Physiological impacts of magnesium-deficiency in Citrus seedlings: photosynthesis, antioxidant system and carbohydrates

Magnesium (Mg)-deficiency affects productivity and quality in agriculture, yet at a physiological level it is not well understood. Citrus grandis and Citrus sinensis seedlings were irrigated for 12?weeks with 0, 50, 500 or 2,000?μM MgSO₄. Thereafter, Mg-deficiency-induced changes in photosynthesis, antioxidant system and carbohydrates were investigated. Mg-deficiency affected CO₂ assimilation more in C. grandis leaves than in C. sinensis ones, but Mg-deficiency-induced accumulation of sugars was not higher in the former except for sucrose. Mg-deficiency-induced photoinhibitory impairment occurring on the whole photosynthetic electron transport chain was more severe in C. grandis leaves than in C. sinensis ones. Mg-deficient leaves had higher or similar activities of antioxidant enzymes and contents of antioxidant metabolites except for catalase (CAT) activity and reduced glutathione (GSH) content. However, Mg-deficiency increased leaf malondialdehyde (MDA) content. In conclusion, the greater decrease in CO₂ assimilation in Mg-deficient C. grandis leaves may be caused by the greater decrease in the photosynthetic electron transport capacity. Mg-deficiency-induced up-regulation in leaf antioxidant system does not provide enough protection to Mg-deficient leaves against the oxidative damage.     

Magnesium deficiency-induced changes in organic acid metabolism of Citrus sinensis roots and leaves

Organic acid (OA) metabolisms are of fundamental importance but very limited data are available on the responses of plant OA metabolisms to Mg-deficiency. Seedlings of Citrus sinensis (L.) Osbeck cv. Xuegan were irrigated with Mg-deficient (0, 50, or 500 μM MgSO₄) or Mg-sufficient (2000 μM MgSO₄) nutrient solution every other day for 12 weeks. Thereafter, we investigated the content of Mg, malate, and citrate as well as the activities of acidmetabolizing enzymes in roots and leaves. Root malate content remained stable except for an increase in the highest Mg content and root citrate content increased with increasing root Mg content. As leaf Mg content increased, leaf malate and malate + citrate content decreased whereas leaf citrate content increased. Mg-deficiency decreased or did not affect activities of citrate synthase (CS), aconitase (ACO), phosphoenolpyruvate carboxylase (PEPC), NADP-isocitrate dehydrogenase (NADP-IDH), NAD-malate dehydrogenase (NAD-MDH), NADP-malic enzyme (NADP-ME), and pyruvate kinase (PK) in roots, whereas phosphoenolpyruvate phosphatase (PEPP) activity slightly increased. In contrast, Mg-deficient leaves had higher or similar activities of enzymes above mentioned except PEPP, NAD-MDH, and NADP-ME. In conclusion, both glycolysis and tricarboxylic acid (TCA) cycle may be up-regulated in Mg-deficient leaves but down-regulated in Mg-deficient roots.     

Spatial patterns of photosynthesis in thin- and thick-leaved epiphytic orchids: unravelling C3–CAM plasticity in an organ-compartmented way

Background and Aims

A positive correlation between tissue thickness and crassulacean acid metabolism (CAM) expression has been frequently suggested. Therefore, this study addressed the question of whether water availability modulates photosynthetic plasticity in different organs of two epiphytic orchids with distinct leaf thickness.

Methods

Tissue morphology and photosynthetic mode (C₃ and/or CAM) were examined in leaves, pseudobulbs and roots of a thick-leaved (Cattleya walkeriana) and a thin-leaved (Oncidium ‘Aloha’) epiphytic orchid. Morphological features were studied comparing the drought-induced physiological responses observed in each organ after 30 d of either drought or well-watered treatments.

Key Results

Cattleya walkeriana, which is considered a constitutive CAM orchid, displayed a clear drought-induced up-regulation of CAM in its thick leaves but not in its non-leaf organs (pseudobulbs and roots). The set of morphological traits of Cattleya leaves suggested the drought-inducible CAM up-regulation as a possible mechanism of increasing water-use efficiency and carbon economy. Conversely, although belonging to an orchid genus classically considered as performing C₃ photosynthesis, Oncidium ‘Aloha’ under drought seemed to express facultative CAM in its roots and pseudobulbs but not in its leaves, indicating that such photosynthetic responses might compensate for the lack of capacity to perform CAM in its thin leaves. Morphological features of Oncidium leaves also indicated lower efficiency in preventing water and CO₂ losses, while aerenchyma ducts connecting pseudobulbs and leaves suggested a compartmentalized mechanism of nighttime carboxylation via phosphoenolpyruvate carboxylase (PEPC) (pseudobulbs) and daytime carboxylation via Rubisco (leaves) in drought-exposed Oncidium plants.

