stress.     

Abscisic acid and osmoticum prevent germination of developing alfalfa embryos,but only osmoticum maintains the synthesis of developmental proteins

Developing seeds of alfalfa (Medicago sativa L.) acquire the ability to germinate during the latter stages of development, the maturation drying phase. Isolated embryos placed on Murashige and Skoog medium germinate well during early and late development, but poorly during mid-development; however, when placed on water they germinate well only during the latter stage of development. Germination of isolated embryos is very slow and poor when they are incubated in the presence of surrounding seed structures (the endosperm or seed coat) taken from the mid-development stages. This inhibitory effect is also achieved by incubating embryos in 10^?5 M abscisic acid (ABA). Endogenous ABA attains a high level during mid-development, especially in the endosperm. Seeds developing in pods treated with fluridone (1-methyl-3-phenyl-5[3-(trifluoromethyl)-phenyl]-4(1H)-pyridinone) contain low levels of ABA during mid-development, and the endosperm and seed coat only weakly inhibit the germination of isolated embryos. However, intact seeds from fluridone-treated pods do not germinate viviparously, which is indicative that ABA alone is not responsible for maintaining seeds in a developing state. Application of osmoticum (e.g. 0.35 M sucrose) to isolated developing embryos prevents their germination. Also, in the developing seed in situ the osmotic potential is high. Thus internal levels of osmoticum may play a role in preventing germination of the embryo and maintaining development. Abscisic acid and osmoticum impart distinctly different metabolic responses on developing embryos, as demonstrated by their protein-synthetic capacity. Only in the presence of osmoticum do embryos synthesize proteins which are distinctly recognizable as those synthesized by developing embryos in situ, i.e. when inside the pod. Abscisic acid induces the synthesis of a few unique proteins, but these arise even in mature embryos treated with ABA. Thus while both osmoticum and ABA prevent precocious germination, their effects on the synthetic capacity of the developing embryo are quite distinct. Since seeds with low endogenous ABA do not germinate, osmotic regulation may be the more important of these two factors in controlling seed development.     

Osmotic regulation of assimilate unloading from seed coats of Vicia faba. Assimilate partitioning to and within attached seed coats.

The significance of the osmotic potential of the seed apoplast sap as a regulator of assimilate transfer to and within coats of developing seed of Vicia faba (cv. Coles Prolific) was assessed using attached empty seed coats and intact developing seed. Following surgical removal of the embryos, through windows cut in the pod walls and underlying seed coats, the resulting attached “empty” seed coats were filled with solutions of known osmotic potentials (–0. 02 versus –0. 75 MPa). Sucrose efflux from the coats was elevated at the higher osmotic potential (high osmotic concentration) for the first 190 min of exchange. Thereafter, this efflux was depressed relative to efflux from coats exposed to the low osmotic potential (high osmotic concentration) solution. This subsequent reversal in efflux was attributable to an enhanced diminution of the coat sucrose pools at the high external osmotic potential. Indeed, when expressed as a proportion of the current sucrose pool size, relative efflux remained elevated for coats exposed to the high osmotic potential solution. Measurement of potassium and sucrose fluxes to and from their respective pools in the coat tissues demonstrated that the principal, fluxes, sensitive to variative in the external osmotic potential, were phloem import into and efflux from the “empty” coats. Phloem import, consistent with a pressure-driven phloem transport mechanism, responded inversely with changes in the external osmotic potential. In contrast, sucrose and potassium efflux from the coats exhibited a positive dependence on the osmotic potential. Growth rates of whole seed were approximately doubled by enclosing selected pods in water jackets held at temperatures of 25°C. compared to 15°C. The osmotic potential of sap collected from the seed apoplast remained constant and independent of the temperature-induced changes in seed growth rates and hence phloem import. Based on these findings, it is proposed that control of phloem import by changes in the external osmotic potential observed with “empty” seed coats has no significance as a regulator of assimilate import by intact seed. Rather, maintenance of the seed apoplast osmotic potential, independent of seed growth rate, suggests that the observed osmotic regulation of efflux from the coats may play a key role in integrating assimilate demand by the embryo with phloem import.     

