Danghui Xu

Photoprotective function of photorespiration in Reaumuria soongorica during different levels of drought stress in natural high irradiance

Diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem 2 (PS2) as well as H2O2 content were analyzed in Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub. The rate of photorespiration was estimated by combined measurement of gas exchange and Chl fluorescence. The rate of photorespiration increased with the increasing drought stress (DS). The ratio of carboxylation electron flow to oxygenation electron flow (Jc/Jo) and the maximal photochemical efficiency of PS2 (variable to maximum fluorescence ratio, Fv/Fm) decreased with the increasing DS. Fv/Fm in isonicotinic acid hydrazide (INH)-sprayed plants was lower than that in normal plants under moderate DS, but no significant difference was observed under severe DS. H2O2 content in INH-sprayed plants was significantly lower than that in normal plants under severe DS. Taken together, photorespiration in R. soongorica consumed excess electrons and protected photosynthetic apparatus under moderate DS, whereas it accelerated H2O2 accumulation markedly and induced the leaf abscission under severe DS.

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.

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.

The effects of phylogeny, life-history traits and altitude on the carbon, nitrogen, and phosphorus contents of seeds across 203 species from an alpine meadow

Seed reserves are very important to the earlier growth and survival of plants, and its variation in nutrient contents might make species to form different germination or seedlings growth strategies. Here, 203 species collected from an alpine meadow on the northeastern Qinghai–Tibet plateau were used to test the effects of phylogenetic groups, life-history traits, and altitude on carbon (C), nitrogen (N) and phosphorus (P) contents of seeds across species. The results showed that (1) seeds of Brassicaceae had the highest C content, those of Fabaceae had the highest N content, and Asteraceae had the highest P content, and family explained independently 32.7%, 46.4%, and 17.9% of the variation in C, N, and P contents of seeds, respectively; (2) the smaller seeds tended to have higher C and P contents, and seed mass explained independently 2.5% of variation in C, 4.3% in N, and 8.1% in P contents; (3) N content was explained 1% independently by life form, seeds N content of perennials was significantly higher than that of annals, but seeds C and P contents had non-significant difference between them; (4) seeds of wind-adapted species had higher N and P contents, and dispersal mode explained independently 1.7% of variation in C, 1.6% in N, and 5.6% in P contents; (5) seeds from high altitude had the highest N and P contents, and altitude explained independently 4.3% of the variation in N and 4% in P contents.

Structural, compositional and trait differences between the mature and the swamp meadow communities

The mature meadows (MMs) and the swamp meadows (SMs) are the two most important ecosystems in the eastern Tibetan Plateau, China. Besides their substantial differences in terms of soil water conditions and thereby the soil oxygen and nutrients, however, little is known about the differences in community composition, structure, traits and productivity between these two meadows. We particularly ask whether light availability mediated by physical structure heterogeneity is a key determinant of the difference in community composition and productivity between these two meadows. We examined the community structure, composition, aboveground net primary productivity (ANPP), light availability in understory and the community-weighted means (CWMs) for leaf morphological and physiological traits in 12 random plots (5 m × 5 m) for each of the studied habitats. The results showed that plant community in the MM had higher variation in both vertical and horizontal structure and thus had more light availability in the understory. The MM had higher species richness and greater ANPP than the SM. The CWMs of leaf morphological and physiological traits for species in the MM featured a fast-growing strategy (i.e. higher height, leaf area and net photosynthesis rate and lower nitrogen:phosphorus ratio), in contrast to those in the SM. We also found that there were significant correlations between the CWM of traits and the ANPP, indicating that some key traits in these habitats have linked to community productivity. Our study also suggests that the heterogeneity in the community structure, which affects light availability in the understory, may play an important role in determining the community composition and productivity. In conclusion, our study revealed significant differences in community structure, composition and traits between the MM and the SM, and the light availability that related closely to community structure is the key factor to determine the composition and productivity of the community of these two habitats.

The effect of light and seed mass on seed germination of common herbaceous species from the eastern Qinghai‐Tibet Plateau

Smaller seeds might encounter more severe selective pressure than larger ones because they have fewer food reserves and are more easily buried; thus, seed mass can be considered to be directly related to the effect of light on germination. To investigate the effect of light on seed germination and associated seed mass variation within a whole plant community, we compiled germination data for common herbaceous species from an alpine meadow on the eastern Qinghai-Tibet plateau. The results showed the following. (i) Light had a general positive effect on seed germination. Under light, the proportion of species with lower germinability was decreased, mean germination percentage was increased by 20% and the speed of germination was doubled. (ii) Irrespective of light environment, species with medium-sized seeds (seed mass ranging from 0.11 to 0.5 mg) had higher germination percentage and speed when compared with species within the largest seed mass group. (iii) The germination of smaller-seeded species was more dependent on light stimulation than larger-seeded ones. In darkness, the species within the smallest seed mass group had the lowest percentage and speed of germination; however, under light, the species within the largest seed mass group had the lowest percentage and speed of germination. Our results suggested that the germination characteristics and especially seeds’ response to light among species in the alpine meadow might be an adaptation to natural selective pressure.

Responses of plant functional groups to natural nitrogen fertility on an alpine grassland in the Qinghai-Tibet plateau

Species distribution is often closely associated with soil nutrients in terrestrial ecosystem. In contrast to most manipulated N (nitrogen) experimental studies, there are few observation experiments examining the distribution of species or functional groups along a natural soil N gradient. Alpine meadows with higher soil spatial heterogeneity at fine scale, which have a large gradient in soil N gradient, provides an ideal system to examine the distribution of species or functional groups. Here we used redundancy analysis (RDA) to examine the relationships between soil and plant properties in northeast of Qinghai-Tibet Plateau over two years. The results showed the relative biomass of forbs increased, while those of legumes and grasses decreased with the soil N availability. This suggests that legumes and grasses had stronger tolerance to infertile soils than forbs, which may due to the N2-fixed for legumes and high nutrient use efficiency for grasses. Furthermore, the positive significant relationships between the percentage of legumes biomass and N: P (phosphorus) ratio were found in the whole community and non-legumes, confirming the presence of legumes improved the vegetation N status even for non-legumes.