Xiangwen Fang

Fruit production of shrub, Caragana korshinskii, following above-ground partial shoot removal: mechanisms underlying compensation

A number of studies have showed that under some conditions plant may partially, fully or overcompensate for tissue loss, however, the mechanisms underlying compensation are not well understood and still need to be researched. We examined the ability of Caragana korshinskii to compensate for fruit production after above-ground partial shoot removal. Fruit production of 30% main shoot length removal (30% RSL) and 25 and 50% main shoot number removal (25% RSN, 50% RSN) resulted in overcompensation and the response of 60% main shoot length removal (60% RSL) was full compensation. Plants’ responses associated with compensation included (1) greater reproduction efficiency (RA); (2) increased fruit set; (3) decreased fruit abortion; (4) increased seed number per pod; and (5) higher individual seed biomass. These responses may have resulted from more nectar production per flower, more sucrose flux per pod and more sucrose flux per seed of clipped plants, which may in turn have resulted from (1) drawing upon more non-structural carbohydrate (TNC) from roots to supply flower bud development and the flush of new foliage; (2) supplying more photosynthetic assimilation to fruit development owing to increases in leaf-level photosynthetic rates. Increases in leaf-level photosynthetic rates may be caused by more nutrient (nitrogen) and water availability per unit area of resource leaves after clipping.

Effect of legume species introduction to early abandoned field on vegetation development

One of the most important areas in ecology is to elucidate the factors that drive succession in ecosystems. The purpose of our study was to assess the effects of legume species (Medicago sativa, Melilotus suaveolens and Astragalus adsurgens) introduction to abandoned arable land on vegetation development in the Loess Plateau, China. Results from our study showed that addition of legume species strongly affected the composition of recently abandoned-field vegetation. Legume species were effective at reducing the number and dominance of natural colonizers (mainly weeds from the seed bank). The introduction of legume species into newly abandoned fields maintained high total cover and above-ground biomass and could improve soil organic carbon and total nitrogen. However, the effects of the treatments were species-specific. Melilotus suaveolens turned out to be severely suppressive to natural colonizers (weed species). Also, Melilotus suaveolens-adding maintained the highest cover and above-ground biomass and was helpful to improve later succession species, e.g. Stipa breviflora and Astragalus polycladus, to invade and establish. Medicago sativa-adding was superior in enhancing the soil organic carbon and total nitrogen. The present results suggested that addition of legume species with greater cover and biomass strongly suppressed the dominance of the weedy species in early succession and the course of old-field succession may be accelerated by introduction of legume species at least temporarily. However, the experimental period was too short to assess to what extent succession may be affected in the longer term.

Responses of Caragana korshinskii Kom. to shoot removal: mechanisms underlying regrowth

Caragana korshinskii Kom. a dominant member of desert flora in north-western China, is often subjected to aboveground shoot destruction but is very successful in its rapid recovery. We investigated the physiological basis for resprouting by comparing shoot elongation, leaf-nutrient content, pre-dawn leaf-water potential (LWP), root non-structural carbohydrate (TNC), and photosynthetic rate of first-year resprouts with those of adjacent undamaged individuals. C. korshinskii resprouts had a significantly higher rate of shoot elongation. Plant responses associated with enhanced shoot elongation included (1) improved water status, (2) drawing upon more TNC from roots to support aboveground shoot regrowth, (3) enhanced photosynthetic rate owing to improved water status and increased nutrient content in leaves, and (4) allocating more photosynthate to vegetative production without reproduction costs. Maintaining an active root system after shoot removal may be the foundation which engenders these mechanisms underlying rapid regrowth of C. korshinskii in the disturbed environment.