Fruit production of shrub, Caragana korshinskii, following above-ground partial shoot removal: mechanisms underlying compensation

A number of studies have showed that under some conditions plant may partially, fully or overcompensate for tissue loss, however, the mechanisms underlying compensation are not well understood and still need to be researched. We examined the ability of Caragana korshinskii to compensate for fruit production after above-ground partial shoot removal. Fruit production of 30% main shoot length removal (30% RSL) and 25 and 50% main shoot number removal (25% RSN, 50% RSN) resulted in overcompensation and the response of 60% main shoot length removal (60% RSL) was full compensation. Plants’ responses associated with compensation included (1) greater reproduction efficiency (RA); (2) increased fruit set; (3) decreased fruit abortion; (4) increased seed number per pod; and (5) higher individual seed biomass. These responses may have resulted from more nectar production per flower, more sucrose flux per pod and more sucrose flux per seed of clipped plants, which may in turn have resulted from (1) drawing upon more non-structural carbohydrate (TNC) from roots to supply flower bud development and the flush of new foliage; (2) supplying more photosynthetic assimilation to fruit development owing to increases in leaf-level photosynthetic rates. Increases in leaf-level photosynthetic rates may be caused by more nutrient (nitrogen) and water availability per unit area of resource leaves after clipping.     

外源水杨酸对NaCl胁迫下白榆幼苗光合作用及离子分配的影响

以1年生白榆幼苗为研究对象,设置0、0.5、1.0和2.0 mmol·L^-1水杨酸(SA)与0、50、100和150 mmol·L^-1 NaCl处理组合,考察盐胁迫下白榆幼苗生物量、光合色素含量、光合作用参数及根叶离子含量、分配、运输的情况,探讨外源SA对NaCl胁迫下白榆幼苗耐盐生理特征的影响。结果表明:(1)NaCl胁迫显著抑制了白榆幼苗的生长、光合色素含量及光合能力,并破坏了白榆体内离子平衡。(2)喷施外源SA使盐胁迫下白榆幼苗的干重和根冠比均不同程度升高,0.5和2.0 mmol·L^-1 SA不同程度提高了50和100 mmol·L^-1 NaCl处理组幼苗叶片光合色素含量。(3)0.5 mmol·L^-1 SA显著提升了50 mmol·L^-1 NaCl处理组白榆幼苗的净光合速率(Pn)、气孔导度(Gs)和蒸腾速率(Tr),1.0和2.0 mmol·L^-1 SA对150 mmol·L^-1 NaCl处理组幼苗净光合速率改善效果较好,外源SA对100 mmol·L^-1 NaCl处理组幼苗的光合作用参数无显著影响。(4)NaCl胁迫下,外源SA处理的白榆幼苗叶和根Na^+含量及Na^+/K^+、Na^+/Ca^2+和Na^+/Mg^2+显著降低,离子选择运输系数SK,Na、SCa,Na和SMg,Na升高,从而促进了幼苗K^+、Ca^2+和Mg^2+由根向叶片的转运;隶属函数分析发现对白榆幼苗叶和根中离子含量改善效果最好的SA浓度分别为1.0和2.0 mmol·L^-1。因此,适宜浓度的外源水杨酸能够有效改善NaCl胁迫下白榆幼苗的光合能力,有效调节白榆幼苗体内离子状态,从而增强白榆对NaCl胁迫的抗性。     

Intra-fruit seed abortion in a wind dispersed tree,Dalbergia sissoo Roxb: Proximate mechanisms

