Jiayin Pang

Xylem ionic relations and salinity tolerance in barley

Control of ion loading into the xylem has been repeatedly named as a crucial factor determining plant salt tolerance. In this study we further investigate this issue by applying a range of biophysical [the microelectrode ion flux measurement (MIFE) technique for non-invasive ion flux measurements, the patch clamp technique, membrane potential measurements] and physiological (xylem sap and tissue nutrient analysis, photosynthetic characteristics, stomatal conductance) techniques to barley varieties contrasting in their salt tolerance. We report that restricting Na(+) loading into the xylem is not essential for conferring salinity tolerance in barley, with tolerant varieties showing xylem Na(+) concentrations at least as high as those of sensitive ones. At the same time, tolerant genotypes are capable of maintaining higher xylem K(+)/Na(+) ratios and efficiently sequester the accumulated Na(+) in leaves. The former is achieved by more efficient loading of K(+) into the xylem. We argue that the observed increases in xylem K(+) and Na(+) concentrations in tolerant genotypes are required for efficient osmotic adjustment, needed to support leaf expansion growth. We also provide evidence that K(+)-permeable voltage-sensitive channels are involved in xylem loading and operate in a feedback manner to maintain a constant K(+)/Na(+) ratio in the xylem sap.

Growth and physiological responses of six barley genotypes to waterlogging and subsequent recovery

In this study, the growth response of 6 barley genotypes of different origin (3 from China, 2 from Australia, 1 from Japan) to waterlogging and subsequent recovery was evaluated in 2 different soil types, an artificial potting mix and a Vertosol. A range of physiological measurements was assessed, to develop a method to aid selection for waterlogging tolerance. Plants at the 3 or 4 expanded leaf stages were subjected to waterlogging for 3 weeks followed by 2 weeks of recovery. Both shoot and root growth was negatively affected by waterlogging. As waterlogging stress developed, chlorophyll content, CO2 assimilation rate, and maximal quantum efficiency of photosystem II (Fv/Fm) decreased significantly. The adverse effect of waterlogging was most severe for genotype Naso Nijo, intermediate for ZP, Gairdner, DYSYH, and Franklin, and least for TX9425 in both trials. Studies of the root anatomy suggested that such a contrasting behaviour may be partially due to a significant difference in the pattern of aerenchyma formation in barley roots. The adverse effects in stressed plants were alleviated after 2 weeks of drainage for all genotypes. In general, TX9425 continued to grow better than other varieties, whereas recovery of Naso Nijo was extremely slow. It is suggested that screening a small number of lines for waterlogging tolerance could be facilitated by selecting genotypes with least pronounced reduction of photosynthetic rate or total chlorophyll content, and for a larger number of lines, chlorophyll fluorescence is the most appropriate tool. Additional keywords: chlorophyll content, photosynthesis, chlorophyll fluorescence, aerenchyma.

Carbon trading for phosphorus gain: The balance between rhizosphere carboxylates and arbuscular mycorrhizal symbiosis in plant phosphorus acquisition

Two key plant adaptations for phosphorus (P) acquisition are carboxylate exudation into the rhizosphere and mycorrhizal symbioses. These target different soil P resources, presumably with different plant carbon costs. We examined the effect of inoculation with arbuscular mycorrhizal fungi (AMF) on amount of rhizosphere carboxylates and plant P uptake for 10 species of low-P adapted Kennedia grown for 23 weeks in low-P sand. Inoculation decreased carboxylates in some species (up to 50%), decreased plant dry weight (21%) and increased plant P content (23%). There was a positive logarithmic relationship between plant P content and the amount of rhizosphere citric acid for inoculated and uninoculated plants. Causality was indicated by experiments using sand where little citric acid was lost from the soil solution over 2 h and citric acid at low concentrations desorbed P into the soil solution. Senesced leaf P concentration was often low and P-resorption efficiencies reached >90%. In conclusion, we propose that mycorrhizally mediated resource partitioning occurred because inoculation reduced rhizosphere carboxylates, but increased plant P uptake. Hence, presumably, the proportion of plant P acquired from strongly sorbed sources decreased with inoculation, while the proportion from labile inorganic P increased. Implications for plant fitness under field conditions now require investigation.

