Assimilate Unloading from Maize ( Zea mays L.) Pedicel Tissues

The osmotic pressure of the cell sap of stalk storage parenchyma of sugarcane (Saccharum spp. hybrids) increases by an order of magnitude during ontogeny to reach molar concentrations of sucrose at maturity. Stalk parenchyma cells must either experience very high turgor at maturation or have an ability to regulate turgor. We tested this hypothesis by using pressure probe techniques to quantify parameters of cell and tissue water relations of sugarcane storage parenchyma during ontogeny. The largest developmental change was in the volumetric elastic modulus, which increased from 6 bars in immature tissue to 43 bars in mature tissue. Turgor was maintained relatively low during sucrose accumulation by the partitioning of solutes between the cell and wall compartments. Membrane hydraulic conductivity decreased from about 12 x 10-7 centimeters per second per bar down to 4.4 x 10-7 centimeters per second per bar. The 2.7-fold decrease in membrane hydraulic conductivity during tissue maturation was accompanied by a 7.8-fold increase in wall elasticity. Integration of the cell wall and membrane properties appears to be by the opposing effects of turgor on hydraulic conductivity and elastic modulus. The changes in these properties during development of sugarcane stalk tissue may be a way for parenchyma cells to develop a capacity for expansive growth and still serve as a strong sink for storing high concentrations of sucrose.Storage parenchyma of sugarcane, beet, and certain fruits, such as grape, accumulate molar concentrations of sugars during maturation (10,17,25). This corresponds to osmotic pressures in excess of 20 bars, yet the few measurements that have been made in such tissues (14,25) indicate that turgor pressures are in the same range as nonstorage parenchyma with much lower osmotic pressures, i.e. in the range of 2 to 8 bars (7,11,16,17,23,24 12 months of age were excised, cut into 1 -m segments, enclosed in a plastic bag, and taken to the field laboratory where they were recut to obtain two experimental segments. The younger segment contained partially to fully expanded internodes numbered 3 to 8 counting down from the node subtending the youngest fully expanded leaf blade. The older segment contained internodes exhibiting near maximum sucrose content (internodes No. 13-18). The ends of each segment were dipped in molten paraffin to reduce moisture loss. The segments were then wrapped in a moist paper towel, sealed in a plastic bag, and shipped by air to Pennsylvania State University where experiments commenced within 60 h of harvest. Turgor and osmotic pressures of the 60-h material were similar to those of fresh tissue.Storage parenchyma tissue was excised from the central portion of internodes held within a humidified chamber or from internodes held on damp paper towels in the open laboratory. Typically, the central 3-to 4-cm length of a given internode was excised, then a sharp razor blade was used to remove the sclerified rind. The remaining core ofparenchyma was split into six or eight pie-sha...

Section: Discussionmentioning

confidence: 99%

“…The older segment contained internodes exhibiting near maximum sucrose content (internodes No. [13][14][15][16][17][18]. The ends of each segment were dipped in molten paraffin to reduce moisture loss.…”

Section: Plant Materialsmentioning

confidence: 99%

Developmental Changes in Cell and Tissue Water Relations Parameters in Storage Parenchyma of Sugarcane

Moore¹,

Cosgrove²

1991

“…Com, however, contrasts with other species in both respects. Solute release from the com pedicel is not affected by metabolic inhibitors or PCMBS (Porter et al, 1985) and is decreased by osmotic treatments (Porter et al, 1987).…”

mentioning

confidence: 99%

Monitoring Phloem Unloading and Post-Phloem Transport by Microperfusion of Attached Wheat Grains