Dalbergia sissoo, a tropical tree with wind-dispersed pods, exhibits a highly positively skewed distribution of seeds per pod with predominantly only one of the four or five ovules maturing into seed. The abortion cannot be attributed to lack of pollen or resources. This study examines the hypothesis that the abortion is due to an intense rivalry among the developing sibs to gain dispersal advantage. Aqueous extract of the dominant embryos at the stigmatic end that generally develop to maturity significantly inhibited the uptake of labelled sucrose by the young developing (subject) embryos in an in vitro assay system. Extracts of tissues such as subordinate (peduncular embryos), unfertilized ovules and pod coat did not cause such inhibition. Aqueous diffusate of dominant embryos also inhibited the uptake of labelled sucrose by subject embryos. The chemical substance responsible for the inhibition appears to be heat-stable and non-proteinaceous. HPLC analysis indicated the presence of two retention time peaks, different from that of standard indole acetic acid, but with considerable overlap. We hypothesize that the compound could be an indole derivative. We propose that the stigmatic embryos have a head start due to earlier fertilization and produce a chemical that either directly (by metabolically killing) or indirectly (by preventing the uptake of assimilates) kills the proximally placed peduncular embryos.     

Sink to source translocation in soybean

The possibility that phloem loading may occur in the reproductive sink tissues of soybeans (Glycine max Merr. cv Chippewa 64) was examined. When [¹⁴C]sucrose was applied to seed coat tissues from which the developing embryo had been surgically removed, 0.1% to 0.5% of the radioactivity was translocated to the vegetative plant parts. This sink to source translocation was largely unaffected by destroying a band of phloem with steam treatment on the stem above and below the labeled pod. The same steam treatment, however, completely abolished translocation of [¹⁴C]sucrose between mature leaves and developing fruits. These results indicate that the movement of nutrients from developing seed coats to the vegetative plant parts occur in the xylem and that phloem loading does not occur in this sink tissue.     

模拟干旱和盐分胁迫对沙枣幼苗PSⅡ活力的影响

通过PEG-6000和NaCl模拟实验研究了干旱和盐分胁迫对沙枣幼苗叶片光系统Ⅱ(PSⅡ)活力的影响。结果显示:PEG胁迫使沙枣幼苗叶片PSⅡ在300μs时相对于FJ-Fo的可变荧光比值(Wk)升高,却降低了单位反应中心密度(RC/CSo)和最大量子产量(Fv/Fm),导致效能指数(PIABS)随水势降低而显著下降,阻碍了电子传递链中供体和受体侧的电子传递,也抑制了叶绿素的合成,从而全面抑制PSⅡ的活力;NaCl胁迫对沙枣叶片PSⅡ活力没有显著影响。比较两种处理等渗溶液下的结果发现,盐离子对沙枣叶片PSⅡ活力具有正效应,它抵消了渗透效应对沙枣叶片PSⅡ活力的抑制作用,这可能与盐离子进入叶片细胞,减轻了渗透胁迫有关。     

Maturation of black spruce somatic embryos: Sucrose hydrolysis and resulting osmotic pressure of the medium

The physiological and osmotic roles of sucrose during black spruce (Picea mariana (Mill.) B.S.P.) embryo maturation were investigated. The results showed that when both sucrose and mannitol were present in the medium, the optimum sucrose concentration varied between 4% and 6%. From these data, mannitol does not apparently replace sucrose during the maturation of somatic embryos and therefore it might not be a suitable osmoticum. For the media supplemented with 4% to 12% sucrose and various concentrations of mannitol, the osmotic pressure of the medium rose during maturation, particularly for the highest sucrose concentrations (7% to 12%). Medium containing 3% each of fructose and glucose produced fewer mature embryos compared to the medium with 6% sucrose. An increment in the osmotic potential was observed in medium with 6% sucrose in contrast to that containing 3% each of fructose and glucose. Sugar analysis revealed that the sucrose hydrolysis in the medium was detectable within 1 week of incubation and continued throughout the maturation period. Moreover, no significant uptake of the sugars was detected, since the total amount of fructose, glucose and sucrose remained constant. Our results indicate that the action of sucrose on embryo maturation is mostly achieved through an osmotic control.     

