The mechanism of boron tolerance for maintenance of root growth in barley (Hordeum vulgare L.)

Cultivar differences in root elongation under B toxic conditions were observed in barley (Hordeum vulgare L.). A significant increase in the length and width of the root meristematic zone (RMZ) was observed in Sahara 3771 (B tolerant) when it was grown under excessive B concentration, compared to when grown at adequate B supply. This coincided with an increase in cell width and cell numbers in the meristematic zone (MZ), whereas a significant decrease in the length and no significant effect on the width of the MZ was observed in Clipper (B intolerant) when it was grown under excessive B supply. This was accompanied by a decrease in cell numbers, but an increase in the length and width of individual cells present along the MZ. Excessive B concentrations led to a significantly lower osmotic potential within the cell sap of the root tip in SloopVic (B tolerant) and Sahara 3771, while the opposite was observed in Clipper. Enhanced sugar levels in the root tips of SloopVic were observed between 48 and 96 h after excess B was applied. This coincided with an increase in the root elongation rate and with a 2.7-fold increase in sucrose level within mature leaf tissue. A significant decrease in reducing sugar levels was observed in the root tips of Clipper under excessive B concentrations. This coincided with significantly lower root elongation rates and lower sucrose levels in leaf tissues. Results indicate a B tolerance mechanism associated with a complex control of sucrose levels between leaf and root tip that assist in maintaining root growth under B toxicity.     

The base of the leaf acts as a localized sink for photosynthate in mature barley leaves

The gradients in photosynthetic and carbohydrate metabolism which persist within the fully expanded second leaf of barley ( Hordeum vulgare ) were examined. Although all regions of the leaf blade were green and photosynthetically active, the basal 5 cm, representing approximately 20% of the leaf area, retained some characteristics of sink tissue. The leaf blade distal from the leaf sheath exhibited characteristics typical of source tissue; the activities of sucrolytic enzymes (invertase and sucrose synthase) were relatively low, whilst that of sucrose phosphate synthase was high. These regions of the leaf accumulated sucrose throughout the photoperiod and starch only in the second half of the photoperiod whilst hexose sugars remained low. By contrast the leaf blade proximal to the leaf sheath retained relatively high activities of sucrolytic enzymes (especially soluble, acid invertase) whilst sucrose phosphate synthase activity was low. Glucose, as well as sucrose, accumulated throughout the photoperiod. Although starch accumulated in the second half of the photoperiod, a basal level of starch was present throughout the photoperiod, by contrast with the rest of the leaf. The ¹⁴CO₂ feeding experiments indicated that a constant amount of photosynthate was partitioned towards starch in this region of the leaf irrespective of irradiance. These findings are interpreted as the base of the leaf blade acting as a localized sink for carbohydrate as a result of sucrose hydrolysis by acid invertase.     

Companion-Cell Specific Localization of Sucrose Synthase in Zones of Phloem Loading and Unloading

An immunohistochemical approach was used in maize (Zea mays) and citrus (Citrus paradisi) to address the previously noted association between sucrose synthase and vascular bundles and to determine the localization of the low but detectable levels of sucrose synthase that remain in leaves after the import-export transition. Sucrose synthase protein was immunolocalized at the light microscope level using paraffin sections reacted with rabbit sucrose synthase polyclonal antisera and gold-conjugated goat anti-rabbit immunoglobulin G. Immunolabel was specifically observed in phloem companion cells of minor and intermediate veins in mature leaves of both species. Similar localization was apparent in the midrib of mature citrus leaves, with additional labeling in selected files of phloem parenchyma cells. A clear companion-cell specificity was evident in the phloem unloading zone of citrus fruit, where high activity of sucrose synthase has been demonstrated in vascular bundles during periods of rapid import. Sucrose synthase protein was not associated with adjacent cells surrounding the vascular strands in this tissue. The companion-cell specificity of sucrose synthase in phloem of both importing and exporting structures of these diverse species implies that this may be a widespread association and underscores its potential importance to the physiology of vascular bundles.     

Growth rate,photosynthetic activity,and leaf development of Brazil pine seedlings (Araucaria angustifolia[Bert.] O. Ktze.)