Conclusions

Water availability modulated CAM expression in an organ-compartmented manner in both orchids studied. As distinct regions of the same orchid could perform different photosynthetic pathways and variable degrees of CAM expression depending on the water availability, more attention should be addressed to this in future studies concerning the abundance of CAM plants.     

What is the most prominent factor limiting photosynthesis in different layers of a greenhouse cucumber canopy?

Background and Aims

Maximizing photosynthesis at the canopy level is important for enhancing crop yield, and this requires insights into the limiting factors of photosynthesis. Using greenhouse cucumber (Cucumis sativus) as an example, this study provides a novel approach to quantify different components of photosynthetic limitations at the leaf level and to upscale these limitations to different canopy layers and the whole plant.

Methods

A static virtual three-dimensional canopy structure was constructed using digitized plant data in GroIMP. Light interception of the leaves was simulated by a ray-tracer and used to compute leaf photosynthesis. Different components of photosynthetic limitations, namely stomatal (S_L), mesophyll (M_L), biochemical (B_L) and light (L_L) limitations, were calculated by a quantitative limitation analysis of photosynthesis under different light regimes.

Key Results

In the virtual cucumber canopy, B_L and L_L were the most prominent factors limiting whole-plant photosynthesis. Diffusional limitations (S_L + M_L) contributed <15 % to total limitation. Photosynthesis in the lower canopy was more limited by the biochemical capacity, and the upper canopy was more sensitive to light than other canopy parts. Although leaves in the upper canopy received more light, their photosynthesis was more light restricted than in the leaves of the lower canopy, especially when the light condition above the canopy was poor. An increase in whole-plant photosynthesis under diffuse light did not result from an improvement of light use efficiency but from an increase in light interception. Diffuse light increased the photosynthesis of leaves that were directly shaded by other leaves in the canopy by up to 55 %.

Conclusions

Based on the results, maintaining biochemical capacity of the middle–lower canopy and increasing the leaf area of the upper canopy would be promising strategies to improve canopy photosynthesis in a high-wire cucumber cropping system. Further analyses using the approach described in this study can be expected to provide insights into the influences of horticultural practices on canopy photosynthesis and the design of optimal crop canopies.     

Recovery dynamics of growth, photosynthesis and carbohydrate accumulation after de-submergence: a comparison between two wetland plants showing escape and quiescence strategies

Background and Aims

The capacity for fast-growth recovery after de-submergence is important for establishment of riparian species in a water-level-fluctuation zone. Recovery patterns of two wetland plants, Alternanthera philoxeroides and Hemarthria altissima, showing ‘escape’ and ‘quiescence’ responses, respectively, during submergence were investigated.

Methods

Leaf and root growth and photosynthesis were monitored continuously during 10 d of recovery following 20 d of complete submergence. Above- and below-ground dry weights, as well as carbohydrate concentrations, were measured several times during the experiment.

Key Results

Both species remobilized stored carbohydrate during submergence. Although enhanced internode elongation depleted the carbohydrate storage in A. philoxeroides during submergence, this species resumed leaf growth 3 d after de-submergence concomitant with restoration of the maximal photosynthetic capacity. In contrast, some sucrose was conserved in shoots of H. altissima during submergence, which promoted rapid re-growth of leaves 2 d after de-submergence and earlier than the full recovery of photosynthesis. The recovery of root growth was delayed by 1–2 d compared with leaves in both species.

Conclusions

Submergence tolerance of the escape and quiescence strategies entails not only the corresponding regulation of growth, carbohydrate catabolism and energy metabolism during submergence but also co-ordinated recovery of photosynthesis, growth and carbohydrate partitioning following de-submergence.     

Mistletoes and mutant albino shoots on woody plants as mineral nutrient traps

Background and Aims

Potassium, sulphur and zinc contents of mistletoe leaves are generally higher than in their hosts. This is attributed to the fact that chemical elements which are cycled between xylem and phloem in the process of phloem loading of sugars are trapped in the mistletoe, because these parasites do not feed their hosts. Here it is hypothesized that mutant albino shoots on otherwise green plants should behave similarly, because they lack photosynthesis and thus cannot recycle elements involved in sugar loading.

Methods

The mineral nutrition of the mistletoe Scurrula elata was compared with that of albino shoots on Citrus sinensis and Nerium oleander. The potential for selective nutrient uptake by the mistletoe was studied by comparing element contents of host leaves on infected and uninfected branches and by manipulation of the haustorium–shoot ratio in mistletoes. Phloem anatomy of albino leaves was compared with that of green leaves.

