Gen-Xuan Wang

Doubled CO2 could improve the drought tolerance better in sensitive cultivars than in tolerant cultivars in spring wheat

Abstract Effects of doubled CO2 concentration on drought stress tolerance were investigated in two spring wheat cultivars (Triticum aestivum L. Longchun 292 and Longchun 8139). Doubled CO2 decreased protein and chlorophyll in the two cultivars, but decreased total SOD activity mainly by decreasing FeSOD and decreased GSH/GSSG and AS/DHA ratios by converting more GSH and AS to GSSG and DHA only in 8139. After stress, as to every parameter, the curve changes were similar both in 292 and in 8139 regardless of ambient or doubled CO2 and could be divided into two stages. Under ambient CO2, the first stages were observed from days 0 to 6 in 292 and from days 0 to 5 in 8139. This result showed that 292 were more tolerant to drought stress than 8139 under ambient CO2. The decreases in protein, chlorophyll, activities of antioxidant enzymes and GSH/GSSG and AS/DHA ratios and the increases in H2O2 contents, TBARS contents and relative leakage ration induced by drought stress were delayed by doubled CO2 and the initial of the second stages was delayed 3 days in 8139 and 2 days in 292, then the first stages were observed from days 0 to 8, both in 292 and 8139. The results indicated that doubled CO2 could enhance antioxidant systems to increase the tolerance to drought stress and this effect was stronger in the cultivars, which are more sensitive to drought under ambient CO2.

Salt stress-induced programmed cell death via Ca2+-mediated mitochondrial permeability transition in tobacco protoplasts

The change in cytosolic free concentration of calcium ([Ca2+]cyt) plays a key role in regulating apoptosis in animal cells. In our experiment, we tried to investigate the function of Ca2+ in programmed cell death (PCD) in tobacco (Nicotiana tobacum, cultivar BY-2) protoplasts induced by salt stress. An obvious increase in [Ca2+]cyt was observed a few minutes after treatment and the onset of a decrease in mitochondrial membrane potential (ΔΨm) was also observed before the appearance of PCD, pre-treatment of protoplasts with EGTA or LaCl3 effectively retarded the increase in [Ca2+]cyt, which was concomitant with the decrease in the percentage of cell death and higher ΔΨm, pre-treatment with cyclosporine A (CsA) also effectively retarded the increase in [Ca2+]cyt, the decrease in ΔΨm and the onset of PCD. All these results suggest that Ca2+ is a necessary element in regulating PCD and the increase in [Ca2+]cyt and the opening of mitochondrial permeability transition pore (MPTP) could promote each other in regulating PCD in tobacco protoplasts induced by salt stress.

Plant height–crown radius and canopy coverage–density relationships determine above-ground biomass–density relationship in stressful environments

Debate continues in theoretical ecology over whether and why the scaling exponent of biomass–density (M–N) relationship varies along environmental gradients. By developing a novel geometric model with assumptions of allometric growth at the individual level and open canopy at the stand level, we propose that plant height–crown radius and canopy coverage–density relationships determine the above-ground M–N relationship in stressful environments. Results from field investigation along an aridity gradient (from eastern to western China) confirmed our model prediction and showed that the above-ground M–N scaling exponent increased with drought stress. Therefore, the ‘universal’ scaling exponents (−3/2 or −4/3) of the M–N relationship predicted by previous models may not hold for above-ground parts in stressful environments.

Cytosolic calcium oscillation may induce stomatal oscillation in Vicia faba

Abstract Whether guard cell cytosolic calcium ([Ca 2+ ] cyt ) oscillation induced stomatal oscillation was investigated in Vicia faba leaf. Extracellular Ca 2+ , ABA and H 2 O 2 treatments induced stomata to close in a dose- and time-dependent way. Both steady-state and rapid exchange treatments could first induce a steep decrease in stomatal aperture and then significantly induce stomatal oscillations at small apertures. Steady-state extracellular Ca 2+ and ABA treatments caused obvious but small and irregular stomatal oscillations, and the oscillations lasted for shorter time, while steady-state H 2 O 2 treatment induced no stomatal oscillation. Intriguingly, rapid exchange treatments of high [K + ] buffer and high [Ca 2+ ] buffer or high [ABA] buffer induced significant, strong, regular and longer time stomatal oscillations, while no oscillation occurred under H 2 O 2 treatment in the same procedure. It was likely that [Ca 2+ ] cyt oscillations induced by extracellular Ca 2+ and ABA could induce stomatal oscillations and stomatal oscillations occurred at small apertures.

Insights into plant size-density relationships from models and agricultural crops

There is general agreement that competition for resources results in a tradeoff between plant mass, M, and density, but the mathematical form of the resulting thinning relationship and the mechanisms that generate it are debated. Here, we evaluate two complementary models, one based on the space-filling properties of canopy geometry and the other on the metabolic basis of resource use. For densely packed stands, both models predict that density scales as M−3/4, energy use as M0, and total biomass as M1/4. Compilation and analysis of data from 183 populations of herbaceous crop species, 473 stands of managed tree plantations, and 13 populations of bamboo gave four major results: (i) At low initial planting densities, crops grew at similar rates, did not come into contact, and attained similar mature sizes; (ii) at higher initial densities, crops grew until neighboring plants came into contact, growth ceased as a result of competition for limited resources, and a tradeoff between density and size resulted in critical density scaling as M−0.78, total resource use as M−0.02, and total biomass as M0.22; (iii) these scaling exponents are very close to the predicted values of M−3/4, M0, and M1/4, respectively, and significantly different from the exponents suggested by some earlier studies; and (iv) our data extend previously documented scaling relationships for trees in natural forests to small herbaceous annual crops. These results provide a quantitative, predictive framework with important implications for the basic and applied plant sciences.