Seedlings of Brazil pine, a large-seeded South American conifer, were grown in a climate chamber to investigate vertical growth pattern and the time course of leaf development. We examined shoot growth, photosynthetic performance and markers of leaf maturation such as contents of soluble sugars and activities of sucrose-phosphate synthase (SPS), neutral invertase (nI) and sucrose synthase (Susy). The daily increment of shoot length showed an optimum curve during the first 70 days after germination. The low growth rate during the first 20 days of development correlated with net CO² emission of the seedling. Analyses of leaf maturation markers in older seedlings revealed low sucrose/hexose ratios and high activities of nI and Susy in the uppermost leaves. Although the SPS/Susy ratio was low in these leaves the extractable SPS activity did not change significantly among leaves of different age. The photosynthetic light compensation points of young leaves were about 2-fold higher than those of mature leaves and their photosynthetic capacity was only 50% as high. Our results indicate that a rapid maturation of leaves of Brazil pine seedlings may reduce the respiratory loss of carbohydrates and that the mobilisation of seed storage compounds supports initial shoot growth under light-limiting conditions which may occur in the forest-grassland succession zone.     

Biochemical Basis for Partitioning of Photosynthetically Fixed Carbon between Starch and Sucrose in Soybean (Glycine max Merr.) Leaves

The control of photosynthetic starch/sucrose formation in leaves of soybean (Glycine max L. Merr.) cultivars was studied in relation to stage of plant development, photosynthetic photoperiod, and nitrogen source. At each sampling, leaf tissue was analyzed for starch content, activities of sucrose-metabolizing enzymes, and labeling of starch and sucrose (by ¹⁴CO₂ assimilation) in isolated cells. In three of the four varieties tested, nodulated plants had lower leaf starch levels and higher activities of sucrose phosphate synthetase (SPS), and isolated mesophyll cells incorporated more carbon (percentage of total ¹⁴CO₂ fixed) into sucrose and less into starch as compared to nonnodulated (nitrate-dependent) plants. The variation among cultivars and nitrogen treatments observed in the activity of SPS in leaf extracts was positively correlated with labeling of sucrose in isolated cells (r = 0.81) and negatively correlated with whole leaf starch content (r = −0.66). The results suggested that increased demand for assimilates by nodulated roots may be accommodated by greater partitioning of carbon into sucrose in the mesophyll cells. We have also confirmed the earlier report (Chatterton, Silvius 1979 Plant Physiol 64: 749-753) that photoperiod affects partitioning of fixed carbon into starch. Within two days of transfer of nodulated soybean Ransom plants from a 14-hour to a 7-hour photoperiod, leaf starch accumulation rates doubled, and this effect was associated with increased labeling of starch and decreased labeling of sucrose in isolated cells. Concurrently, activities of SPS, sucrose synthase, and uridine diphosphatase in leaves were decreased.     

Sucrose Synthase in Developing Maize Leaves: Regulation of Activity by Protein Level during the Import to Export Transition

The maize (Zea mays) leaf is a valuable system to study the sucrose import to sucrose export transition at the cellular level. Rapidly growing and fully heterotrophic cells in the basal part of the young leaf showed a high sucrose synthase (SS) activity. Leaf SS has been purified to homogeneity. By comparison with purified kernel SS isozymes, the leaf SS has been identified as SS₂. SS₁ protein and SS₂ protein were clearly separated by electrophoresis and the two monomers differed in size by 6 kilodaltons. Nevertheless, kinetic parameters of both enzymes were very similar. Immunodetection of SS protein showed that in young heterotrophic tissues SS₂ was a major protein accounting for 3% of the total protein. Concurrent with greening, SS activity decreased and the change of activity was explained by regulation of the protein level. In mature green tissues, which are synthetizing sucrose as evidenced by the presence of sucrose phosphate synthase activity, SS activity was almost completely absent. Results suggested that down regulation of SS₂ enzyme protein level was an early event in the transition from import to export status of the leaf.     

Subdomains of the octopine synthase upstream activating element direct cell-specific expression in transgenic tobacco plants.

