The initiation, progression, and natural variation of autumn senescence in European aspen (Populus tremula) was investigated by monitoring chlorophyll degradation in (1) trees growing in natural stands and (2) cloned trees growing in a greenhouse under various light regimes. The main trigger for the initiation of autumn senescence in aspen is the shortening photoperiod, but there was a large degree of variation in the onset of senescence, both within local populations and among trees originating from different populations, where it correlated with the latitude of their respective origins. The variation for onset of senescence with a population was much larger than the variation of bud set. Once started, autumn senescence was accelerated by low temperature and longer nights, and clones that started to senescence late had a faster senescence. Bud set and autumn senescence appeared to be under the control of two independent critical photoperiods, but senescence could not be initiated until a certain time after bud set, suggesting that bud set and growth arrest are important for the trees to acquire competence to respond to the photoperiodic trigger to undergo autumn senescence. A timetable of events related to bud set and autumn senescence is presented.

No abstract

The genus Populus is currently the main model system for genetic, genomic, and physiological research in trees. Phenotypic variation in aspen (Populus tremula) populations growing in different environments across Sweden is expected to reflect genetic variation that is important for local adaptation. To analyze such natural phenotypic and genetic variation, the Swedish Aspen (SwAsp) Collection was established. Trees were taken from 12 different populations across Sweden, from 56°to 66°l atitude north and planted in two common gardens in Ekebo (55.9°N) and Sävar (63.4°N). Data related to phenological and growth traits were collected during the second year of growth. Some traits like the date of bud set and leaf area duration showed strong clinal variation patterns with latitude in both field trials, but the date of bud flush did not change along a latitudinal cline. The phenological traits showed moderate within-populations heritabilities, although growth traits showed weaker clinal patterns and lower heritabilities than the phenological traits. This research forms the starting point for the development of the SwAsp collection, a resource facilitating analysis of the natural genetic variation in aspen, the elucidation of the structure and dynamics of aspen populations, and the future identification of the genes controlling adaptive traits using association mapping of selected candidate genes.

The initiation of growth cessation and dormancy represents a critical ecological and evolutionary tradeoff between survival and growth in most forest trees. The most important environmental cue regulating the initiation of dormancy is a shortening of the photoperiod and phytochrome genes have been implicated in short-day-induced bud set and growth cessation in Populus. We characterized patterns of DNA sequence variation at the putative candidate gene phyB2 in 4 populations of European aspen (Populus tremula) and scored single-nucleotide polymorphisms in an additional 12 populations collected along a latitudinal gradient in Sweden. We also measured bud set from a subset of these trees in a growth chamber experiment. Buds set showed significant clinal variation with latitude, explaining 90% of the population variation in bud set. A sliding-window scan of phyB2 identified six putative regions with enhanced population differentiation and four SNPs showed significant clinal variation. The clinal variation at individual SNPs is suggestive of an adaptive response in phyB2 to local photoperiodic conditions. Three of four SNPs showing clinal variation were located in regions with excessive genetic differentiation, demonstrating that searching for regions of high genetic differentiation can be useful for identifying sites putatively involved in local adaptation.

We investigated the utility of association mapping to dissect the genetic basis of naturally occurring variation in bud phenology in European aspen (Populus tremula). With this aim, we surveyed nucleotide polymorphism in 13 fragments spanning an 80-kb region surrounding the phytochrome B2 (phyB2) locus. Although polymorphism varies substantially across the phyB2 region, we detected no signs for deviations from neutral expectations. We also identified a total of 41 single nucleotide polymorphisms (SNPs) that were subsequently scored in a mapping population consisting of 120 trees. We identified two nonsynonymous SNPs in the phytochrome B2 gene that were independently associated with variation in the timing of bud set and that explained between 1.5 and 5% of the observed phenotypic variation in bud set. Earlier studies have shown that the frequencies of both these SNPs vary clinally with latitude. Linkage disequilibrium across the region was low, suggesting that the SNPs we identified are strong candidates for being causally linked to variation in bud set in our mapping populations. One of the SNPs (T608N) is located in the ''hinge region,'' close to the chromophore binding site of the phyB2 protein. The other SNP (L1078P) is located in a region supposed to mediate downstream signaling from the phyB2 locus. The lack of population structure, combined with low levels of linkage disequilibrium, suggests that association mapping is a fruitful method for dissecting naturally occurring variation in Populus tremula.