Caragana korshinskii seedlings maintain positive photosynthesis during short-term, severe drought stress

Seedling performance may determine plant distribution, especially in water-limited environments. Plants of Caragana korshinskii commonly grow in arid and semiarid areas in northwestern China, and endure water shortage in various ways, but little is known about their performance when water shortage occurs at early growth stages. The water relations, photosynthetic activity, chlorophyll (Chl) content and proline accumulation were determined in 1-year-old seedlings growing in a 1:1 mixture of Loess soil and Perlite and subjected to (1) a water deficit for 20 days and (2) kept adequately watered throughout. The water deficit induced low (−6.1 MPa) predawn leaf water potentials (LWP), but did not induce any leaf abscission. Stomatal conductance (gs), leaf transpiration rate (E), and net photosynthetic rate (PN) decreased immediately following the imposition of the water deficit, while the maximal photochemical efficiency of photosystem II (PSII) (Fv/Fm) and the effective quantum yield of PSII (ΦPSII) decreased 15 days later. An early and rapid decrease in gs, reduced E, increased Chl (a+b) loss, increased the apparent rate of photochemical transport of electrons through PSII (ETR)/PN, as well as a gradual increase in non-photochemical quenching of fluorescence (NPQ) and proline may have contributed to preventing ΦPSII from photodamage. C. korshinskii seedlings used a stress-tolerance strategy, with leaf maintenance providing a clear selective advantage, considering the occasional rainfall events during the growing season.

Limits to the height growth of Caragana korshinskii resprouts

Predawn leaf water potential (LWP), the LWP between 09:00 and 10:30 h (termed minimum LWP), stem xylem hydraulic conductivity, foliar nitrogen, leaf gas exchange and leaf traits were measured on the same days in adults and 1-year-old to 7-year-old resprouts that had regrown after removing all the aboveground shoots. Height growth and accumulation of aboveground biomass quickly decreased with resprout age and there was no difference between 7-year-old resprouts and the uncut adults. Predawn LWP showed no significant difference between resprouts and adults, but the minimum LWP decreased gradually from -2.0 MPa in 1-year-old resprouts to -3.0 MPa in 7-year-old resprouts. The decrease in minimum LWP was associated with increased hydraulic resistance, as indicated by the gradual decrease in leaf area-specific hydraulic conductivity (KL) and sapwood area-specific hydraulic conductivity (KS) and the associated increase in stem native percentage loss of hydraulic conductivity in older than 2-year-old resprouts. The leaf nitrogen content per unit area (Narea) also decreased steadily from 3.6 g m(-2) in 1-year-old resprouts to 1.7 g m(-2) in 7-year-old resprouts. With the decline in LWP and Narea, the rate of leaf photosynthesis per unit area (Aarea) decreased from 20 μ mol m(-2) s(-1) in 1-year-old resprouts to 11 μ mol m(-2) s(-1) in 7-year-old resprouts. In adults, although KS decreased further compared with 7-year-old resprouts, the minimum LWP, KL, Narea and the rate of photosynthesis increased by 0.3 MPa, 29, 34 and 23%, respectively. The results show that a progressive loss of stem hydraulic conductivity and a steady decrease in foliar nitrogen with age were associated with a decrease in the photosynthetic rate of Caragana korshinskii Kom. resprouts, possibly changing the allocation of photosynthetic assimilates and slowing resprout height growth.

Photosynthetic activity of poikilochlorophyllous desiccation tolerant plant Reaumuria soongorica during dehydration and re-hydration

Diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem 2 (PS2) as well as Chl content were analyzed in Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub during dehydration and rehydration. The net photosynthetic rate (PN), maximum photochemical efficiency of PS2 (variable to maximum fluorescence ratio, Fv/Fm), quantum efficiency of non-cyclic electron transport of PS2, and Chl content decreased, but non-photochemical quenching of fluorescence and carotenoid content increased in stems with the increasing of drought stress. 6 d after re-hydration, new leaves budded from stems. In the re-watered plants, the chloroplast function was restored and Chl a fluorescence returned to a similar level as in the control plants. This improved hydraulic adjustment in plant triggered a positive effect on ion flow in the tissues and increased shoot electrical admittance. Thus R. soongorica plants are able to sustain drought stress through leaf abscission and keep part of Chl content in stems.