Wang

Fisher

1994

Phloem unloading and post-phloem transport in developing wheat (Trificum aestivum 1.) grains were investigated by perfusing the endosperm cavities of attached grains. Relative unloading ratio (RUR) and the rate of sucrose release into the endosperm cavity (SRR) were calculated, respectively, from 14C import and from sucrose washout from the cavity. RUR and SRR continued at or near in vivo rates over a wide range of cavity sap osmolality (90 to approximately 500 milliosmolal) and sucrose concentration (14-430 mM) and for long times (29 h). These are much greater ranges than have been observed for the endosperm cavity in vivo (230-300 milliosmolal, and 40-120 mM, respectively), indicating that neither the cavity sap osmolality nor sucrose concentration are controlling factors for the rate of assimilate import into the cavity. The maintenance of in vivo transport rates over a wide range of conditions strongly implicates the role of transport processes within the maternal tissues of the wheat grain, rather than activities of the embryo or endosperm, in determining the rate of assimilate import into the grain. RUR was decreased by high concentrations of sucrose and sorbitol, but not of mannitol. By plasmolyzing some chalazal cells, sorbitol appeared to block symplastic transport across the crease tissues, but neither sucrose nor mannitol caused plasmolysis in maternal tissues of attached grains. The inhibition of RUR by KCN and carbonyl cyanide m-chlorophenyl (CCCP) and the continued import of sucrose into grains against its concentration gradient suggest that solute movement into the endosperm cavity might occur by active membrane transport. However, the evidence is weak, since KCN and CCCP appeared to act primarily on some aspect of symplastic (i.e. nonmembrane) transport. Also, sucrose could move from the endosperm cavity into the maternal tissues (i.e. opposite to the normal direction of sucrose movement), suggesting that transmembrane movement in the nucellus may be a reversible process. Pressure-driven flow into the grain could account for movement against a concentration gradient.

“…Since no vascular or plasmodesmatal connections exist between the maternal tissues and the developing endosperm cells in maize, a similar assimilate pathway to that of legumes must be followed in maize kernels (3,20). We conclude from available evidence (3,12,15,16,20) that sugars move passively from the pedicel symplast of maize as sucrose and are then hydrolyzed to monosaccharides within the apoplast. Wolswinkel and Ammerlaan (24) noted that sink tissues may utilize energy during unloading from the sieve elements, uptake into storage cells, or for biosynthetic and respiratory transformations.…”

mentioning

confidence: 99%

“…Since no vascular or plasmodesmatal connections exist between the maternal tissues and the developing endosperm cells in maize, a similar assimilate pathway to that of legumes must be followed in maize kernels (3,20). We conclude from available evidence (3,12, 15,16,20) PA. Plant culture and experimental conditions for these greenhouse studies have been described (15). Plants which were 21 or 22 d postpollination were utilized for all experiments.…”

mentioning

confidence: 99%

Assimilate Unloading from Maize (Zea mays L.) Pedicel Tissues

Porter¹,

Knievel²,

Shannon³

1987

Self Cite

ABSTRACrSugar, amino acid, and "C-assimilate release from attached maize (Zea mays L.) pedicels was studied following treatment with several chemical inhibitors. In the absence of these agents, sugar release was nearly linear over a 7-hour period. At least 13 amino acids were released with glutamine comprising over 30% of the total. Release was not affected by potassium concentration, 10-minute pretreatments with p-chloromercuribenzene sulfonic acid (PCMBS) or dithiothreitol, and low concentrations of CaC12. Three hours or more exposure to PCMBS, dinitrophenol, N-ethylmaleimide, or 2,4,6-trinitrobenzene sulfonic acid strongly inhibited "C-assimilate, sugar, and amino acid release from the pedicel. These treatments also reduced 'C-assimilate movement into the kernel bases.It is, therefore, likely that reduced unloadings caused by these relatively long-term exposures to chemical inhibitors, was related to reduced translocation of assimilates into treated kernels. Whether this effect is due to disruption of kernel metabolism and sieve element function or reduced assimilate unloading and subsequent accumulation of unlabeled assimilates within the pedicel tissues cannot be determined at this time.Assimilate unloading processes are difficult to understand because unloading mechanisms apparently differ among crop species and among the various sink tissues within a plant (21). In growth sinks, such as root tips and expanding leaves, there is evidence that unloading is via the symplast (5, 19). Within stem tissues, unloading occurs into the apoplast from which assimilates may be taken up for storage or reloaded into the phloem (7,11). In reproductive sinks, such as legume seeds, unloading occurs from the maternal seed coat tissue into the apoplast which surrounds the developing embryo (21). Assimilates must then be taken up into the embryo. Since no vascular or plasmodesmatal connections exist between the maternal tissues and the developing endosperm cells in maize, a similar assimilate pathway to that of legumes must be followed in maize kernels (3,20). We conclude from available evidence (3,12, 15,16,20) PA. Plant culture and experimental conditions for these greenhouse studies have been described (15). Plants which were 21 or 22 d postpollination were utilized for all experiments. Kernels from these plants were in the linear phase of dry matter accumulation and had attained less than 50% oftheir final dry weight. Kernel water content ranged from 55 to 65% by weight. The experiments reported herein were initiated between 9:00 and