Chemolithoautotrophic Nitrifiers in the Phyllosphere of a Spruce Ecosystem Receiving High Atmospheric Nitrogen Input

Evidence is presented for the first time that chemolithoautotrophic ammonia oxidizers (CAO) and chemolithoautotrophic nitrite oxidizers (CNO) colonize in appreciable cell numbers the phyllosphere of spruce trees in a forest ecosystem exposed for decades to high levels of atmospheric nitrogen (The Höglwald Forest, Bavaria, Germany). The results strongly indicate that both, CAO and CNO are predominantely located inside the spruce needles, most likely within the stomatal cavity. These results are further supported by field experiments of NH₃ uptake into twigs on intact spruce trees in the presence and absence of 10 Pa acetylene, an inhibitor of the ammonia monooxygenase of CAO. It is clearly demonstrated for the first time that in situ uptake of NH₃ from the atmosphere into spruce needles exposed to high levels of atmospheric N is not catalyzed exclusively by the tree, but is the result of combined activities of both, the spruce trees and the chemolithoautotrophic nitrifiers colonizing the needles.     

Impact of waterlogging on the N-metabolism of flood tolerant and non-tolerant tree species

The present study was conducted to characterize the N‐metabolism of important European tree species with different degrees of flooding tolerance. The roots of Fagus sylvatica (sensitive to flooding), Quercus robur (moderately flood tolerant) and Populus tremula × P. alba (flood tolerant) saplings were exposed to different flooding regimes and N uptake, amino acid, protein and chlorophyll concentrations as well as gas exchange were measured. The effects of these treatments on the tree species varied distinctly. In general, the N metabolism of beech was severely affected whereas less impacts were observed on oaks and almost no effects on poplars. The concentrations of amino compounds, particularly of Asp, Asn, Glu and Gln, were lower in the roots of flooded trees than in controls. By contrast, γ‐amino butyric acid concentrations increased. Root protein concentrations remained unaffected in oak and poplar but decreased in beech in response to flooding. The concentrations of pigments remained unaffected by flooding in all tree species investigated. However, photosynthesis and transpiration were severely affected in beech but much less in oak and poplar. The data obtained show a clear correlation between the different flooding tolerances of the trees investigated and the impacts of flooding on N uptake and N metabolism.     

Consequences of Sphaeropsis tip blight disease for the phytohormone profile and antioxidative metabolism of its pine host

Phytopathogenic fungi infections induce plant defence responses that mediate changes in metabolic and signalling processes with severe consequences for plant growth and development. Sphaeropsis tip blight, induced by the endophytic fungus Sphaeropsis sapinea that spreads from stem tissues to the needles, is the most widespread disease of conifer forests causing dramatic economic losses. However, metabolic consequences of this disease on bark and wood tissues of its host are largely unexplored. Here, we show that diseased host pines experience tissue dehydration in both bark and wood. Increased cytokinin and declined indole‐3‐acetic acid levels were observed in both tissues and increased jasmonic acid and abscisic acid levels exclusively in the wood. Increased lignin contents at the expense of holo‐cellulose with declined structural biomass of the wood reflect cell wall fortification by S. sapinea infection. These changes are consistent with H₂O₂ accumulation in the wood, required for lignin polymerization. Accumulation of H₂O₂ was associated with more oxidized redox states of glutathione and ascorbate pools. These findings indicate that S. sapinea affects both phytohormone signalling and the antioxidative defence system in stem tissues of its pine host during the infection process.     

Photosynthesis, Soluble and Structural Carbon Compounds in Two Mediterranean Oak Species (Quercus pubescens and Q. ilex) after Lifetime Growth at Naturally Elevated CO2 Concentrations

Abstract: To study physiological responses of mature forest trees to elevated CO₂ after lifetime growth under elevated atmospheric CO₂ concentrations ( p CO₂), photosynthesis, Rubisco content, foliar concentrations of soluble sugars and starch, sugar concentrations in transport tissues (phloem and xylem), structural biomass, and lignin in leaves and branches were investigated in 30- to 50-year-old Quercus pubescens and Q. ilex trees grown at two naturally elevated CO₂ springs in Italy. Ribulose-1,5-bisphosphate carboxylase/oxygenase content was decreased in Q. pubescens grown under elevated CO₂ concentrations, but not in Q. ilex. Photosynthesis was consistently higher in Q. pubescens grown at elevated CO₂ as compared with "control" sites, whereas the response in Q. ilex was less pronounced. Stomatal conductance was lower in both species leading to decreased transpiration and increased instantaneous water use efficiency in Q. pubescens. Overall mean sugar + starch concentrations of the leaves were not affected by elevated p CO₂, but phloem exudates contained higher concentrations of soluble sugars. This finding suggests increased transport to sinks. Qualitative changes in major carbon-bearing compounds, such as structural biomass and lignins, were only found in bark but not in other tissues. These results support the concept that the maintenance of increased rates of photosynthesis after long-term acclimation to elevated p CO₂ provides a means of optimization of water relations under arid climatic conditions but does not cause an increase in aboveground carbon sequestration per unit of tissue in Mediterranean oak species.     

