Hui-Bai Huang

The growth potential generated in citrus fruit under water stress and its relevant mechanisms

A mild water stress was imposed upon potted tangerine trees (Citrus sinensis Blanco. cv. Zhuju) by water withholding during early juice sac expansion stage. While fruit growth was inhibited by water stress a growth potential was built up inside the fruit, which was not expressed until rewatering. The more powerful water uptake force of the stressed fruit was caused by its more negative fruit water potential. The mechanisms involved were both passive and active in nature: more water loss from fruit to transpiring leaves during water stress and some active adaptive physiological responses of fruit to water stress. The physiological responses involved both osmotic adjustment and cell wall adjustment. The former was reflected in higher soluble solute contents in both fruit juice and fruit skin (on a dry weight basis) resulting in the drop of osmotic potential (cs). The latter was reflected in the cell wall loosening of fruit skin in response to water stress causing a further fruit turgor (cs) drop. These two responses further reduced fruit water potential, which promoted post-stress fruit expansion growth. # 2000 Elsevier Science B.V. All rights reserved.

Litchi fruit abscission: its patterns, effect of shading and relation to endogenous abscisic acid

Abstract In litchi (lychee), three waves of fruit drop were generally designated when the intensity of abscission was expressed as a relative rate. An additional wave of pre-harvest drop was detected in cultivars with aborted seeds. Fruit drop accentuated by shading treatment indicates that reduced incident solar radiation by overcast weather in the post-bloom period could be a major hindrance to the set. High leaves-to-panicle ratio was found to be needed for a normal set. Shading reduced photosynthesis but also enhanced the abscisic acid levels in the fruits. The abscisic acid levels were positively correlated with abscission, regardless of whether it was caused by non-pollination or by shading. Seed abscisic acid peaks were concomitant with the waves of fruit-drop. It is suggested that abscisic acid promotes litchi fruit abscission antagonistically to the role of auxin.

Sugar and acid compositions in the arils of Litchi chinensis Sonn.: cultivar differences and evidence for the absence of succinic acid

Summary Quantitative and qualitative changes in sugars and organic acids were investigated in two litchi (Litchi chinensis Sonn.) cultivars, ‘Feizixiao’ and ‘Nuomici’, during aril development, and their levels measured in another six cultivars (‘Guiwei’, ‘Sanyuehong’, ‘Jizuili’, ‘Xuehuaizi’, ‘Dahongli’ and ‘Yuhebao’) at maturity. Glucose, fructose and sucrose were the predominant sugars. Litchi cultivars could be classified into three types based on their sugar composition: 1) monosaccharide-prevalent types including cvs. ‘Feizixiao’, ‘Xuehuaizi’ and ‘Yuhebao’; 2) disaccharide-prevalent types including cvs. ‘Guiwei’, ‘Jizuili’ and ‘Nuomici’; and 3) intermediate types including cvs. ‘Sanyuehong’ and ‘Dahongli’. Differences in sugar compositions between cultivars were associated with differences in the activities of certain key enzymes. Succinic acid, which was previously reported to be one of the major organic acids in litchi arils, was found to be absent in all cultivars tested in this study. The major organic acid in the litchi aril was malic acid; others included tartaric, citric and ascorbic acids. Malic acid levels increased during the early stages of aril development, followed by a dramatic decrease as the fruit approached maturity. The ratio of malic acid to tartaric acid varied from 2.6–5.7 between cultivars. The concentration of ascorbic acid decreased with fruit development until 2 weeks before harvest, when it started to increase slightly as the fruit approached full maturity. Ascorbic acid levels varied considerably among the eight cultivars tested. Monosaccharide-prevalent cultivars contained more ascorbic acid than disaccharide-prevalent cultivars.

