CO<Subscript>2</Subscript> enrichment,nitrogen fertilization and development of freezing tolerance in Norway spruce

Plant growth and adaptation to cold and freezing temperatures in a CO₂-enriched atmosphere have received little attention despite the predicted effects of elevated CO₂ on plant distribution and productivity. Norway spruce [Picea abies (L.) Karst.] seedlings from latitudinally distinct seed sources (66°N and 60°N) were grown for one simulated growth season under controlled conditions in an atmosphere enriched in CO₂ (70 Pa) and at ambient CO₂ (40 Pa), combined factorially with low (3.6 mM) or high (15.7 mM) concentrations of nitrogen fertilization. There was a clear difference between the two provenances in height growth, in the timing of bud set, and in freezing tolerance. Nitrogen fertilization increased height growth in both provenances, while CO₂ enrichment stimulated height growth only in the southern provenance. We found no significant effects of elevated CO₂ or nitrogen fertilization on the timing of bud set. During cold acclimation, freezing tolerance increased from –10°C to –35°C, and there was a marked increase in all soluble sugars except inositol. Elevated CO₂ in combination with high nitrogen led to a slight increased freezing tolerance in both provenances during the early stages of cold acclimation. However, towards the end of cold acclimation, elevated CO₂ and high nitrogen led to reduced freezing tolerance in the southern provenance, while elevated CO₂ and low nitrogen reduced freezing tolerance in the northern provenance. These results suggest that CO₂ enrichment influences the development of freezing tolerance, and that these responses differ with available nitrogen and between provenances.     

The role of the photosynthetic apparatus in cold acclimation of <Emphasis Type="Italic">Lolium multiflorum</Emphasis>. Characteristics of novel genotypes low-sensitive to PSII over-reduction

During cold acclimation by higher plants, temperature perception via changes in redox state of Photosystem II (PSII) and subsequent acclimation of the photosynthetic apparatus to cold is very important for achieving freezing tolerance. These properties were studied in two groups (A and B) of the same backcross 3 (BC₃) progeny derived from a triploid hybrid of Festuca pratensis (2×) × Lolium multiflorum (4×) backcrossed three times onto diploid L. multiflorum cultivars. Leaves of Group A plants formed at 20°C at medium-low light were unable to acclimate their photosynthetic apparatus to cold. Compared to Group B, the Group A plants were also more frost sensitive. This acclimation ability correlated with the freezing tolerance of the plants. However, leaves of the same Group A plants developed at 20°C, but under higher-light conditions had increased ability to acclimate their photosynthetic apparatus to cold. It was concluded that Group A plants may have impaired PSII temperature perception, and this then resulted in their poor capability to cold acclimate.     

Colinearity and Similar Expression Pattern of Rice DREB1s Reveal Their Functional Conservation in the Cold-Responsive Pathway

The clustered genes C-repeat (CRT) binding factor (CBF)1/ dehydration-responsive element binding protein (DREB)1B, CBF2/DREB1C, and CBF3/DREB1A play a central role in cold acclimation and facilitate plant resistance to freezing in Arabidopsis thaliana. Rice (Oryza sativa L.) is very sensitive to low temperatures; enhancing the cold stress tolerance of rice is a key challenge to increasing its yield. In this study, we demonstrate chilling acclimation, a phenomenon similar to Arabidopsis cold acclimation, in rice. To determine whether rice CBF/DREB1 genes participate in this cold-responsive pathway, all putative homologs of Arabidopsis DREB1 genes were filtered from the complete rice genome through a BLASTP search, followed by phylogenetic, colinearity and expression analysis. We thereby identified 10 rice genes as putative DREB1 homologs: nine of these were located in rice genomic regions with some colinearity to the Arabidopsis CBF1–CBF4 region. Expression profiling revealed that six of these genes (Os01g73770, Os02g45450, Os04g48350, Os06g03670, Os09g35010, and Os09g35030) were similarly expressed in response to chilling acclimation and cold stress and were co-expressed with genes involved in cold signalling, suggesting that these DREB1 homologs may be involved in the cold response in rice. The results presented here serve as a prelude towards understanding the function of rice homologs of DREB1 genes in cold-sensitive crops.     