Formation of methanethiol from methionine by leaf tissue

Leaf discs, but not detached leaves, exposed to L-methionine or S-methyl-L-cysteine emitted a volatile sulphur compound identified as methanethiol by different trapping systems and by GC. Methanethiol emission was analyzed using pumpkin (Cucurbita pepo) leaf discs. Emission was observed in darkness or light, however methanethiol emission was greately stimulated by light. Light-dependent emission started after a lag-time of 5–6 hr with an emission peak after 36–40 hr. Maximum rates obtained were in the range of 200 pmol methanethiol/min/cm² leaf area. After a period of 42 hr about 60–80% of total methionine sulphur added was released as methanethiol. Addition of chloramphenicol did not alter the induction period nor the maximum emission rate of methanethiol in response to L-methionine. Emission was also observed in response to S-methyl-L-cysteine; however, the shorter lag-period for methanethiol formation suggests metabolism via a different enzyme system. In a cell-free system of pumpkin leaves methanethiol formation occured in response to L-methionine. Feeding experiments with L-[³⁵S]methionine to leaf discs showed that more than 80% of methanethiol emitted was derived from the labelled methionine fed. These findings suggest that plants have the capacity to degrade L-methionine to methanethiol. Whole leaves fed L-methionine by the petiole system do not emit methanethiol, but this compound is formed and transported into the feeding solution. Thus, methanethiol is also produced by the intact leaf, but, in contrast to sulphide, is not released into the atmosphere. It is suggested that translocation of methanethiol may function as a signal for the regulation of sulphate uptake.     

Growth and protection against oxidative stress in young clones and mature spruce trees (Picea abies L.) at high altitudes

Clones of Norway spruce (Picea abies L.) were grown for several years on an altitudinal gradient (1750 m, 1150 m and 800 m above sea level) to study the effects of environmental × genetic interactions on growth and foliar metabolites (protein, pigments, antioxidants). Clones at the tree line showed 4.3-fold lower growth rates and contained 60% less chlorophyll (per gram of dry matter) than those at valley level. The extent of growth reduction was clone-dependent. The mortality of the clones was low and not altitude-dependent. At valley level, but not at high altitude, needles of mature spruce trees showed lower pigment and protein concentrations than clones. In general, antioxidative systems in needles of the mature trees and young clones did not increase with increasing altitude. Needles of all trees at high altitude showed higher concentrations of dehydroascorbate than at lower altitudes, indicating higher oxidative stress. In one clone, previously identified as sensitive to acute ozone doses, this increase was significantly higher and the growth reduction was stronger than in the other genotypes. This clone also displayed a significant reduction in glutathione reductase activity at high altitude. These results suggest that induction of antioxidative systems is apparently not a general prerequisite to cope with altitude in clones whose mother plants originated from higher altitudes (about 650–1100 m above sea level, Hercycnic-Carpathian distribution area), but that the genetic constitution for maintenance of high antioxidative protection is important for stress compensation at the tree line. Received: 13 October 1998 / Accepted: 22 June 1999     

S-methylmethionine plays a major role in phloem sulfur transport and is synthesized by a novel type of methyltransferase.

All flowering plants produce S-methylmethionine (SMM) from Met and have a separate mechanism to convert SMM back to Met. The functions of SMM and the reasons for its interconversion with Met are not known. In this study, by using the aphid stylet collection method together with mass spectral and radiolabeling analyses, we established that l-SMM is a major constituent of the phloem sap moving to wheat ears. The SMM level in the phloem ( approximately 2% of free amino acids) was 1.5-fold that of glutathione, indicating that SMM could contribute approximately half the sulfur needed for grain protein synthesis. Similarly, l-SMM was a prominently labeled product in phloem exudates obtained by EDTA treatment of detached leaves from plants of the Poaceae, Fabaceae, Asteraceae, Brassicaceae, and Cucurbitaceae that were given l-(35)S-Met. cDNA clones for the enzyme that catalyzes SMM synthesis (S-adenosylMet:Met S-methyltransferase; EC 2.1.1.12) were isolated from Wollastonia biflora, maize, and Arabidopsis. The deduced amino acid sequences revealed the expected methyltransferase domain ( approximately 300 residues at the N terminus), plus an 800-residue C-terminal region sharing significant similarity with aminotransferases and other pyridoxal 5'-phosphate-dependent enzymes. These results indicate that SMM has a previously unrecognized but often major role in sulfur transport in flowering plants and that evolution of SMM synthesis in this group involved a gene fusion event. The resulting bipartite enzyme is unlike any other known methyltransferase.     

Nitrogen nutrition of beech forests in a changing climate: importance of plant-soil-microbe water,carbon, and nitrogen interactions

Background

For 15+ years, a beech (Fagus sylvatica L.) dominated forest on calcareous soil was studied on two opposing slopes with contrasting microclimate in Tuttlingen, Swabian Alb, Germany. The cool-humid NE aspect of these slopes represents the majority of beech forests under current climate, the warmer and drier SW aspect represents beech forests under future climate conditions. The field studies were supplemented by investigations under controlled conditions.

Scope

The research program aimed to provide a comprehensive understanding of plant-soil-microbe water, carbon and nitrogen feedbacks in a changing climate and a holistic view of the sensitivity of beech to climate change.

Conclusions

The results of comparative and experimental studies underpin the high vulnerability of adult beech and its natural regeneration on calcareous soil to both direct climate change effects on plant physiology and indirect effects mediated by soil biogeochemical cycles. Mechanisms contributing to this vulnerability at the ecosystem and organismic level indicate a high significance of competitive interactions of beech with other vegetation components and soil microbial communities. Obvious forest management practices such as selective felling did not necessarily counteract negative effects of climate change.