Previous work has shown that the octopine synthase (ocs) gene encoded by the Agrobacterium tumefaciens Ti-plasmid contains an upstream activating sequence necessary for its expression in plant cells. This sequence is composed of an essential 16-bp palindrome and flanking sequences that modulate the level of expression of the ocs promoter in transgenic tobacco calli. In this study, we have used RNA gel blot analysis of RNA extracted from transgenic tobacco plants to show that the octopine synthase gene is not constitutively expressed in all plant tissues and organs. This tissue-specific pattern of expression is determined, to a large extent, by the 16-bp palindrome. Histochemical analysis, using an ocs-lacZ fusion gene, has indicated that the 16-bp palindrome directs the expression of the ocs promoter in specific cell types in the leaves, stems, and roots of transgenic tobacco plants. This expression is especially strong in the vascular tissue of the leaves, leaf mesophyll cells, leaf and stem guard cells, and the meristematic regions of the shoots and roots. Sequences surrounding the palindrome in the upstream activating sequence restrict the expression of the ocs promoter to fewer cell types, resulting in a reduced level of expression of beta-galactosidase activity in the central vascular tissue of leaves, certain types of leaf trichomes, and the leaf primordia.     

Diurnal Changes in Maize Leaf Photosynthesis : III. Leaf Elongation Rate in Relation to Carbohydrates and Activities of Sucrose Metabolizing Enzymes in Elongating Leaf Tissue

Maize (Zea mays L. cv. Pioneer 3184) leaf elongation rate was measured diurnally and was related to diurnal changes in the activities of sucrose metabolizing enzymes and carbohydrate content in the elongating portion of the leaf. The rate of leaf elongation was greatest at midday (1300 hours) and was coincident with the maximum assimilate export rate from the distal portion of the leaf. Leaf elongation during the light period accounted for 70% of the total observed increase in leaf length per 24 hour period. Pronounced diurnal fluctuations were observed in the activities of acid and neutral invertase and sucrose phosphate synthase. Maximum activities of sucrose phosphate synthase and acid invertase were observed at 0900 hours, after which activity declined rapidly. The activity of sucrose phosphate synthase was substantially lower than that observed in maize leaf source tissue. Neutral invertase activity was greatest at midday (1200 hours) and was correlated positively with diurnal changes in leaf elongation rate. There was no significant change in the activity of sucrose synthase over the light/dark cycle. Sucrose accumulation rate increased during a period when leaf elongation rate was maximal and beginning to decline. Maximum sucrose concentration was observed at 1500 hours, when the activities of sucrose metabolizing enzymes were low. At no time was there a significant accumulation of hexose sugars. The rate of starch accumulation increased after the maximum sucrose concentration was observed, continuing until the end of the light period. There was no delay in the onset of starch mobilization at the beginning of the dark period, and essentially all of the starch was depleted by the end of the night. Mobilization of starch in the elongating tissue at night could account for a significant proportion of the calculated increase in the tissue dry weight due to growth. Collectively, the results suggested that leaf growth may be controlled by the activities of certain sucrose metabolizing enzymes and may be coordinated with assimilate export from the distal, source portion of the leaf. Results are discussed with reference to diurnal photoassimilation and export in the distal, source portion of the leaf.     

Carbohydrate,metabolic, and osmotic changes in scaled-up liquid cultures of <Emphasis Type="Italic">Narcissus</Emphasis> leaves

Summary The use of scaled-up liquid cultures could be an efficient system for mass propagation of Narcissus, as it can greatly reduce the costs involved with manual handling. Induction of hyperhydric meristematic leaf section clusters and proliferation were carried out in an ancymidol (ANC)-containing liquid medium in flasks and disposable presterilized plastic bioreactors. Non-hyperhydric bulblets started to develop from hyperhydric meristematic leaf section clusters after subculture on a 0.8% agar strength medium, and young bulbs formed after 10 mo. in vivo acclimatization with a 98% survival rate. The present study reveals that in Narcissus leaf sections cultured in liquid medium, morphogenetic changes in leaf sections were associated with metabolic changes. The changes in carbohydrate, protein, and water potential of the liquid media and leaf sections were found to be closely related to meristematic center initiation on Narcissus hyperhydric leaf sections. Starch, sucrose, and glucose were significantly higher in the hyperhydric leaf sections cultured in ANC medium. The water potential was signifieantly higher in ANC-treated leaf sections and significantly lower in the medium containing ANC, at the stage shortly before or after hyperhydricity and meristematic centers hegan forming on the leaf sections. A 30kDa protein was found to be present in the hyperhydric leaf sections. Based on the present study, a largescale micropropagation protocol of Narcissus in agar and liquid cultures is proposed.     

