Chemical and rheological properties of polysaccharides from litchi pulp.

Litchi polysaccharide (LP) was extracted from litchi pulp. Its chemical composition, microstructure, zeta potential, flow and viscoelastic behavior were investigated. LP contained uronic acid (41.18%), neutral sugar (42.23%), and protein (2.72%). The monosaccharide composition was mostly arabinose, galactose, and a small amount of mannose, rhamnose and glucose. Scanning electron microscopy (SEM) showed LP was porous network structure. LP concentration had no effect on its zeta potential value while salts reduced them. LP showed shear-thinning behavior during the tested shear rate range. The power-law model was used to evaluate the flow behavior of LP; both its flow behavior index and consistency index changed with different concentrations. The viscosity of LP increased under acidic conditions (pH2-4), but was stable with heat treatment. The LP dispersion displayed as a liquid viscoelastic behavior in 1% and 2% concentrations and behaved as an elastic gel at 3% concentration as well as the addition of NaCl and CaCl2.

Sugar uptake in the Aril of litchi fruit depends on the apoplasmic post-phloem transport and the activity of proton pumps and the putative transporter LcSUT4.

The post-phloem unloading pathway and the mechanism of sugar accumulation remain unclear in litchi fruit. A combination of electron microscopy, transport of phloem-mobile symplasmic tracer (carboxyfluorescein, CF) and biochemical and molecular assays was used to explore the post-phloem transport pathway and the mechanism of aril sugar accumulation in litchi. In the funicle, where the aril originates, abundant plasmodesmata were observed, and CF introduced from the peduncle diffused to the parenchyma cells. In addition, abundant starch and pentasaccharide were detected and the sugar concentration was positively correlated with activities of sucrose hydrolysis enzymes. These results clearly showed that the phloem unloading and post-phloem transport in the funicle were symplastic. On the other hand, imaging of CF showed that it remained confined to the parenchyma cells in funicle tissues connecting the aril. Infiltration of both an ATPase inhibitor [eosin B (EB)] and a sucrose transporter inhibitor [p-chloromercuribenzene sulfonate (PCMBS)] inhibited sugar accumulation in the aril. These results indicated an apoplasmic post-phloem sugar transport from the funicle to the aril. Although facilitated diffusion might help sucrose uptake from the cytosol to the vacuole in cultivars with high soluble invertase, membrane ATPases in the aril, especially tonoplast ATPase, are crucial for aril sugar accumulation. The expression of a putative aril vacuolar membrane sucrose transporter gene (LcSUT4) was highly correlated with the sugar accumulation in the aril of litchi. These data suggest that apoplasmic transport is critical for sugar accumulation in litchi aril and that LcSUT4 is involved in this step.

Distribution changes of calcium and programmed cell death in the pistil of litchi (Litchi chinensis Sonn.) flower during its development.

Potassium pyroantimonate precipitation method was used for investigating calcium distribution and cell ultrastructure change during development of pistils of litchi male and female flower. The results showed that at the megasporocyte stage of female flowers, calcium precipitates was located mainly at cell wall and intercellular space of inner integument near the micropyle and style cells, and to a lesser extent in vacuoles. Vascular tissues also contained much calcium precipitates. In inner integument cells near the micropyle of male flowers, the vacuole contained most of the calcium precipitates. Calcium precipitates in style cell and vascular tissues of male flowers was sparse and seldom seen. After meiosis of megasporocyte, pistils of female flowers continued to grow and those of male flowers aborted. In female flowers, calcium precipitates concentration became lower and calcium precipitates was probably transported to the places for future pollen bourgeoning and fertilization. Cell wall calcium precipitates concentration increased in the inner integument cells near the micropyle. Calcium precipitates concentration increased from topper style cells to lower ones. In male flowers, inner integument cells near the micropyle underwent the programmed cell death (PCD): flow of calcium from vacuoles into nucleus might had triggered the PCD process. A continuous channel was formed between perinuclear space and cytoplasm membrane lumen, and calcium flowed freely between nuclear membrane and plasma membrane. At certain time and locations, calcium precipitates was newly appeared at some organelles like endoplasimic reticulum, mitochondria and peroxisomes. This calcium redistribution in cells might trigger and regulate the process of PCD. In male flowers, style cells containing no calcium precipitation soon began to degenerate.

[Changes in cell ultrastructure during sexual determination of litchi staminate flower].

The ultrastructural changes of meristematic cell during the degeneration of gynoecium primordium leading to the formation of staminate flower of litchi were followed. Degradation of the cells and transport of the dissolved cytoplasmic components were well ordered. Configurations of rough endoplasmic reticulum (RER) changed significantly. ER played an important role in degenerative processes of gynoecium primordiuml cells. The degenerative processes started with the appearance of long RER cisternae throughout the cytoplasm. Some long RER cut or enclosed the cytoplasm. Some RER connected nucleus and mitochondria of adjacent cells, formed a ridge-like connection. Later the RER formed concentric patterns and then became irregular stacks. RER and golgiosome produced many vesicles, which were importance to protoplasmic degradation and intercellular transport of the cellular debris. The number of mitochondria increased up to the time when they began to degrade in batches. Peroxisomes appeared temporarily at the middle stage near the nucleus. The nucleolus disintegrated at the beginning of degeneration of nucleus. Then fragments of chromatin aggregated at the periphery of nuclear membrane and diffused outward. In some nuclei the perinuclear membrane became dilated and puffs were formed. As cell degeneration progressed, the protoplasm disintegrated and dissipated in an orderly fashion, i.e. ribosomes became disorganized first, followed by peroxisomes, ER, golgiosoms, mitochondria and nucleus. Eventually, gynoecium primordium cells digested all of the cytoplasm, leaving only cell wall with high electron density. Most of the products of degeneration of gynoecium primordium cells were removed through either symplastic or apoplastic pathways. Programmed cell death (PCD) may be involved in the degeneration of meristematic cells at the gynoecium primodium.