Clinal variation in the non-acclimated and cold-acclimated freezing tolerance of Arabidopsis thaliana accessions

Arabidopsis thaliana is a geographically widely spread species consisting of local accessions differing both genetically and phenotypically. These differences may constitute environmental adaptations and a latitudinal cline in freezing tolerance has been shown previously. Many plants, including Arabidopsis, exhibit increased freezing tolerance after cold exposure (cold acclimation). Here we present evidence for geographical clines (both latitudinal and longitudinal) in acclimated (ACC) and non-acclimated (NA) freezing tolerance, estimated from electrolyte leakage measurements on 54 accessions. Leaf Pro contents were not correlated with freezing tolerance, while sugar contents (Glc, Fru, Suc, Raf) were in the ACC, but not the NA state. Expression levels of 14 cold-induced genes were investigated before and after 2 weeks of cold acclimation by quantitative RT-PCR. Expression of the CBF1, 2 and 3 genes was not correlated with freezing tolerance. The expression of some CBF-regulated (COR) genes, however, was correlated specifically with ACC freezing tolerance. A tight correlation between CBF and COR gene expression was only observed under non-acclimating conditions, where CBF and COR expression were also correlated with the expression of PRR5, a component of the circadian clock. Collectively, this study sheds new light on the molecular determinants of plant-freezing tolerance and cold acclimation and their geographical dependence.     

Cold acclimation of forage grasses in relation to pink snow mould (Microdochium nivale) resistance

The aim of our work is to investigate the changes in phenolic level, PAL activity and heat production rate induced during pre-hardening at 12°C and cold acclimation at 2°C of the forage grasses Festulolium, meadow fescue, tall fescue and Italian ryegrass in relation to their resistance to snow mould caused by Microdochium nivale. Meadow fescue and tall fescue were most resistant to M. nivale infection, while Italian ryegrass demonstrated the least resistance to this fungus inoculation. Festulolium, meadow fescue and tall fescue responded similarly to low temperature, while Italian ryegrass demonstrated considerable disturbance of energy balance and lower phenolic concentration, which could explain a higher susceptibility of the latter species to infection by M. nivale. The enhanced level of phenolic compounds, probably utilised for cell wall lignification as well as equilibrium of the metabolic activity observed in meadow fescue and tall fescue, is very important for both cold and pathogen-resistance mechanisms. The studied Festulolium cultivar ‘Felopa’, a hybrid of the Lolium multiflorum and Festuca pratensis genomes, was characterised by changes in biochemical parameters similar to the resistant meadow fescue and tall fescue.     

Does long-term cultivation of saplings under elevated CO2 concentration influence their photosynthetic response to temperature?

Background and Aims Plants growing under elevated atmospheric CO₂ concentrations often have reduced stomatal conductance and subsequently increased leaf temperature. This study therefore tested the hypothesis that under long-term elevated CO₂ the temperature optima of photosynthetic processes will shift towards higher temperatures and the thermostability of the photosynthetic apparatus will increase.Methods The hypothesis was tested for saplings of broadleaved Fagus sylvatica and coniferous Picea abies exposed for 4–5 years to either ambient (AC; 385 µmol mol⁻¹) or elevated (EC; 700 µmol mol⁻¹) CO₂ concentrations. Temperature response curves of photosynthetic processes were determined by gas-exchange and chlorophyll fluorescence techniques.Key Results Initial assumptions of reduced light-saturated stomatal conductance and increased leaf temperatures for EC plants were confirmed. Temperature response curves revealed stimulation of light-saturated rates of CO₂ assimilation (A_max) and a decline in photorespiration (R_L) as a result of EC within a wide temperature range. However, these effects were negligible or reduced at low and high temperatures. Higher temperature optima (T_opt) of A_max, Rubisco carboxylation rates (V_Cmax) and R_L were found for EC saplings compared with AC saplings. However, the shifts in T_opt of A_max were instantaneous, and disappeared when measured at identical CO₂ concentrations. Higher values of T_opt at elevated CO₂ were attributed particularly to reduced photorespiration and prevailing limitation of photosynthesis by ribulose-1,5-bisphosphate (RuBP) regeneration. Temperature response curves of fluorescence parameters suggested a negligible effect of EC on enhancement of thermostability of photosystem II photochemistry.Conclusions Elevated CO₂ instantaneously increases temperature optima of A_max due to reduced photorespiration and limitation of photosynthesis by RuBP regeneration. However, this increase disappears when plants are exposed to identical CO₂ concentrations. In addition, increased heat-stress tolerance of primary photochemistry in plants grown at elevated CO₂ is unlikely. The hypothesis that long-term cultivation at elevated CO₂ leads to acclimation of photosynthesis to higher temperatures is therefore rejected. Nevertheless, incorporating acclimation mechanisms into models simulating carbon flux between the atmosphere and vegetation is necessary.     