Cell wall modifications in the pericarp of litchi (Litchi chinensis Sonn.) cultivars that differ in their resistance to cracking

Summary Changes in structural calcium and galacturonan concentrations, the degree of methylesterification of pectins, the activities of pectin methylesterase (PME) and soluble and wall-bound peroxidases (POD), and amino acid compositions of structural proteins in the fruit pericarp (skin) were studied in cracking-resistant ‘Huaizhi’ and cracking-susceptible ‘Nuomici’ litchi. Structural calcium concentrations were higher in ‘Huaizhi’ than in ‘Nuomici’ and decreased from 22 d to 52 d after anthesis (DAA), then increased. Galacturonan concentrations increased over time and were higher in ‘Huaizhi’. Methylesterification of pectins increased from 50% at 22 DAA, to 55% at 78 DAA in ‘Huaizhi’, but fluctuated around 48% in ‘Nuomici’. PME activity was significantly higher in ‘Nuomici’ than in ‘Huaizhi’ in the later stages of fruit development (> 50 DAA). The activities of soluble (SPOD) and ionically wall-bound POD (IWBPOD) increased with fruit development. Maximum SPOD activity was similar in both cultivars, whereas the peak activity of IWBPOD was higher in ‘Nuomici’ than in ‘Huaizhi’. Total concentrations of SDS-soluble amino acids were similar in the two cultivars. Total structural proteins (SDS-insoluble amino acids) decreased with fruit development. Among the SDS-insoluble amino acids, hydroxyproline was the only one that increased over time. The results suggest that higher levels of structural calcium and galacturonans may contribute to cracking resistance in ‘Huaizhi’, while a higher activity of IWBPOD, which catalyses the formation of phenolic cross-linkages and an irreversible increase in the rigidity of cell walls, may be associated with cracking susceptibility in ‘Nuomici’. Structural proteins may not be involved in resistance to cracking in litchi cultivars.

A study of the cause of the mango black tip disorder

Abstract Causative factors of the fruit black tip disorder (BT) in mango (Mangifera indica L.) have been studied in Guangdong Province, China. Dipping fruits in 150 and 600 mg kg−1 fluoride solutions induced symptoms similar to that of the black tip disorder. The F content of the fruits with artificially induced symptoms was about 25% higher than that of control (healthy) fruits. The F content of BT fruits from an affected orchard was twice as much as that of the normal (healthy) fruits from an unaffected orchard. The leaf F content of affected trees was about six times as much as that of the control. This study has provided evidence that fluorine in fumes emitted from adjacent brick kilns was the direct causative factor of the mango black tip disorder.

The roles of cytokinins and abscisic acid in the pericarp of litchi (Litchi chinensis Sonn.) in determining fruit size

Summary Variations in the concentrations of cytokinins (CTKs) and abscisic acid (ABA) were studied in the pericarp of litchi (Litchi chinensis Sonn.) fruit using the large-fruited cv. ‘Erdanli’ (55.3 g per fruit) and the small-fruited cv. ‘Huaizhi’ (20.9 g per fruit), as well as large fruit (32.4 g) from early blooms and small (20.8 g) fruit from late blooms on the same inflorescences of cv. ‘Feizixiao’ from the same commercial orchard in Guangdong, China in 2000. ‘Erdanli’ had higher concentrations of CTKs than cv. ‘Huaizhi’ on three out of six sampling dates during fruit development, and lower concentrations of abscisic acid (ABA) from 40 d after anthesis. In cv. ‘Feizixiao’, fruit from early blooms had higher concentration of CTKs than fruit from late blooms at all sampling dates, and lower concentrations of ABA on three out of five sampling dates. Therefore, the former had a higher CTK:ABA ratio. These data suggest that a high CTK:ABA ratio favours fruit growth in litchi.

Early calcium accumulation may play a role in spongy tissue formation in litchi pericarp