NaCl胁迫下外源NO供体硝普钠（SNP）对盐地碱蓬种子萌发的影响

用添加与不添加0．1mm01．L^-1NO供体硝普钠（sNP）的800mmol．L^-1NaCl溶液处理盐地碱蓬种子后,800mmol·L^-1NaCl处理下盐地碱蓬种子的萌发率、含水量和吸水速率显著增加,胚中脯氨酸的含量降低,但对Na^＋、K^＋和可溶性糖含量无显著影响。表明0．1mmol．L^-1SNP缓解800mmol．L^-1NaCl对盐地碱蓬种子萌发抑制的主要原因是盐地碱蓬种子含水量的提高,从而缓解了盐的渗透胁迫。     

Ionic and osmotic effects of NaCl-induced inactivation of photosystems I and II in Synechococcus sp

We report here that osmotic effects and ionic effects are both involved in the NaCl-induced inactivation of the photosynthetic machinery in the cyanobacterium Synechococcus sp. PCC 7942. Incubation of the cyanobacterial cells in 0.5 M NaCl induced a rapid and reversible decline and subsequent slow and irreversible loss of the oxygen-evolving activity of photosystem (PS) II and the electron transport activity of PSI. An Na(+)-channel blocker protected both PSII and PSI against the slow, but not the rapid, inactivation. The rapid decline resembled the effect of 1.0 M sorbitol. The presence of both an Na(+)-channel blocker and a water-channel blocker protected PSI and PSII against the short- and long-term effects of NaCl. Salt stress also decreased cytoplasmic volume and this effect was enhanced by the Na(+)-channel blocker. Our observations suggested that NaCl had both osmotic and ionic effects. The osmotic effect decreased the amount of water in the cytosol, rapidly increasing the intracellular concentration of salts. The ionic effect was caused by an influx of Na(+) ions through potassium/Na(+) channels that also increased concentrations of salts in the cytosol and irreversibly inactivated PSI and PSII.     

Soybean Seed Water Relations during in Situ and in Vitro Growth and Maturation

Water, osmotic, and pressure potentials of soybean (Glycine max [L.] Merrill) embryos and related maternal tissues were measured during periods of seed growth and maturation to test the involvement of embryo water relations in seed maturation. Seeds were matured in situ or in an in vitro liquid culture medium in detached pods or as isolated seeds. Changes in water relations of embryo tissues were independent of maternal tissues. During seed maturation in situ, water and osmotic potentials in both embryo and maternal tissues declined sharply near the time of maximum dry weight. During in vitro seed culture with and without pods, water and osmotic potentials in axis and cotyledon tissues declined continuously during growth. Water and osmotic potentials of the seed coat, which was present only during in vitro seed culture with pods, changed little during the culture period. Positive turgor in the embryo was maintained beyond maximum dry weight and the loss of green color during in vitro culture but declined to zero at maturity in situ. The osmotic potential in embryo tissues declined from −1.1 megapascals at early pod fill to between −1.65 and −2.2 megapascals at maximum seed dry weight across all maturation environments. It is suggested that the decreasing osmotic potential in the growing soybean embryo reaches a threshold level that is associated with cessation of growth and onset of seed maturation.     

Effects of Salt Stress on Carbohydrate Metabolism in Desert Soil Alga Microcoleus vaginatus Gom.

The effects of salt stress on carbohydrate metabolism in Microcoleus vaginatus Gom., a cyanobacterium isolated from desert algal crusts, were investigated in the present study. Extracellular total carbohydrates and exopolysaccharides （EPS） in the culture medium produced by M. vaginatus increased significantly during the growth phase and reached a maximum during the stationary phase. The production of extracellular carbohydrates also significantly increased under higher salt concentrations, which was attributed to an increase in low molecular weight carbohydrates. In the presence of NaCI, the production of cellular total carbohydrates decreased and photosynthetic activity was impaired, whereas cellular reducing sugars, water-soluble sugars and sucrose content and sucrose phosphate synthase activity increased, reaching a maximum in the presence of 200 mmol/L NaCI. These parameters were restored to original levels when the algae were transferred to a non-saline medium. Sodium and K＋ concentrations of stressed cells decreased significantly and H＋-ATPase activity increased after the addition of exogenous sucrose or EPS. The results suggest that EPS and sucrose are synthesized to maintain the cellular osmOtic equilibrium between the intra-and extracellular environment, thus protecting algal cells from osmotic damage, which was attributed to the selective exclusion of cellular Na＋ and K＋ by H＋-ATPase.     