Influences of nitrogen, phosphorus and silicon addition on plant productivity and species richness in an alpine meadow

Plots in an alpine meadow fertilized with Si in combination with either N or P had higher aboveground primary productivity (APP) and higher species richness than when fertilized with N or P alone. Our finding highlights the importance of Si in improving APP and alleviating N fertilization-induced biodiversity loss in grasslands, and will help improve our ability to predict community composition and biomass dynamics in alpine meadow ecosystems subject to changing nutrient availability.

Ecophysiological responses of Caragana korshinskii Kom. under extreme drought stress: Leaf abscission and stem survives

Caragana korshinskii Kom. is a perennial xerophytic shrub, well known for its ability to resist drought. In order to study ecophysiological responses of C. korshinskii under extreme drought stress and subsequent rehydration, diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem II as well as Chl content were analyzed. Plant responses to extreme drought included (1) leaf abscission and using stem for photosynthesis, (2) improved instantaneous water-use efficiency, (3) decreased photosynthetic rate and partly closed stomata owing to leaf abscission and low water status, (4) decreased maximum photochemical efficiency of photosystem II (PSII) (variable to maximum fluorescence ratio, Fv/Fm), quantum efficiency of noncyclic electron transport of PSII, and Chl a and Chl b. Four days after rehydration, new leaves budded from stems. In the rewatered plants, the chloroplast function was restored, the gas exchange and Chl fluorescence returned to a similar level as control plant. The above result indicated that maintaining an active stem system after leaf abscission during extreme drought stress may be the foundation which engenders these mechanisms rapid regrowth for C. korshinskii in arid environment.

Activities of starch hydrolytic enzymes and starch mobilization in roots of Caragana korshinskii following above-ground partial shoot removal

In many resprouting plants, carbohydrates are stored as starch in roots and will be mobilized to support above-ground tissue regrowth after shoot damage. Our objective was to determine how activities of starch hydrolytic enzymes change damage-induced starch mobilization in Caragana korshinskii roots after above-ground tissue loss. Zero percent (control), 30% (30% RSL), 60% (60% RSL) of main shoot length, and 25% (25% RSN), 50% (50% RSN), and 100% (100% RSN) of main shoot number were removed. Compared with control plants, clipping accelerated the reduction of starch in the roots, increased sucrose flux per flower per hour and nectar production per flower per day in 30% RSL, 60% RSL, 25% RSN, and 50% RSN treatments, and improved vegetative growth in 100% RSN treatment. All treatments had similar total nonstructural carbohydrate (TNC) concentrations in leaves, shoots, and stems with the exception of 100% RSN with higher TNC concentration in shoots. Both α-, and β-amylase activities were enhanced by clipping, the former being more strongly correlated with starch degradation in the roots than the latter. The other two possible starch-breaking enzymes, α-glucosidase, and starch phosphorylase showed no significant differences in the activities between treatments. The results suggest that starch degradation in the roots of C. korshinskii was regulated by α-amylase activity and more mobilized starch was used to support vegetative growth in 100% RSN treatment and support sexual reproduction followed by other clipping treatments.

Seed germination of Caragana species from different regions is strongly driven by environmental cues and not phylogenetic signals

Seed germination behavior is an important factor in the distribution of species. Many studies have shown that germination is controlled by phylogenetic constraints, however, it is not clear whether phylogenetic constraints or environmental cues explain seed germination of a genus from a common ancestor. In this study, seed germination under different temperature- and water-regimes [induced by different osmotic potentials of polyethylene glycol (PEG)] was investigated in the phylogenetically-related Caragana species that thrive in arid, semiarid, semihumid and humid environments. The results showed that the final percentage germination (FPG) decreased from 95% in species from arid habitats to 0% in species from humid habitats, but with no significant phylogenetic signal. Rather, the response of seed germination to temperature and PEG varied greatly with species from arid to humid habitats and was tightly linked to the ecological niche of the species, their seed coat structure and abscisic acid concentration. The findings are not consistent with the hypothesis that within a family or a genus, seed germination strategies can be a stable evolutionary trait, thus constraining interspecific variation, but the results clearly show that seed germination of Caragana species distributed across a range of habitats has adapted to the environment of that habitat.