Effects of hypoxia and elevated carbon dioxide concentration on water flux through Populus roots

Source-sink relations of sunflower plants as affected by a parasite modifies carbon allocations and leaf traits

Sunflower broomrape (Orobanche cumana) is a root holoparasitic plant causing major damage to sunflower (Helianthus annuus L.). Parasite infection initiates source-sink relations between the parasite (sink) and the host (source), allocating carbohydrates, water and nutrients to the parasite. The primary aim of the current study was to explore responses of sunflower to broomrape parasitism, specifically to examine alternations in leaf area, leaf mass per area (LMA), mesophyll structure and root hydraulic conductivity. Leaf changes revealed modifications similar to described previously in shade adapted plants, causing larger and thinner leaves. These traits were accompanied with significantly higher root hydraulics. These changes were caused by carbohydrate depletion due to source-sink relationships between the host and parasite. An Imazapic herbicide (ALS inhibitor) was used for controlling broomrape attachments and by to investigate the plasticity of the traits found. Broomrape infected plants which were treated with Imazapic had leaves similar to non-infected plants, including mesophyll structure and carbon assimilation rates. These results demonstrated source-sink effects of broomrape which cause a low-light-like acclimation behavior which is reversible.

Effect of hypoxic acclimation on anoxia tolerance in Vitis roots: response of metabolic activity and K+ fluxes

The effect of a hypoxic pre-treatment (HPT) on improving tolerance to prolonged anoxia conditions in two contrasting Vitis species (V. riparia, anoxia tolerant; V. rupestris, anoxia sensitive) was evaluated. The energy economy of root cells was studied by measuring heat production, the activity of pyruvate decarboxylase (PDC) and alcohol dehdrogenase (ADH), ethanol and ATP production, and K(+) fluxes. The results showed that HPT is an effective tool in order to maintain a sustainable metabolic performance in both the species under anoxia conditions, especially in sensitive species such as V. rupestris. Our results showed that the improved tolerance was mainly driven by: (i) an enhanced activity of key enzymes in alcohol fermentation (ADC and PDC); (ii) the capability to maintain a higher level of respiration, evidenced by a lesser decrease in heat development and ATP production; and (iii) the maintenance of a better ion homeostasis (highlighted by measurement of K(+) fluxes) and K(+) channel functionality.

Radial growth of Pinus sylvestris growing on alluvial terraces is sensitive to water-level fluctuations

Along the Alpine river Lech (Tyrol, Austria), poorly grown Scots pine (Pinus sylvestris) stands dominate on dry alluvial terraces, which are made up of coarse calcareous gravel. Here we evaluated the impact of environmental factors, such as precipitation, temperature and water table, on annual variability of radial growth. Tree-ring chronologies from six stands comprising different age classes were developed by extracting two core samples from more than 15 trees per plot. Pearson correlations were applied to determine the influence of environmental factors. Close inverse correlations (r2 > 0.35) between maximum water table during the growing period and annual increments of adult stands indicated that water surplus in the rooting zone was the primary growth-limiting factor. Drought stress reduced growth only in some extremely dry years. Results provide evidence that dominance of P. sylvestris on gravelly alluvial terraces is caused not only by tolerance of soil dryness and nutrient deficiency, as previously assumed, but also by a dynamic multilayered root system which allows adaptation to highly variable soil-moisture conditions.

Laboratory-scale measurements of N2O and CH4 emissions from hybrid poplars (Populus deltoides x Populus nigra)

The purpose of this study was to determine whether or not young hybrid poplar (Populus deltoides x Populus nigra) could transport landfill biogas internally from the root zone to the atmosphere, thereby acting as conduits for landfill gas release. Fluxes of methane (CH4) and nitrous oxide (N2O) from the seedlings to the atmosphere were measured under controlled conditions using dynamic flux chambers and a tunable diode laser trace gas analyser (TDLTGA). Nitrous oxide was emitted from the seedlings, but only when extremely high soil N2O concentrations were applied to the root zone. In contrast, no detectable emissions of CH4 were measured in a similar experimental trial. Visible plant morphological responses, characteristic of flood-tolerant trees attempting to cope with the negative effects of soil hypoxia, were observed during the CH4 experiments. Leaf chlorosis, leaf abscission and adventitious roots were all visible plant responses. In addition, seedling survival was observed to be highest in the biogas 'hot spot' areas of a local municipal solid waste landfill involved in this study. Based on the available literature, these observations suggest that CH4 can be transported internally by Populus deltoides x Populus nigra seedlings in trace amounts, although future research is required to fully test this hypothesis.

Assimilate movement dictates remote sites of wound-induced gene expression in poplar leaves

When a single leaf on a young poplar tree is mechanically wounded, wound-induced (win) mRNAs are detected in the unwounded portion of that leaf and in specific leaves that are remote from the wounded leaf. Shortly after wounding (6-8 hr), the remote leaves in which win genes are expressed can be predicted by a knowledge of photoassimilate movement patterns in vivo. When assimilate movement from a wounded leaf is blocked or the direction of assimilate movement is altered by shading, win gene expression in remote leaves is similarly blocked or altered. These data illustrate how the long-distance transduction of wound-induced signals can be manipulated in plants by altering carbon allocation.

Root hydraulic conductivity and xylem sap levels of zeatin riboside and abscisic acid in ectomycorrhizal Douglas fir seedlings

Mechanistic hypotheses to explain mycorrhizal enhancement of root hydraulic conductivity (L_p ) suggest that phosphorus (P) nutrition, plant growth substances and/or altered morphology may be responsible. Such ideas are based on work with VA (vesicular-arbuscular) mycorrhizas. Since VA mycorrhizas and ectomycorrhizas differ in many respects, they may alter host plant water uptake via different mechanisms. This paper examines L_T in various ectomycorrhizal associations while considering factors which are important to the VA mycorrhizal effect on L_p . Douglas fir Pseudotsuga menziesii (Mirb.) Franco] seedlings inoculated with the ectomycorrhizal fungi Laccaria bicolor (Maire) Orton and Hebeloma crustuliniforme (Bull, ex St. Amans) Quel. and non-inoculated seedlings infected naturally with Thelephora were grown under three low levels of P fertilization (1, 10 and 100 fim P). Seedling morphology, tissue P levels, L_p and plant growth substance levels in xylem sap were measured after nine months growth. Increased tissue P and decreased root/shoot ratio correlated with increased L_p in each of the mycorrhizal treatments. When adjusted for the effect of these two factors, L_p of Laccaria and Hebeloma seedlings was still lower than the Thelephora seedlings. In a subsequent experiment, the L_p of seedlings with Hebeloma and Rhizopogon vinicolor Smith mycorrhizas was compared to the L_p of non-mycorrhizal seedlings (grown at 100 mM P) and no differences were found among treatments. The lack of an ectomycorrhizal effect on L_p is quite different from the enhancement of host L_p by VA mycorrhizas. Zeatin riboside concentrations of Thelephora- and Hebeloma-iniected seedlings were similar, yet higher than with Laccaria. There was no relationship between plant growth substances and L_p in ectomycorrhizal Douglas fir, despite lower zeatin riboside concentrations for Laccaria-inoculated plants.