Carbohydrate status during somatic embryo maturation in Norway spruce

Summary The development of Norway spruce (Picea abies (L.) Karst.) somatic embryos on a maturation medium was accompanied by changes in nonstructural carbohydrate status. During embryo maturation, the content of total soluble sugars in the embryonal suspensor mass decreased and the partitioning between sucrose and hexoses changed considerably in favor of sucrose. Developing somatic embryos were mainly responsible for these changes. Osmotic stress caused by the presence of 3.75% polyethylene glycol (PEG) in the maturation medium (decrease in osmotic potential by 52.5 kPa) resulted in dramatic changes in the content of endogenous saccharides. There was a lower total carbohydrate content in the embryonal suspensor mass grown on the medium containing PEG in comparison with the untreated control. Isolated embryos from later stages of embryo development contained mainly sucrose with a small amount (20%) of fructose and nearly no glucose. A further increase in PEG concentration in the medium (7.5%; decrease in osmotic potential by 112.5 kPa compared to the maturation medium) led to a large increase in the total endogenous sugar content. This increase in sugars was a result of the enhanced content of sucrose, fructose, and glucose. The increased glucose content was in contrast to embryos grown on the medium with lower or no PEG content.     

Role of 5-aminolevulinic acid (ALA) on active oxygen-scavenging system in NaCl-treated spinach (Spinacia oleracea)

ALA is a key precursor in the biosynthesis of porphyrins such as chlorophyll and heme, and was found to induce temporary elevations in the photosynthesis rate, APX, and CAT; furthermore, treatment with ALA at a low concentration might be correlated to the increase of NaCl tolerance of spinach plants. The photosynthetic rate and the levels of active oxygen-scavenging system in the 3rd leaf of spinach (Spinacia oleracea) plants grown by foliar treatment with 0, 0.18, 0.60 and 1.80 mmol/L 5-aminolevulinic acid under 50 and 100 mmol/L NaCl were analyzed. Plants treated with 0.60 and 1.80 mmol/L ALA showed significant increases in the photosynthetic rate at 50 and 100 mmol/L NaCl, while that of 0.18 mmol/L ALA did not show any changes at 50 mmol/L NaCl and a gradual decrease at 100 mmol/L NaCl. In contrast, the rate with 0 mmol/L ALA showed reduction at both concentrations of NaCl. The increase of hydrogen peroxide content by treatment with 0.60 and 1.80 mmol/L ALA were more controlled than that of 0 mmol/L ALA under both NaCl conditions. These ALA-treated spinach leaves also exhibited a lower oxidized/reduced ascorbate acid ratio and a higher reduced/oxidized glutathione ratio than the 0 mmol/L-treated spinach leaves when grown at both NaCl conditions. With regard to the antioxidant enzyme activities in the leaves, ascorbate peroxidase, catalase, and glutathione reductase activities were enhanced remarkably, most notably at day 3, by treatment with 0.60 and 1.80 mmol/L ALA under both NaCl conditions in comparison to that of 0 and 0.18 mmol/L ALA. These data indicate that the protection against oxidative damage by higher levels of antioxidants and enzyme activities, and by a more active ascorbate-glutathione cycle related to the increase of the photosynthesis rate, could be involved in the increased salt tolerance observed in spinach by treatment with 0.60 to 1.80 mmol/L ALA with NaCl.     