Physiological aspects of vetiver grass for rehabilitation in abandoned metalliferous mine wastes

Physiological aspects of why vetiver grass (Vetiveria zizanioides L.) can be tolerant to heavy metals and be used as an alternative method for rehabilitation of abandoned metalliferous mine wastelands have been investigated. The results showed that high proportions of lead and zinc (Pb/Zn) tailing greatly inhibited the leaf growth, dry matter accumulation, and photosynthesis of leaves, but stimulated the accumulation of proline and abscisic acid (ABA), and enhanced the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), implying that different mechanisms to detoxify active oxygen species (AOS) existed in different parts of plants. Physiological responses to heavy metal treatments differed greatly between roots and shoots. Nitrogen fertilizer application could greatly alleviate the adverse effects of high proportions of Pb/Zn tailing on vetiver grass growth.

Wheat genotypes with high early vigour accumulate more nitrogen and have higher photosynthetic nitrogen use efficiency during early growth

Genotypic differences in early growth and nitrogen (N) uptake among 24 wheat (Triticum aestivum L.) genotypes were assessed in a field trial. At late tillering, large genetic variation was observed for shoot biomass (23-56gm-2 ground area) and N uptake (1.1-1.8gm-2 ground area). A strong correlation between aboveground biomass and N uptake was observed. Variation around this relationship was also found, with some genotypes having similar N uptake but large differences in aboveground biomass. A controlled environment experiment was conducted to investigate the underlying mechanisms for this variation in aboveground biomass using three vigorous genotypes (38-19, 92-11 and CV97) and a non-vigorous commercial cultivar (Janz). Vigorous genotypes had lower specific leaf N in the youngest fully expanded leaf than Janz. However, there was no difference in chlorophyll content, maximum Rubisco activity or the rate of electron transport per unit area. This suggests that Janz invested more N in non-photosynthetic components than the vigorous lines, which could explain the higher photosynthetic N use efficiency of the vigorous genotypes. The results suggest that the utilisation of wheat genotypes with high early vigour could improve the efficiency of N use for biomass production in addition to improving N uptake during early growth.

Contrasting responses to drought stress in herbaceous perennial legumes

Background and aimsMedicago sativa L. is widely grown in southern Australia, but is poorly adapted to dry, hot summers. This study aimed to identify perennial herbaceous legumes with greater resistance to drought stress and explore their adaptive strategies.MethodsTen herbaceous perennial legume species/accessions were grown in deep pots in a sandy, low-phosphorus field soil in a glasshouse. Drought stress was imposed by ceasing to water. A companion M. sativa plant in each pot minimised differences in leaf area and water consumption among species. Plants were harvested when stomatal conductance of stressed plants decreased to around 10% of well watered plants.ResultsA range of responses to drought stress were identified, including: reduced shoot growth; leaf curling; thicker pubescence on leaves and stems; an increased root:shoot ratio; an increase, decrease or no change in root distribution with depth; reductions in specific leaf area or leaf water potential; and osmotic adjustment. The suite of changes differed substantially among species and, less so, among accessions.ConclusionsThe inter- and intra-specific variability of responses to drought-stress in the plants examined suggests a wide range of strategies are available in perennial legumes to cope with drying conditions, and these could be harnessed in breeding/selection programs.

