Glycine and serine are potential sources of nitrogen for the aquatic resurrection plant Chamaegigas intrepidus Dinter in the rock pools that provide its natural habitat. The pathways by which these amino acids might be utilized were investigated by incubating C. intrepidus roots and maize (Zea mays) root tips with [(15)N]glycine, [(15)N]serine and [2-(13)C]glycine. The metabolic fate of the label was followed using in vivo NMR spectroscopy, and the results were consistent with the involvement of the glycine decarboxylase complex (GDC) and serine hydroxymethyltransferase (SHMT) in the utilization of glycine. In contrast, the labelling patterns provided no evidence for the involvement of serine:glyoxylate aminotransferase in the metabolism of glycine by the root tissues. The key observations were: (i) the release of [(15)N]ammonium during [(15)N]-labelling experiments; and (ii) the detection of a characteristic set of serine isotopomers in the [2-(13)C]glycine experiments. The effects of aminoacetonitrile, amino-oxyacetate, and isonicotinic acid hydrazide, all of which inhibit GDC and SHMT to some extent, and of methionine sulphoximine, which inhibited the reassimilation of the ammonium, supported the conclusion that GDC and SHMT were essential for the metabolism of glycine. C. intrepidus was observed to metabolize serine more readily than the maize root tips and this may be an adaptation to its nitrogen-deficient habitat. Overall, the results support the emerging view that GDC is an essential component of glycine catabolism in non-photosynthetic tissues.
The role of submerged and floating leaves in plant photosynthetic performance of the aquatic resurrection plant Chamaegigas intrepidus Dinter was investigated by monitoring chlorophyll fluorescence under the fluctuating natural field conditions that characterise the extreme habitat of this species. The performance of the two different leaf types during desiccation–rehydration cycles in the field was examined. PSII quantum efficiency indicates a similar regeneration capacity in both leaf types after water stress. Electron transport rates under controlled light conditions were 3–4 times higher in floating leaves than in submerged leaves. The two leaf types showed specific adaptations to their ambient photosynthetic photon flux densities (PPFD), shade tolerance in the submerged leaves and adaptation to high PPFD in floating leaves. These results imply a significant role of the floating leaves for total plant carbon gain. It is concluded that the combination of high N content of floating leaves and a high availability of CO2 and light at the water surface contributes to the importance of this leaf type for photosynthesis in C. intrepidus.
Following a precultivation with pedospheric nitrogen nutrition, nitrate or ammonium solutions were supplied to the shoots of Ricinus plants by spraying (during the experimental period) resulting in an increase of biotic/organic and abiotic/inorganic particles on the surface, which significantly increased wetting of the leaf surfaces. The distribution of particles on the surface of sprayed leaves, in particular crystals around and in stomata, indicated the possible entry of nutrients via thin water films through the stomatal pores in addition to diffusion through the cuticle. Ammonium was taken up more readily than nitrate by the foliage, but both at relatively low rates which caused N limitation. Interestingly, the inorganic N, both in the form of nitrate and even ammonium, was entirely assimilated in the shoots; phloem transport of inorganic N to the root was negligible. The flows of malate, and the acidification of the apoplastic washing solution of leaves in ammonium‐sprayed plants pointed to the role of metabolism of malate and excretion of protons in maintaining pH during ammonium assimilation in the shoot. Ammonium‐sprayed plants incorporated the N in the same amounts in shoots and roots, only 38% of the shoot‐borne N being recycled in the xylem. In nitrate‐sprayed plants the root was not only favoured in N partitioning, but even a net export of previously incorporated N from the shoots occurred which reflected the N limitation. The N limitation also affected carbon metabolism, in particular the flows of C, incorporation in the shoot and photosynthesis, which were decreased when compared with data from recent experiments with pedospheric well fed Ricinus . However, there was little difference in C flows between nitrate and ammonium‐sprayed plants with respect to respiration, C partitioning and, most interestingly, in relative stimulation of root growth. The loss of C from dark respiration of the shoots was high on a f. wt basis as well as in relative terms, owing to exclusive N assimilation in the shoot. In general the plants invested untargeted increases in root growth as a result of N limitation irrespective of the imposed artificial treatment which made the shoot the site of mineral N uptake.
In this study the impact of salt stress on the physiology and wood structure of the salt-sensitive Populus £ canescens was investigated. Two weeks of salt stress altered wood anatomy signiWcantly. The xylem diVerentiation zone was reduced and the resulting vessels exhibited reduced lumina. To understand this phenomenon, ion composition, levels of corresponding transcripts and of the stress hormone ABA were analysed. With increasing sodium and chloride concentrations, a general reduction of potassium was found in roots and shoots, but not in leaves. Consequently, the corresponding K + channel transcripts in roots favoured K + release. The overall osmolarity in leaves was up to fourfold higher than in roots or shoots. Therefore, adjustment of the K + /Na + balance seemed not to be required in leaves. Sodium increased gradually from roots to shoots and then to leaves indicating that sodium storage took place Wrst in roots, then in shoots, and Wnally in leaves to protect photosynthesis from salt eVects as long as possible. Since leaf abscisic acid levels markedly increased, stomatal clo- sure seemed to limit CO 2 uptake. As a consequence, dimin- ished nutrient supply to the cambium in combination with lowered shoot K + content led to decreased vessel lumina, and a reduction of the radial cambium was observed. Thus, xylem diVerentiation was curtailed and the development of full size vessels was impaired.
Using modified compartmental analysis the unidirectional fluxes of abscisic acid (ABA) and their cytoplasmic and vacuolar contents in 3H-ABA preloaded barley root segments (Hordeum distichon cv. Aura) have been studied. When root segments were stressed osmotically with sorbitol (osmotic potential of the media π0 = 0.2 MPa) cytoplasmic and vacuolar contents of ABA were enhanced. Under increased stress cytoplasmic and vacuolar contents were much lower than in the unstressed controls. ABA fluxes were very sensitive to osmotic stress and ABA transport from the cytoplasm of the xylem parenchyma to the xylem vessels (φcx) was rapidly inhibited. The cultivar Aura has higher cytoplasmic and vacuolar ABA contents than the barley cultivar Kocherperle. This correlates well with the higher stress tolerance of the Aura cultivar.
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