Differential characteristics of photochemical acclimation to cold in two contrasting sweet sorghum hybrids

Sweet sorghum has a photosynthetic system which is highly sensitive to cold stress and hence strongly limits its development in temperate environments; therefore, the identification of key exploitable cold tolerance traits is imperative. From a preliminary field trial, two dissimilar sweet sorghum hybrids (ICSSH31 and Bulldozer), in terms of early vigor and productivity, were selected for a controlled‐environment trial aiming at identifying useful traits related to acclimation mechanisms to cold stress. The higher cold tolerance of Bulldozer was partially related to a more efficient photochemical regulation mechanism of the incoming light energy: the higher tolerance of photosystem II (PSII) to photo‐inactivation was because of a more effective dissipation capacity of the excess of energy and to a more balanced diversion of the absorbed energy into alternative energy sinks. ICSSH31 increased the dissipation and accumulation of a large amount of xanthophylls, as in Bulldozer, but, at the same time, inactivated the oxygen evolving complex and the re‐synthesis of chlorophyll (Chl) a and b, thus, leading to an overproduction of CO₂ fixation enzymes after re‐warming. In summary, in Bulldozer, the acclimation adjustments of the photosynthetic apparatus occurred through an efficient control of energy transfer toward the reaction centers, and this likely allowed a more successful seedling establishment; ICSSH31, conversely, exhibited a fast re‐synthesis of Chl pigments, which appears to divert photosynthates from dry matter accumulation. Such broad acclimation traits may constitute a source for selecting higher genetic gain traits relevant for enlarging the growing season of promising biomass sorghum ideotypes in temperate climates.     

Photosynthesis of Helianthus annuus does not acclimate toelevated CO2 regardless of N supply

A number of studies have shown that relatively long-term exposure to elevated levels of CO₂ can lead to the downward acclimation of photosynthesis. Although the exact mechanisms are not clearly understood, it has been suggested that such a downward adjustment may be more common under limited N availability. Here we examined the effect of N supply on the photosynthetic acclimation response of Helianthus annuus L. cv. Teddy Bear plants to elevated CO₂ at three growth stages – 18, 38 and 56 d after emergence corresponding to vegetative, pre-flowering and flowering stages. Plants were grown at CO₂ partial pressures of 37 or 70 Pa, and supplied with 0.5, 2.5 or 5 mol·m^–3 N. After 18 d of treatment, photosynthetic capacity of H. annuus as evaluated by parameters derived from the A-C_i data (Rubisco carboxylation capacity, V_c,max; electron transport capacity, J_max; and capacity for triose phosphate utilization, TPU) showed no acclimation to elevated CO₂. The leaf nitrogen concentration, [N], and total non-structural carbohydrates, [TNC], were also comparable between ambient- and elevated-CO₂-grown plants. However, all these photosynthetic parameters as well as leaf [N], but not [TNC], significantly increased in response to N supply. Similarly, after 38 and 56 d of exposure to CO₂ treatments, photosynthetic capacities, foliar [N] and [TNC] did not significantly differ between ambient- and elevated-CO₂ plants. These results suggest that H. annuus plants maintained their photosynthetic capacity during long-term exposure to elevated CO₂ because of their capacity to maintain leaf N-status. It is further suggested that plant capacity to maintain the balance between C and N acquisition rather than simply N-supply level, may determine whether photosynthetic acclimation in response to elevated CO₂ occurs or not.