Oxidative stress protection and stomatal patterning as components of salinity tolerance mechanism in quinoa (Chenopodium quinoa)

Two components of salinity stress are a reduction in water availability to plants and the formation of reactive oxygen species. In this work, we have used quinoa (Chenopodium quinoa), a dicotyledonous C3 halophyte species displaying optimal growth at approximately 150 mM NaCl, to study mechanisms by which halophytes cope with the afore-mentioned components of salt stress. The relative contribution of organic and inorganic osmolytes in leaves of different physiological ages (e.g. positions on the stem) was quantified and linked with the osmoprotective function of organic osmolytes. We show that the extent of the oxidative stress (UV-B irradiation) damage to photosynthetic machinery in young leaves is much less when compared with old leaves, and attribute this difference to the difference in the size of the organic osmolyte pool (1.5-fold difference under control conditions; sixfold difference in plants grown at 400 mM NaCl). Consistent with this, salt-grown plants showed higher Fv/Fm values compared with control plants after UV-B exposure. Exogenous application of physiologically relevant concentrations of glycine betaine substantially mitigated oxidative stress damage to PSII, in a dose-dependent manner. We also show that salt-grown plants showed a significant (approximately 30%) reduction in stomatal density observed in all leaves. It is concluded that accumulation of organic osmolytes plays a dual role providing, in addition to osmotic adjustment, protection of photosynthetic machinery against oxidative stress in developing leaves. It is also suggested that salinity-induced reduction in stomatal density represents a fundamental mechanism by which plants optimize water use efficiency under saline conditions.     

NaCl胁迫对2种表型盐地碱蓬种子萌发的渗透效应和离子效应研究

采用高低2个浓度的NaCl、LiCl及等渗甘露醇溶液处理紫红色表型(紫色型)和绿色表型(绿色型)盐地碱蓬种子,通过测定它们的种子萌发率、吸胀速率和胚内离子含量,研究NaCl胁迫对2种表型种子萌发的离子效应和渗透效应.结果表明:(1)2种表型盐地碱蓬种子萌发率在高浓度(300 mmol/L)和低浓度(100 mmol/L)NaCl处理下均显著降低,紫色型种子萌发率在低浓度下显著低于绿色型,而在高浓度下却显著高于绿色型;绿色型种子萌发率在高浓度(30 mmol/L)和低浓度(10 mmol/L)LiCl处理下均未受到显著影响,但紫色型种子萌发率却均极显著降低;2种表型盐地碱蓬种子萌发率在低浓度等渗甘露醇处理下均极显著低于低浓度NaCl处理,而高浓度等渗甘露醇处理却均与高浓度NaCl处理无显著差异.(2)2表型种盐地碱蓬种子的吸胀速率在低浓度NaCl处理下没有受到显著影响,但高浓度NaCl处理及与之等渗的高浓度甘露醇处理下都显著降低,而且紫色型种子的吸胀速率在等渗甘露醇处理时显著高于绿色型.(3)2种表型盐地碱蓬种子胚中的Na 含量和Na /K 在对照和低浓度NaCl处理下无显著差异,但紫色型种子胚中的Na 、K 含量在高浓度NaCl处理时都显著高于对照,且K 含量增加的幅度远大于Na 含量,导致紫色型种子胚中的Na /K 显著低于绿色型.研究发现,盐地碱蓬种子萌发在低浓度NaCl胁迫下主要受离子效应抑制,而高浓度NaCl胁迫下则主要受渗透效应抑制,紫色型种子萌发率在高浓度NaCl胁迫下高于绿色型的原因之一是前者能维持更低的Na /K 比.     

Comparing carbohydrate status during norway spruce seed development and somatic embryo formation

Summary The carbohydrate status of developing seeds of Picea abies was examined in order to provide a frame of reference for the evaluation of changes in carbohydrate content in maturing somatic embryos of the same species. Samples were taken at weekly intervals from 12 May 1998 (estimated time of pollination) until 20 October 1998. The total non-structural carbohydrate content was high (≈150–180 μg mg⁻¹ dry weight) at the time of the first samples and the carbohydrate spectrum consisted of sucrose, glucose, fructose, and pinitol. A dramatic decrease in carbohydrate content took place from June 6 onwards, that was accompanied by changes in carbohydrate partitioning to favor sucrose over hexoses and the disappearance of pinitol. Raffinose and stachyose were first detected on July 28, and their content gradually increased thereafter. Isolated embryos and remaining megagametophytes were analyzed starting with September 1. Carbohydrate content was higher in isolated zygotic embryo than in the rest of the seed, with a slowly increasing fraction of raffinose and stachyose. Comparisons of presented data with the results of our previous study of somatic embryo carbohydrate status (Lipavská et al., 2000) revealed the following common features: (1) a decrease in total carbohydrate content and (2) an increase in sucrose:hexose ratios in developing seeds and embryonal suspensor mass. Marked differences were observed in carbohydrate spectra: (1) somatic embryo development was not accompanied by pinitol accumulation in any phase; (2) mature zygotic embryos, in contrast to mature somatic embryos, contained raffinose and stachyose. These observations will provide a solid basis for improvement of protocols for somatic embryogenesis in Picea.     