Key Results

Both mistletoes and albino leaves had higher contents of potassium, sulphur and zinc than hosts or green leaves, respectively. Hypothetical discrimination of nutrient elements during the uptake by the haustorium is not supported by our data. Anatomical studies of albino leaves showed characteristics of release phloem.

Conclusions

Both albino shoots and mistletoes are traps for elements normally recycled between xylem and phloem, because retranslocation of phloem mobile elements into the mother plant or the host is low or absent. It can be assumed that the lack of photosynthetic activity in albino shoots and thus of sugars needed in phloem loading is responsible for the accumulation of elements. The absence of phloem loading is reflected in phloem anatomy of these abnormal shoots. In mistletoes the evolution of a parasitic lifestyle has obviously eliminated substantial feeding of the host with photosynthates produced by the mistletoe.     

Peroxynitrite (ONOO?) is endogenously produced in arabidopsis peroxisomes and is overproduced under cadmium stress

Background and Aims

Peroxisomes are subcellular compartments involved in multiple cellular metabolic pathways. Peroxynitrite (ONOO⁻) is a nitric oxide-derived molecule which is a nitrating species that causes nitration of proteins. This study used cell biology techniques to explore the potential presence of peroxynitrite in peroxisomes and evaluated its content under stress conditions (excess cadmium).

Methods

Peroxynitrite, nitric oxide and superoxide anion were studied using cell-permeable specific fluorescent probes by confocal laser scanning microscopy in Arabidopsis thaliana transgenic plants expressing cyan fluorescent protein through the addition of peroxisomal targeting signal 1 (PTS1), which enables peroxisomes to be visualized in vivo.

Key Results

When no stress was applied, peroxynitrite was clearly localized in the peroxisomes of roots and stomatal guard cells. Under cadmium (150 μm) stress, the generation of peroxynitrite, nitric oxide and the superoxide anion (O₂^·–) increased and was localized in peroxisomes and the cytosol, participating in the generation of nitro-oxidative stress.

Conclusions

The results show that peroxisomes are an endogenous source of peroxynitrite, which is over-produced under cadmium stress, suggesting that the metabolism of reactive nitrogen species in peroxisomes could participate in the mechanism of the response to this heavy metal.     

Cluster-root formation and carboxylate release in three Lupinus species as dependent on phosphorus supply,internal phosphorus concentration and relative growth rate

Background and Aims

Some Lupinus species produce cluster roots in response to low plant phosphorus (P) status. The cause of variation in cluster-root formation among cluster-root-forming Lupinus species is unknown. The aim of this study was to investigate if cluster-root formation is, in part, dependent on different relative growth rates (RGRs) among Lupinus species when they show similar shoot P status.

Methods

Three cluster-root-forming Lupinus species, L. albus, L. pilosus and L. atlanticus, were grown in washed river sand at 0, 7·5, 15 or 40 mg P kg⁻¹ dry sand. Plants were harvested at 34, 42 or 62 d after sowing, and fresh and dry weight of leaves, stems, cluster roots and non-cluster roots of different ages were measured. The percentage of cluster roots, tissue P concentrations, root exudates and plant RGR were determined.

Key Results

Phosphorus treatments had major effects on cluster-root allocation, with a significant but incomplete suppression in L. albus and L. pilosus when P supply exceeded 15 mg P kg⁻¹ sand. Complete suppression was found in L. atlanticus at the highest P supply; this species never invested more than 20 % of its root weight in cluster roots. For L. pilosus and L. atlanticus, cluster-root formation was decreased at high internal P concentration, irrespective of RGR. For L. albus, there was a trend in the same direction, but this was not significant.

Conclusions

Cluster-root formation in all three Lupinus species was suppressed at high leaf P concentration, irrespective of RGR. Variation in cluster-root formation among the three species cannot be explained by species-specific variation in RGR or leaf P concentration.     

Proteomic changes of Citrus roots in response to long-term manganese toxicity

Key message

Fifty-three and thirty-nine differentially expressed protein spots were isolated from Mn-toxic Citrus sinensis and Citrus grandis roots, respectively. Mn-toxicity-induced changes in protein profiles greatly differed between the two species.