Summary Changes in the micro-distribution of calcium in litchi (Litchi chinensis Sonn.) pericarp (fruit skin) were studied with an X-ray dispersive spectrometer (electron probe) and the structures of the spongy tissue-forming cells were observed with a transmission electron microscope (TEM) in litchi cvs. ‘Huaizhi’ (Wai Chee) and ‘Nuomici’ (No Mai Chee), which differ in cracking susceptibility. Before spongy tissue was visible, calcium had accumulated at the sites of spongy tissue formation, which formed calcium-rich zones in the inner mesocarp. These zones, which included groups of cells, were larger in cracking-resistant ‘Huaizhi’ than in susceptible ‘Nuomici’. ‘Huaizhi’ pericarp also had a thicker spongy tissue. Cells forming the spongy tissue started to die, with a breakdown of the plasma membrane and tonoplast, plasmolysis, loss of cell contents, swelling and autolysis of the walls. The spongy tissue was initially formed by localized cell death, and later by an increase of intercellular spaces due to the breakdown of middle lamellae. The early accumulation of calcium in spongy tissue-forming cells might have signalled this localized cell death, which falls into the category of programmed cell death (PCD). Calcium-rich zones at the site of spongy tissue formation largely disappeared as spongy tissue was formed, with the endocarp becoming “calcium rich”, indicating that intercellular re-localization of calcium may have occurred within the pericarp. The relations between calcium accumulation, spongy tissue formation, and fruit cracking resistance are discussed. We conclude that, in addition to its structural role, calcium also plays a non-structural role in conferring cracking resistance. The accumulation of Ca2+ in early fruit development signals PCD that leads to spongy tissue formation, a process essential for pericarp extension during aril growth.

The presence of oxalate in the pericarp and fruit pedicel is not linked to a shortage of fruit calcium and increase in cracking incidence in litchi

Summary Concentrations of oxalate and calcium in soluble or structural forms, as calcium oxalate in the pericarp, or as oxalate in the fruit pedicel, and the distribution of calcium in the pericarp and fruit pedicel were studied in litchi (Litchi chinensis Sonn.) using cracking-resistant cv. ‘Huaizhi’ and cracking-susceptible cv. ‘Nuomici’. Oxalate in the pericarp existed mainly in a soluble form and was significantly (P < 0.05) higher in ‘Huaizhi’ than in ‘Nuomici’. The concentration of oxalate was higher than the amount of calcium oxalate. Calcium was first mobilised for the construction of cell walls in the pericarp, despite the high concentration of oxalate in the tissue. ‘Huaizhi’ fruit accumulated more structural calcium and calcium oxalate in the pericarp than ‘Nuomici’ fruit. There was no difference in soluble calcium between the two cultivars. Calcium oxalate was located chiefly in the epidermis of the pericarp, and increased in concentration within 50 d after anthesis (DAA) in ‘Nuomici’ and 64 DAA in ‘Huaizhi’, then decreased as structural calcium increased, indicating that calcium sequestered by oxalate could be remobilised.The fruit pedicel had higher concentrations of oxalate than the pericarp. X-ray microanalysis showed abundant calcium in pedicel tissue adjoining the fruit base, and less calcium in tissues at the fruit base and in pedicel tissue 1 cm from the fruit, suggesting that the pedicel serves as a bottleneck to calcium movement. This bottleneck is not likely to be caused by oxalate in the pedicel. Oxalate in the pericarp and in the fruit pedicel is not linked to a shortage of fruit calcium or to an increase in cracking incidence in litchi.

The stunted fruit disorder—A physiological anomaly in mango (Mangifera indica L.) caused by a fluorine pollutant

SummaryThe stunted mango fruit disorder (SFD) is characterized by small-sized fruit, stiffened pulp tissue and failure to ripen at harvest. A cabinet fumigation trial with potted fruiting mango trees was carried out to investigate the cause of this disorder. Treatment with hydrogen fluoride (12 μg HF m–3 and 24 μg HF m–3) succeeded in inducing the typical symptoms of SFD, while treatment with sulphur dioxide (800 μg SO2 m–3) failed. Inhibition of cell expansion growth was observed in SFD fruits. The cells of affected fruits were much smaller than those of normal fruits at the same growth stage. Although reduction of photosynthesis of mango leaves was seen under both HF and SO2 fumigation treatments, it did not seem to limit fruit growth. The leaf and fruit fluorine (F) or sulfur (S) contents increased in HF or SO2 fumigation treatments. Fluorine pollutant in the smoke emitting from brick-kilns or aluminum factories must be the direct cause of SFD.