Nitrogen nutrition and metabolic interconversions of nitrogenous solutes in developing cowpea fruits

Budgets for import and utilization of ureide, amides, and a range of amino acids were constructed for the developing first-formed fruit of symbiotically dependent cowpea (Vigna unguiculata [L.] Walp. cv Vita 3). Data on fruit total N economy, and analyses of the xylem and phloem streams serving the fruit, were used to predict the input of various solutes while the compositions of the soluble and protein pools of pod, seed coat, and embryo were used to estimate the net consumption of compounds. Ureides and amides provided virtually all of the fruit's N requirements for net synthesis of amino compounds supplied inadequately from the parent plant. Xylem was the principal source of ureide to the pod, while phloem was the major source of amides to pod and seed. All fruit parts showed in vitro activity of urease (EC 3.5.1.5), allantoinase (EC 3.5.2.5), asparaginase (EC 3.5.11), ammonia-assimilating enzymes and aspartate and alanine aminotransferases (EC 2.61.1 and EC 2.6.1.1.2). Asparagine:pyruvate aminotransferase (EC 2.6.1.14) was recovered only from the pod. The pod was initially the major site for processing and incorporating N; later seed coats and finally embryos became predominant. Ureides were broken down mainly in the pod and seed coat. Amide metabolism occurred in all fruit organs, but principally in the embryo during much of seed growth. Seed coats released N to embryos mainly as histidine, arginine, glutamine, and asparagine, hardly at all as ureide. Amino compounds delivered in noticeably deficient amounts to the fruit were arginine, histidine, glycine, glutamate, and aspartate, while seeds received insufficient arginine, histidine, serine, glycine, and alanine. Quantitatively based schemes are proposed depicting the principal metabolic transformation accompanying N-flow between seed compartments during development.     

Development of leaf photosynthetic parameters in Betula pendula Roth leaves: correlations with photosystem I density

The global modelling of photosynthesis is based on exact knowledge of the leaf photosynthetic machinery. The capacities of partial reactions of leaf photosynthesis develop at different rates, but it is not clear how the development of photoreactions and the Calvin cycle are co-ordinated. We investigated the development of foliar photosynthesis in the temperate deciduous tree Betula pendula Roth. using a unique integrated optical/gas exchange methodology that allows simultaneous estimation of photosystem I and II (PS I and PS II) densities per leaf area, interphotosystem electron transport activities, and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) kinetic properties. We combined these measurements with in vitro determinations of Rubisco, soluble protein and chlorophyll contents. We observed a strong increase in leaf photosynthetic capacity in developing leaves per leaf area, as well as per dry mass, that was paralleled by accumulation of leaf Rubisco. Enhanced mesophyll conductance was the outcome of increased carboxylation capacity and increased CO(2) diffusion conductance. However, Rubisco was only partly activated in the leaves, according to in vivo measurements of Rubisco kinetics. The amount of active Rubisco increased in proportion with development of PS I, probably through a direct link between Rubisco activase and PS I electron transport. Since the kinetics for post-illumination P700 re-reduction did not change, the synthesis of cytochrome b(6)f complex was also proportional to PS I. The synthesis of PS II began later and continued for several days after reaching the full PS I activity, but leaf chlorophyll was shared equally between the photosystems. Due to this, the antenna of PS II was very large and not optimally organized, leading to greater losses of excitation and lower quantum yields in young leaves. We conclude that co-ordinated development of leaf photosynthesis is regulated at the level of PS I with subordinated changes in PS II content and Rubisco activation.