Ear photosynthesis may be an important source of C for grain growth in water-stressed plants of cereals. The main objectives of this work were to determine the stability of the photosynthetic apparatus and the photochemical efficiency of ears in plants subjected to post-anthesis drought. Plants of wheat (Triticum aestivum L. cv. Granero INTA) were grown in pots under a rain shelter and subjected to water stress (soil water potential around À0.6 to À0.8 MPa) starting 4 days after anthesis. Post-anthesis drought substantially accelerated the loss of chlorophyll, Rubisco and the light-harvesting complex of photosystem II (LHCII) in the flag leaf, but the degradation of these photosynthetic components was much less affected by water deficit in awns and ear bracts. Quantum yield of PSII (F PSII ) decreased in leaves of water-stressed plants. In contrast, ear bracts had a higher F PSII than leaves, and F PSII of ear bracts did not decrease at all in response to drought. Removing the grains immediately before fluorescence measurements (less than 30 min) slightly reduced F PSII , indicating that CO 2 supplied by grain respiration may contribute to the high photochemical efficiency of ears in droughted plants. However, other factors may be involved in maintaining high F PSII , since even in the absence of grains F PSII remained much higher in ear bracts than in the flag leaf. The relative stability of ear photosynthetic components and their relatively high photochemical efficiency may help to maintain ear photosynthesis during the grain filling period in droughted plants.Abbreviations -Chl, chlorophyll; DAA, days after anthesis; F v /F m , potential quantum yield of PSII; LHCII, light-harvesting complex of PSII; PPFD, photosynthetic photon flux density; F PSII , quantum yield of PSII.

Climate change will increase the occurrence of flash floods as a consequence of extreme rain events, creating alternate periods of drought and flooding during the growing season. We analyzed the responses of two willow clones with contrasting responses to flooding (clone B, Salix matsudana × Salix alba hybrid; clone Y, Salix alba) to different combinations of stress treatments: continuous flooding or drought for six weeks, or cyclic treatments of two weeks of stress separated by two weeks of watering at field capacity. Drought reduced growth, stomatal conductance, and total leaf area in both clones, but flooding did not. Flooding reduced the root to shoot ratio in both clones. The hydraulic conductivity of the main stem was significantly reduced by drought only in clone Y. The area of the vessels was decreased by both drought and flooding, but the number was increased only by drought. The occurrence of drought before flooding reduced the vessel area, but the opposite treatment did not. An episode of drought after one of flooding is more stressful than the opposite situation, especially for clone Y, which could not adjust its water transport capacity during the drought period.

The natural variation in leaf and plant longevity in Arabidopsis thaliana was analysed in a set of 45 ecotypes and 155 recombinant inbred lines derived from a Cape Verde Islands (Cvi) x Landsberg erecta (Ler) cross. Post-bolting longevity was inversely related to time to flowering and rosette leaf number in the set of 45 ecotypes, with Cvi having the longest and Ler the shortest post-bolting longevity. The recombinant inbred line population was tested under low or high soil nutrient levels (LN or HN, respectively). Three quantitative trait loci (QTL), one in chromosome 3 and two in chromosomes 1 and 5, were associated with longevity of the 6th rosette leaf under LN and HN, respectively. Four QTL for post-bolting longevity were found in chromosomes 1, 3, 4, and 5, and two in chromosomes 1 and 5 under LN and HN, respectively. An epistatic interaction affecting post-bolting longevity under LN, but not HN, was detected. Ler and Cvi carry a mix of increasing and decreasing alleles for the QTL affecting longevity of the 6th leaf and post-bolting longevity. Longevity of the 6th rosette leaf was associated with different QTL than post-bolting longevity, and it was affected by different QTL depending on nutrient availability. By contrast, the major QTL affecting post-bolting longevity exerted significant effects irrespective of soil nutrient availability.