Electrical signalling and cytokinins mediate effects of light and root cutting on ion uptake in intact plants

Nutrient acquisition in the mature root zone is under systemic control by the shoot and the root tip. In maize, exposure of the shoot to light induces short-term (within 1-2 min) effects on net K+ and H+ transport at the root surface. H+ efflux decreased (from -18 to -12 nmol m(-2) s(-1)) and K+ uptake (approximately 2 nmol m(-2) s(-1)) reverted to efflux (approximately -3 nmol m(-2) s(-1)). Xylem probing revealed that the trans-root (electrical) potential drop between xylem vessels and an external electrode responded within seconds to a stepwise increase in light intensity; xylem pressure started to decrease after a approximately 3 min delay, favouring electrical as opposed to hydraulic signalling. Cutting of maize and barley roots at the base reduced H+ efflux and stopped K+ influx in low-salt medium; xylem pressure rapidly increased to atmospheric levels. With 100 mm NaCl added to the bath, the pressure jump upon cutting was more dramatic, but fluxes remained unaffected, providing further evidence against hydraulic regulation of ion uptake. Following excision of the apical part of barley roots, influx changed to large efflux (-50 nmol m(-2) s(-1)). Kinetin (2-4 microM), a synthetic cytokinin, reversed this effect. Regulation of ion transport by root-tip-synthesized cytokinins is discussed.

Commensalism in an agroecosystem: hydraulic redistribution by deep-rooted legumes improves survival of a droughted shallow-rooted legume companion

We investigated commensalism of water use among annual shallow-rooted and perennial deep-rooted pasture legumes by examining the effect of hydraulic lift by Cullen pallidum (N.T.Burb.) J.W.Grimes and Medicago sativa on growth, survival and nutrient uptake of Trifolium subterraneum L. A vertically split-root design allowed separate control of soil water in top and bottom soil. Thirty-five days after watering ceased in the top tube, but soil remained at field capacity in the bottom tube, an increase in shallow soil water content by hydraulic lift was 5.6 and 5.9 g kg(-1) soil overnight for C. pallidum and M. sativa, respectively. Trifolium subterraneum in this treatment maintained higher leaf water potentials (with M. sativa) or exhibited a slower decline (with C. pallidum) than without companion perennial plants; and shoot biomass of T. subterraneum was 56% (with C. pallidum) and 67% (with M. sativa) of that when both top and bottom tubes were at field capacity. Uptake of rubidium (a potassium analog) and phosphorus by T. subterraneum was not facilitated by hydraulic lift. Interestingly, phosphorus content was threefold greater, and shoot biomass 1.5-3.3-fold greater when T. subterraneum was interplanted with C. pallidum compared with M. sativa, although dry weight of C. pallidum was much greater than that of M. sativa. This study showed that interplanting with deep-rooted perennial legumes has benefited the survival of T. subterraneum.

Response of chickpea (Cicer arietinum L.) to terminal drought: leaf stomatal conductance, pod abscisic acid concentration, and seed set

Highlight Exposed to terminal drought, the soil water content at which chickpea seed set ceased coincided with that at which stomatal conductance began to decrease and pod abscisic acid concentration increased.

Amelioration of detrimental effects of waterlogging by foliar nutrient sprays in barley

Six barley (Hordeum vulgare L.) cultivars contrasting in their waterlogging tolerance were subjected to waterlogging for 2 weeks under glasshouse conditions. The adverse effects of waterlogging were significantly alleviated by the foliar spray of nutrients (quarter- or full-strength Hoagland solution) in all cultivars. Beneficial effects of nutrient sprays included improved shoot and root growth and reduced leaf senescence. Chlorophyll content was increased, as was net CO2 assimilation, photochemical efficiency of PSII, and adventitious root production compared with waterlogged plants with no added foliar nutrients. Auxin was found accumulated at the shoot base in waterlogged plants, and the spray of foliar nutrients significantly increased this accumulation in waterlogged plants after 14 days of treatment. Foliar application of 1-NAA also promoted the production of adventitious roots. The highest concentration of auxin was measured in the stem region between 1.2 and 1.6 cm above the shoot-root junction. Foliar nutrient application also improved nitrogen and potassium content in both shoot and root, but calcium content was hardly affected. It is concluded that the improvement of waterlogged plant growth by foliar nutrient sprays could be related to both the improved plant nutrition and increased auxin accumulation in the shoot base, responsible for development of adventitious roots.