Abstract

Limited information is available on the manganese (Mn)-toxicity-responsive proteins in plant roots. ‘Sour pummelo’ (Citrus grandis) and ‘Xuegan’ (Citrus sinensis) seedlings were irrigated for 17 weeks with 2 (control) or 600 μM (Mn-toxic) MnSO₄. C. sinensis displayed more tolerance to Mn-toxicity than C. grandis, which may be related to more Mn accumulation in roots and less Mn distribution in shoots. Using two-dimensional electrophoresis (2-DE), we isolated 11 up-regulated and 42 down-regulated protein spots from Mn-toxic C. sinensis roots, and 25 up-regulated and 14 down-regulated protein spots from Mn-toxic C. grandis roots. This indicated more metabolic flexibility in C. sinensis roots, thus contributing to the Mn-tolerance of C. sinensis. According to the biological functional properties, these differentially expressed proteins in the two species were classified into the following categories: protein metabolism, nucleic acid metabolism, carbohydrate and energy metabolism, stress responses, cell wall and cytoskeleton, cell transport, signal transduction and fatty acid metabolism. Under Mn-toxicity, proteins involved in nucleic acid metabolism, glycolysis and cell transport were up-regulated in nontolerant C. grandis roots, and down-regulated in tolerant C. sinensis roots. The notable down-regulation of proteins in Mn-toxic C. sinensis roots with less accumulation of carbohydrates may provide an advantage to the net carbon balance by lowering related metabolic processes, and enhancing the Mn-tolerance of C. sinensis. To conclude, there are many important differences in Mn-toxicity-induced changes in protein profiles and metabolic responses between the two species.     

Characterization of a rice variety with high hydraulic conductance and identification of the chromosome region responsible using chromosome segment substitution lines

Background and Aims

The rate of photosynthesis in paddy rice often decreases at noon on sunny days because of water stress, even under submerged conditions. Maintenance of higher rates of photosynthesis during the day might improve both yield and dry matter production in paddy rice. A high-yielding indica variety, ‘Habataki’, maintains a high rate of leaf photosynthesis during the daytime because of the higher hydraulic conductance from roots to leaves than in the standard japonica variety ‘Sasanishiki’. This research was conducted to characterize the trait responsible for the higher hydraulic conductance in ‘Habataki’ and identified a chromosome region for the high hydraulic conductance.

Methods

Hydraulic conductance to passive water transport and to osmotic water transport was determined for plants under intense transpiration and for plants without transpiration, respectively. The varietal difference in hydraulic conductance was examined with respect to root surface area and hydraulic conductivity (hydraulic conductance per root surface area, L_p). To identify the chromosome region responsible for higher hydraulic conductance, chromosome segment substitution lines (CSSLs) derived from a cross between ‘Sasanishiki’ and ‘Habataki’ were used.

Key Results

The significantly higher hydraulic conductance resulted from the larger root surface area not from L_p in ‘Habataki’. A chromosome region associated with the elevated hydraulic conductance was detected between RM3916 and RM2431 on the long arm of chromosome 4. The CSSL, in which this region was substituted with the ‘Habataki’ chromosome segment in the ‘Sasanishiki’ background, had a larger root mass than ‘Sasanishiki’.

Conclusions

The trait for increasing plant hydraulic conductance and, therefore, maintaining the higher rate of leaf photosynthesis under the conditions of intense transpiration in ‘Habataki’ was identified, and it was estimated that there is at least one chromosome region for the trait located on chromosome 4.     

Aquatic adventitious root development in partially and completely submerged wetland plants Cotula coronopifolia and Meionectes brownii

Background and Aims

A common response of wetland plants to flooding is the formation of aquatic adventitious roots. Observations of aquatic root growth are widespread; however, controlled studies of aquatic roots of terrestrial herbaceous species are scarce. Submergence tolerance and aquatic root growth and physiology were evaluated in two herbaceous, perennial wetland species Cotula coronopifolia and Meionectes brownii.

Methods

Plants were raised in large pots with ‘sediment’ roots in nutrient solution and then placed into individual tanks and shoots were left in air or submerged (completely or partially). The effects on growth of aquatic root removal, and of light availability to submerged plant organs, were evaluated. Responses of aquatic root porosity, chlorophyll and underwater photosynthesis, were studied.

Key Results

Both species tolerated 4 weeks of complete or partial submergence. Extensive, photosynthetically active, aquatic adventitious roots grew from submerged stems and contributed up to 90 % of the total root dry mass. When aquatic roots were pruned, completely submerged plants grew less and had lower stem and leaf chlorophyll a, as compared with controls with intact roots. Roots exposed to the lowest PAR (daily mean 4·7 ± 2·4 µmol m⁻² s⁻¹) under water contained less chlorophyll, but there was no difference in aquatic root biomass after 4 weeks, regardless of light availability in the water column (high PAR was available to all emergent shoots).

Conclusions

Both M. brownii and C. coronopifolia responded to submergence with growth of aquatic adventitious roots, which essentially replaced the existing sediment root system. These aquatic roots contained chlorophyll and were photosynthetically active. Removal of aquatic roots had negative effects on plant growth during partial and complete submergence.