Stachyose Synthesis in Source Leaf Tissues of the CAM Plant Xerosicyos danguyi H. Humb

Leaf tissues from Xerosicyos danguyi H. Humb., a succulent member of the Cucurbitaceae, were found to possess both galactinol synthase activity and the capacity for photosynthetic production of stachyose, the phloem transport oligosaccharide common to other nonsucculent cucurbits. The amounts of stachyose isolated from leaf tissues, and the extractable activity of galactinol synthase, were somewhat higher in leaf tissues obtained from plants operating in the Crassulacean acid metabolism (CAM) mode (well watered plants) compared to leaf tissues from plants operating in the CAM-idling mode (water-stressed plants). In contrast, in leaf discs, the photosynthetic incorporation of label into stachyose following pulse labeling with ¹⁴CO₂ was similar for stressed and for nonstressed tissues. Stachyose could be extracted from, and was synthesized photosynthetically by, leaf discs which contained no vascular tissues, indicating that synthesis of stachyose can occur in photosynthetic mesophyll cells of Xerosicyos.     

Photosynthetic Determinants of Growth in Maize Plants: Effects of Nitrogen Nutrition on Growth, Carbon Fixation and Photochemical Features

Maize(Zea mays L.) plants were grown in a greenhouse with differentlevels of nitrate-N (2 to 20 millimolar). Nitrogen nutritionhad dramatic effects on plant growth and photosynthetic characteristicsof mature leaves. Increasing nitrogen resulted in greater biomassproduction, shoot/root ratios, and rates of leaf expansion duringthe day. The elongating zone of high-N plants had higher activities(per gram fresh weight) of sucrose synthase and neutral invertasethan low-N plants, suggesting that increased leaf growth wasrelated to a greater biochemical capacity for sucrose metabolism. Mature leaves of high-N plants had higher rates of photosynthesisand assimilate export (sucrose formation), and partitioned morecarbon into sucrose relative to starch. Increased photosyntheticrates (leaf area basis) were associated with higher levels ofribulose-l,5-bisphosphate carboxylase, phosphoenolpyruvate carboxylaseand pyruvate, phosphate dikinase (determined immunochemically).In addition, N-nutrition affected the functional organizationof chlorophyll in the leaves. Large increases in the numberof PS I reaction centers were observed which fully accountedfor increases in leaf chlorophyll content with increasing nitratesupply. Collectively, the results suggest that increased growth of maizeplants at high light and optimal nitrogen nutrition is relatedto greater capacity for photosynthesis and translocation inmature leaves, and possibly increased capacity for sucrose metabolismin expanding leaves. (Received May 22, 1989; Accepted August 28, 1989)     

Leaf Growth in Cotton (Gossypium hirsutum L.) 1. Growth in Area of Main-stem and Sympodial Leaves

A model is presented for growth of individual and successivemain-stem leaves of cotton, based on a series of indoor experimentsand data sets from the literature. Three variable parametersare used to describe individual leaf growth: relative growthrate of meristematic tissue (R¹), relative rate of approachof final area (R₂) and a ‘position parameter’ (t_0.5)which governs the transition from meristematic to extensiongrowth. Final area of a leaf does not occur in the model asa deterministic quantity but it is a result of the processesduring growth. The model generates successive mainstem leavesand sympodial leaves as an integrated system. Assimilate shortagesoccurring in the plant operate on R₁ leading to the characteristicchange of final leaf area along the mainstem. Gossypium hirsutumL., cotton, leaf growth, relative growth rate, meristematic tissue, extension growth, mathematical model     

Growth Rates and Carbohydrate Fluxes within the Elongation Zone of Tall Fescue Leaf Blades

Investigations were performed to better understand the carbon economy in the elongation zone of tall fescue leaf blades. Plants were grown at constant 21°C and continuous 300 micromoles per square meter per second photosynthetic photon flux density where leaf elongation was steady for several days. Elongation occurred in the basal 20 mm of the blade (0-20 millimeters above the ligule) and was maximum at 9 to 12 millimeters. Eight 3-millimeter long segments were sampled along the length of the elongation zone and analyzed for water-soluble carbohydrates. Sucrose concentration was high in the zone of cell division (0-6 millimeters) whereas monosaccharide concentration was high at and distal to the location where cell elongation terminated (20 millimeters). Fructan concentration increased in the basal part, then remained constant at about 85% of the total mass of water-soluble carbohydrates through the remainder of the elongation zone. Data on spatial distribution of growth velocities and substance contents (e.g. microgram fructan per millimeter leaf length) were used to calculate local net rates of substance deposition (i.e. excess rates of substance synthesis and/or import over substance degradation and/or export) and local rates of sucrose import. Rates of sucrose import and net deposition of fructan were positively associated with local elongation rate, whereas net rates of sucrose deposition were high in the zone of cell division and those of monosaccharide were high near the termination of elongation. At the location of most active elongation imported sucrose (29.5 milligrams per square decimeter per hour) was used largely for synthesis of structural components (52%) and fructan (41%).     

