Jiaru Li

Senescence of aerial parts is impeded by exogenous gibberellic acid in herbaceous perennial Paris polyphylla.

The effects of gibberellin A(3) (GA(3)) on natural senescence and the relationship between gibberellins (GAs), abscisic acid (ABA), and senescence are not fully understood. For example, it is still unclear whether GA and ABA act antagonistically. There are only few reports on senescence-related changes in physiological parameters of herbaceous perennials. This study was designed to investigate the effects of exogenous GA(3) on the senescence of aerial parts in a herbaceous perennial species, Paris polyphylla, and to test the hypothesis that GA and ABA display antagonistic effects in this process. Physiological changes associated with senescence, in particular of the hormonal and oxidative metabolisms, were also investigated. GA(3) was sprayed on mature leaves at weekly intervals, which significantly impeded senescence of aerial parts and slowed the decline of pigments and total soluble protein. Treated plants suffered less oxidative stress as revealed by reduced lipid peroxidation, a lower hydrogen peroxide level and modified activities of superoxide dismutase, peroxidase, ascorbate peroxidase, and their respective isozyme profiles. In GA(3) treated plants GA(4)+GA(7) (GAs) levels increased progressively and became significantly higher than those of control plants, whereas ABA increased in controls. When plants were treated with GA-synthesis inhibitor paclobutrazol (PCB), GAs decreased, ABA increased, and senescence was promoted. Application of a mixture of GA(3) and PCB restored the accumulation of GAs, reduced ABA, and ultimately senescence was delayed. These results suggest that GA and ABA play antagonistic roles in the senescence of aerial parts in P. polyphylla, and this process is associated with oxidative stress and regulated by endogenous hormones and extrinsic factors. Possible mechanisms that control this GA(3)-mediated inhibition of senescence are discussed.

Gibberellin retards chlorophyll degradation during senescence of Paris polyphylla

Chlorophyll (Chl) degradation was found to be related to the endogenous gibberellin (GA) content in shoots during senescence in the perennial plant Paris polyphylla var. yunnanensis (Franch.). Treatment with gibberellic acid (GA3) significantly increased the content of endogenous GAs (GA4 + GA7), retarded the senescence of shoots, and the degradation of proteins and Chl. Chlorophyllase, Mg-dechelation and peroxidase activities increased more in control plants than in those treated with GA3. GA3 treatment also protected lipoxygenase activity, which decreased significantly in control plants.

Improving rhizome yield and quality of Paris polyphylla through gibberellic acid-induced retardation of senescence of aerial parts.

Senescence in perennials has not been extensively studied compared to that in annual plants, and the effects of delaying senescence on plant biomass and metabolic features in herbaceous perennials has also been poorly reported. We recently examined the effects of gibberellin A3 (GA3) on senescence of aerial parts of Paris polyphylla, which characterize the metabolic changes associated with senescence, and found antagonistic effects of GA and abscisic acid during this process. Rhizome yield and quality (saponin content) of Paris polyphylla were both improved by GA3-induced retardation of senescence. We propose that GA3-induced retardation of senescence increases green leaf area and prolongs the duration of photosynthesis, leading to increased rhizome yield. The increased saponin accumulation in GA3-treated plants may be explained by the longer growth phase and ensuing increased environmental stress.

Function of leafy sepals in Paris polyphylla: photosynthate allocation and partitioning to the fruit and rhizome

Paris polyphylla Smith var. yunnanensis (Franch.) Hand.-Mazz. is a rhizomatous, herbaceous, perennial plant that is used as a medicinal plant with a variety of pharmacological activities. However, the functions of the green, leafy sepal of this plant are poorly understood. The main objectives of this study were to: (a) test the hypothesis that sepals make measurable contributions to fruit development and rhizome growth; and (b) investigate the allocation and partitioning of photosynthates produced by sepals and leaves to fruit and rhizome. Net photosynthetic rate, photosynthetic pigment composition and δ13C values were similar for sepals and leaves. Sepal-darkening and sepal-removal treatments resulted in smaller fruit size and decreased rhizome biomass, whereas fruit removal led to a decrease in calyx size and an increase in rhizome yield and saponin content. Fruit and seed mass were positively and linearly related to calyx size. These results indicate that photosynthates produced by sepals are involved in the fruit growth and seed development and that developing fruit and rhizomes compete for the photosynthates exported by leaves. We propose that the sepals of P. polyphylla function partly as leaves to compensate for reproductive costs. Fruit removal increased carbon partitioning to the rhizome and improved rhizome yield and quality, offering a useful strategy for the domestication of this valuable medicinal plant.

Sodium Bisulfite Improves Rhizome Yield and Quality in Paris polyphylla

Rhizomes of the perennial herb Paris polyphylla have been used in traditional Chinese medicine for hundreds of years. Agricultural production of the rhizomes requires 7-10 years, which is too long to meet the demand of the medicinal industry. Therefore, studies on improving the yield of the herb and shortening the culturing period are imperative. The present work aimed to investigate the effect of sodium bisulfite (NaHSO (3)) on rhizome yield and quality, as well as some related metabolic features of P. polyphylla. The rhizome yield was improved by NaHSO (3) treatment in long-term experiments conducted during 2006 and 2007, with 2 mM NaHSO (3) giving the highest yield. HPLC analysis revealed that NaHSO (3) treatment increased the total saponin content (49 %), including three pennogenin glycosides and two diosgenin glycosides. In a short-term experiment, NaHSO (3) treatment resulted in an enhanced net photosynthetic rate (Pn) for about 4 days without significant changes in the chlorophyll or carotenoid content. The total soluble sugars and sucrose contents in the leaves also significantly increased after 2 mM NaHSO (3) treatment, whereas the starch content changed only slightly. The activities of the enzymes involved in ammonium assimilation (glutamine synthetase [GS] and glutamate dehydrogenase [GDH]) were not significantly influenced. In a long-term experiment, chlorophylls and carotenoids were not significantly affected, and neither was the starch content in leaves, but the total soluble sugars and sucrose contents in leaves increased significantly. The NaHSO (3) treatment significantly increased GS and GDH activities. These results indicate that NaHSO (3) treatment improved the rhizome yield in P. polyphylla, not only through enhancement of Pn but also by improving carbohydrate accumulation and ammonium assimilation. The increased saponin content after NaHSO (3) treatment was indicative of high rhizome quality.

Nitrogen remobilization in shoots of Paris polyphylla is altered by gibberellic acid application during senescence

Nitrogen remobilization during senescence has been studied in perennial herb Paris polyphylla. We analyzed changes in N content, amino acids, N-remobilization enzymes and effects of gibberellic acid (GA) during natural senescence. There was a gradual decrease in the contents of N, chlorophyll and soluble proteins and activities of glutamine synthetase (GS; EC 6.3.1.2) and glutamate dehydrogenase (GLDH; EC 1.4.1.2). Activity staining and Western blots showed that GS2 activity decreased, whereas GS1 activity was relatively stable over time. In contrast, the C/N ratio and total amino acid content increased. Among individual amino acids, the proportions of glutamine (Gln) and asparagine (Asn) increased, and proportions of arginine, aspartate and glycine decreased. Treatment with GA slowed the senescence and retarded decreases in the activities of GS and GLDH and the contents of N, chlorophyll and soluble proteins. Conversely, this treatment slowed increases in the C/N ratio, total free amino acid content, and proportions of Gln and Asn. We conclude that low N resorption efficiency during senescence of P. polyphylla results from a sharp decrease in N remobilization enzyme activity.