Application of external calcium in improving the PEG-induced water stress tolerance in liquorice cells

Calcium (Ca2+) may be involved in plant tolerance to water stress by regulating antioxidant metabolism. This study was designed to examine whether external Ca2+ treatment would improve drought tolerance in liquorice cells. The results showed that water stressed treatment induced by 10% PEG could reduce significantly the FW and RWC of liquorice cells, but external Ca2+ treatment considerably increased the two factors after 10-days stress. In addition, lesser amounts of MDA and H2O2 accumulated in Ca2+-treated cells than in untreated cells, and the activities of CAT, SOD and POD in Ca2+-treated cells were higher than in untreated cells during the stress period. The measured parameters treated by 40 mmol L-1CaCl2 were higher than those treated by 10 mmol L-1CaCl2. The changes in CAT, SOD and POD activities under stressed conditions were significantly larger than those in non-stressed conditions. Under stressed conditions, the trend of SOD activities was similar to that of CAT, and the activity of CAT was larger than that of SOD. CAT activity changed in relation to H2O2 content. It was indicated that water stress induced oxidative stress in liquorice cells, and application of external calcium (40 mmol L-1) significantly improved water stress tolerance in those cells. In addition, the measured parameters were different between Ca2+-treated cells under stressed and non-stressed conditions, and it is possible that calcium signals were different coming from different stimulations. The investigations also showed that the effect of external Ca2+ on the measured parameters was not due to the regulation of osmotic potential and osmotic adjustment in liquorice cells. The mechanism that allowed extracellular Ca2+ to improve adaptation of liquorice cells to drought was mediated by mitigating oxidative stress

Trade-Offs between the Metabolic Rate and Population Density of Plants

The energetic equivalence rule, which is based on a combination of metabolic theory and the self-thinning rule, is one of the fundamental laws of nature. However, there is a progressively increasing body of evidence that scaling relationships of metabolic rate vs. body mass and population density vs. body mass are variable and deviate from their respective theoretical values of 3/4 and −3/4 or −2/3. These findings questioned the previous hypotheses of energetic equivalence rule in plants. Here we examined the allometric relationships between photosynthetic mass (M p) or leaf mass (M L) vs. body mass (β); population density vs. body mass (δ); and leaf mass vs. population density, for desert shrubs, trees, and herbaceous plants, respectively. As expected, the allometric relationships for both photosynthetic mass (i.e. metabolic rate) and population density varied with the environmental conditions. However, the ratio between the two exponents was −1 (i.e. β/δ = −1) and followed the trade-off principle when local resources were limited. Our results demonstrate for the first time that the energetic equivalence rule of plants is based on trade-offs between the variable metabolic rate and population density rather than their constant allometric exponents.

The difference between above- and below-ground self-thinning lines in forest communities

Quantifying the self-thinning process in various plant communities has been a long-standing issue in both theoretical and empirical studies. Most studies on plant self-thinning have centered only on aboveground parts, and rarely on belowground parts. There is still a general lack of comparison between above- and belowground self-thinning processes, especially for forest communities. The fundamental mechanistic difference and the functional association between above- and belowground competition indicate that the self-thinning process of belowground parts may be different from that of aboveground parts. We investigated the self-thinning lines for above-ground (MA), below-ground (MB), and total biomass (MT), respectively, across forest communities in China. The results showed that neither the classical self-thinning rule (−3/2 exponent) nor the universal scaling rule (−4/3 exponent) can apply to all the self-thinning relationships across these forest communities and that the self-thinning lines for belowground biomass were flatter and lower than those for aboveground biomass across most of these forest communities.

Scaling relationship between tree respiration rates and biomass

The WBE theory proposed by West, Brown and Enquist predicts that larger plant respiration rate, R, scales to the three-quarters power of body size, M. However, studies on the R versus M relationship for larger plants (i.e. trees larger than saplings) have not been reported. Published respiration rates of field-grown trees (saplings and larger trees) were examined to test this relationship. Our results showed that for larger trees, aboveground respiration rates RA scaled as the 0.82-power of aboveground biomass MA, and that total respiration rates RT scaled as the 0.85-power of total biomass MT, both of which significantly deviated from the three-quarters scaling law predicted by the WBE theory, and which agreed with 0.81–0.84-power scaling of biomass to respiration across the full range of measured tree sizes for an independent dataset reported by Reich et al. (Reich et al. 2006 Nature 439, 457–461). By contrast, R scaled nearly isometrically with M in saplings. We contend that the scaling exponent of plant metabolism is close to unity for saplings and decreases (but is significantly larger than three-quarters) as trees grow, implying that there is no universal metabolic scaling in plants.

Cell Polarity Signaling: Focus on Polar Auxin Transport

Polar auxin transport, which is required for the formation of auxin gradients and directional auxin flows that are critical for plant pattern formation, morphogenesis, and directional growth response to vectorial cues, is mediated by polarized sub-cellular distribution of PIN-FORMED Proteins (PINs, auxin efflux carriers), AUX1/AUX1-like proteins (auxin influx facilitators), and multidrug resistance P-glycoproteins (MDR/PGP). Polar localization of these proteins is controlled by both developmental and environmental cues. Recent studies have revealed cellular (endocytosis, transcytosis, and endosomal sorting and recycling) and molecular (PINOID kinase, protein phosphatase 2A) mechanisms underlying the polar distribution of these auxin transport proteins. Both TIR1-mediated auxin signaling and TIR1-independent auxin-mediated endocytosis have been shown to regulate polar PIN localization and auxin flow, implicating auxin as a self-organizing signal in directing polar transport and directional flows.