小麦开花后旗叶中蔗糖合成与籽粒中蔗糖降解

在小麦开花后,旗叶中蔗糖磷酸合成酶（SPS）活性在开花后14d内一直维持较高水平,蔗糖合成酶（SS）的活性在开花后14-28d较高,蔗糖的含量与SPS活性呈显著正相关,籽粒中蔗糖合成酶（SS）在开花后28d内一直维持较高的活性;与此相对应,籽粒蔗糖的含量在开花后28d内呈明显的下降趋势。而旗叶和籽粒中SS活性均与籽粒淀粉的积累速率呈极显著正相关。     

Regulation by sucrose of storage compounds breakdown in germinating seeds of yellow lupine (Lupinus luteus L.), white lupine (Lupinus albus L.) and Andean lupine (Lupinus mutabilis Sweet): I. Mobilization of storage protein

Research of the regulatory function of sucrose in storage protein breakdown was conducted on isolated embryo axes, excised cotyledons and whole seedlings of three lupine species grown in vitro on medium with 60 mM sucrose or without the sugar. Sucrose stimulated growth of yellow, white and Andean lupine isolated embryo axes and cotyledons but growth of seedlings was inhibited. Dry matter content was higher in sucrose-fed isolated organs and in seedling organs. Ultrastructure research revealed that lack of sucrose in the medium caused enhancement in storage protein breakdown. Protein deposits in cotyledons were smaller as well as soluble portion content in all studied organs was lower when there was no sucrose in the medium. In the same conditions, the activity of glutamate dehydrogenase was significantly higher. Increase in vacuolization of cells of white lupine root meristematic zone cells was observed and induction of autophagy in young carbohydrate-starved embryo axes is discussed.     

Responses of Ribulose-1,5-Bisphosphate Carboxylase,Cytochrome f,and Sucrose Synthesis Enzymes in Rice Leaves to Leaf Nitrogen and Their Relationships to Photosynthesis

The photosynthetic gas-exchange rates and various biochemical components of photosynthesis, including ribulose-1,5-bisphosphate carboxylase (Rubisco) content, cytochrome (Cyt) f content, and the activities of two sucrose synthesis enzymes, were examined in young, fully expanded leaves of rice (Oryza sativa L.) grown hydroponically in different nitrogen concentrations. The light-saturated rate of photosynthesis at an intercellular CO2 pressure of 20 Pa (CO2-limited photosynthesis) was linearly dependent on leaf nitrogen content, but curvilinearly correlated with Rubisco content. This difference was due to a greater than proportional increase in Rubisco content relative to leaf nitrogen content and the presence of a CO2 transfer resistance between the intercellular air spaces and the carboxylation sites. CO2-limited photosynthesis was proportional to Cyt f content, one of the key components of electron transport, but was not proportional to the activities of cytosolic fructose-1,6-bisphosphatase and sucrose phosphate synthase, the two regulatory enzymes of sucrose synthesis. Light-saturated photosynthesis above an intercellular CO2 pressure of 60 Pa (CO2-saturated photosynthesis) was curvilinearly dependent on leaf nitrogen content. This CO2-saturated photosynthesis was proportional to Cyt f content in the low- and normal-nitrogen leaves, and correlated better with the activities of cytosolic fructose-1,6-bisphosphatase and sucrose phosphate synthase in the high-nitrogen leaves. The increase in the activities of these two enzymes with increasing leaf nitrogen was not as great as the increase in Cyt f content. Thus, as leaf nitrogen increased, the limitation caused by the activities of sucrose synthesis enzymes came into play, which resulted in the curvilinear relationship. However, this limitation by sucrose synthesis enzymes did not affect photosynthesis under normal ambient air.