The responses of Populus euphratica Oliv. plants to soil water deficit were assessed by analyzing gene expression, protein profiles, and several plant performance criteria to understand the acclimation of plants to soil water deficit. Young, vegetatively propagated plants originating from an arid, saline field site were submitted to a gradually increasing water deficit for 4 weeks in a greenhouse and were allowed to recover for 10 d after full reirrigation. Time-dependent changes and intensity of the perturbations induced in shoot and root growth, xylem anatomy, gas exchange, and water status were recorded. The expression profiles of approximately 6,340 genes and of proteins and metabolites (pigments, soluble carbohydrates, and oxidative compounds) were also recorded in mature leaves and in roots (gene expression only) at four stress levels and after recovery. Drought successively induced shoot growth cessation, stomatal closure, moderate increases in oxidative stress-related compounds, loss of CO2 assimilation, and root growth reduction. These effects were almost fully reversible, indicating that acclimation was dominant over injury. The physiological responses were paralleled by fully reversible transcriptional changes, including only 1.5% of the genes on the array. Protein profiles displayed greater changes than transcript levels. Among the identified proteins for which expressed sequence tags were present on the array, no correlation was found between transcript and protein abundance. Acclimation to water deficit involves the regulation of different networks of genes in roots and shoots. Such diverse requirements for protecting and maintaining the function of different plant organs may render plant engineering or breeding toward improved drought tolerance more complex than previously anticipated.

To investigate early salt acclimation mechanisms in a salt-tolerant poplar species (Populus euphratica), the kinetics of molecular, metabolic, and physiological changes during a 24-h salt exposure were measured. Three distinct phases of salt stress were identified by analyses of the osmotic pressure and the shoot water potential: dehydration, salt accumulation, and osmotic restoration associated with ionic stress. The duration and intensity of these phases differed between leaves and roots. Transcriptome analysis using P. euphratica-specific microarrays revealed clusters of coexpressed genes in these phases, with only 3% overlapping salt-responsive genes in leaves and roots. Acclimation of cellular metabolism to high salt concentrations involved remodeling of amino acid and protein biosynthesis and increased expression of molecular chaperones (dehydrins, osmotin). Leaves suffered initially from dehydration, which resulted in changes in transcript levels of mitochondrial and photosynthetic genes, indicating adjustment of energy metabolism. Initially, decreases in stress-related genes were found, whereas increases occurred only when leaves had restored the osmotic balance by salt accumulation. Comparative in silico analysis of the poplar stress regulon with Arabidopsis (Arabidopsis thaliana) orthologs was used as a strategy to reduce the number of candidate genes for functional analysis. Analysis of Arabidopsis knockout lines identified a lipocalin-like gene (AtTIL) and a gene encoding a protein with previously unknown functions (AtSIS) to play roles in salt tolerance. In conclusion, by dissecting the stress transcriptome of tolerant species, novel genes important for salt endurance can be identified.

To investigate physiological and transcriptomic regulation mechanisms underlying the distinct net fluxes of NH₄⁺ and NO₃⁻ in different root segments of Populus species under low nitrogen (N) conditions, we used saplings of Populus × canescens supplied with either 500 (normal N) or 50 (low N) μM NH₄NO₃. The net fluxes of NH₄⁺ and NO₃⁻, the concentrations of NH₄⁺, amino acids and organic acids and the enzymatic activities of nitrite reductase (NiR) and glutamine synthetase (GS) in root segment II (SII, 35–70 mm to the apex) were lower than those in root segment I (SI, 0–35 mm to the apex). The net NH₄⁺ influxes and the concentrations of organic acids were elevated, whereas the concentrations of NH₄⁺ and NO₃⁻ and the activities of NiR and GS were reduced in SI and SII in response to low N. A number of genes were significantly differentially expressed in SII vs SI and in both segments grown under low vs normal N conditions, and these genes were mainly involved in the transport of NH₄⁺ and NO₃⁻, N metabolism and adenosine triphosphate synthesis. Moreover, the hub gene coexpression networks were dissected and correlated with N physiological processes in SI and SII under normal and low N conditions. These results suggest that the hub gene coexpression networks play pivotal roles in regulating N uptake and assimilation, amino acid metabolism and the levels of organic acids from the tricarboxylic acid cycle in the two root segments of poplars in acclimation to low N availability.

The response of nursery seedlings and wildings of oriental beech (Fagus orientalis Lipsky) to canopy gap size was studied in a climax beech forest in northern Iran with respect to seedling survival, shoot growth and vitality. These parameters were followed for 2 years after planting in gaps of 50, 200 and 600 m², as well as in the open field. Foliage coloration was used as a criterion for vitality. The survival rate of nursery seedlings and wildings dropped rapidly with increasing gap size from 84 per cent in 50-m² canopy openings to ∼55 per cent in the 600-m² gaps and the open field 2 years after planting. Shoot growth of wildings and nursery seedlings differed in response to gap size variation. Shoot growth of wildings was higher in small gaps than in larger openings, whereas for nursery seedlings the inverse was observed. Most beech seedlings were not discoloured or slightly discoloured under gaps of 50, 200 and 600 m² and were slightly to moderately discoloured in the open. There was a general tendency for higher discoloration in larger openings. It is concluded that small non-regenerated canopy gaps (≤200 m²) of beech forests in northern Iran can be restored by planting beech wildings as well as nursery seedlings in small openings, whereas nursery seedlings are preferred in larger gaps (≥600 m²).