Autumn senescence in aspen is not triggered by day length

Autumn senescence in mature aspens, grown under natural conditions, is initiated at almost the same date every year. The mechanism of such precise timing is not understood but we have previously shown that the signal must be derived from light. We studied variation in bud set and autumn senescence in a collection of 116 natural Eurasian aspen (Populus tremula) genotypes, from 12 populations in Sweden and planted in one northern and one southern common garden, to test the hypothesis that onset of autumn senescence is triggered by day length. We confirmed that, although bud set seemed to be triggered by a critical photoperiod/day length, other factors may influence it. The data on initiation of autumn senescence, on the other hand, were incompatible with the trigger being the day length per se, hence the trigger must be some other light‐dependent factor.     

Natural phenological variation in aspen (Populus tremula): the SwAsp collection

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° latitude 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.     

Clinal variation in phyB2, a candidate gene for day-length-induced growth cessation and bud set, across a latitudinal gradient in European aspen (Populus tremula)

The initiation of growth cessation and dormancy represents a critical ecological and evolutionary trade-off 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 approximately 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.     

GIGANTEA influences leaf senescence in trees in two different ways

GIGANTEA (GI) genes have a central role in plant development and influence several processes. Hybrid aspen T89 (Populus tremula x tremuloides) trees with low GI expression engineered through RNAi show severely compromised growth. To study the effect of reduced GI expression on leaf traits with special emphasis on leaf senescence, we grafted GI-RNAi scions onto wild-type rootstocks and successfully restored growth of the scions. The RNAi line had a distorted leaf shape and reduced photosynthesis, probably caused by modulation of phloem or stomatal function, increased starch accumulation, a higher carbon-to-nitrogen ratio, and reduced capacity to withstand moderate light stress. GI-RNAi also induced senescence under long day (LD) and moderate light conditions. Furthermore, the GI-RNAi lines were affected in their capacity to respond to “autumn environmental cues” inducing senescence, a type of leaf senescence that has physiological and biochemical characteristics that differ from those of senescence induced directly by stress under LD conditions. Overexpression of GI delayed senescence under simulated autumn conditions. The two different effects on leaf senescence under LD or simulated autumn conditions were not affected by the expression of FLOWERING LOCUS T. GI expression regulated leaf senescence locally—the phenotype followed the genotype of the branch, independent of its position on the tree—and trees with modified gene expression were affected in a similar way when grown in the field as under controlled conditions. Taken together, GI plays a central role in sensing environmental changes during autumn and determining the appropriate timing for leaf senescence in Populus.

In the autumn deciduous trees respond to environmental signals and induce leaf senescence, and two different senescence pathways can be distinguished     

Impacts of bud set and lammas phenology on root:shoot biomass partitioning and carbon gain physiology in poplar

Key message

Timing of bud set and occurrence of lammas in trees can alter growth partitioning (i.e., root:shoot ratios), while only bud set effectively modifies carbon gain by increasing photosynthesis-related physiological traits.

Abstract

Bud set and lammas (second bud flushing) phenology may strongly influence growth, physiology, and biomass in trees. To test effects of these phenological events, 54 individuals from 16 genotypes of black cottonwood poplar (Populus trichocarpa) were grown in a potted trial under greenhouse conditions (with extended daylengths promoting growth), followed by open-air cultivation (with natural daylengths promoting bud set and/or lammas). Trees were monitored for phenology, repeatedly measured for photosynthesis-related traits, harvested for biomass, and assessed for growth partitioning (separating above- and belowground parts). We grouped trees by phenology for comparisons: (1) trees with early summer bud set, (2) trees with early summer bud set that underwent lammas, (3) trees with late summer bud set (August), and (4) trees with bud set occurring in autumn (September). We found that bud set timing positively affected growth partitioning where earlier bud set resulted in shorter trees with higher root:shoot biomass ratios (by increasing root mass). Lammas growth altered these ratios by significantly increasing shoot growth relative to belowground growth. Trees with bud set occurring in late summer also had higher root:shoot biomass ratios (by increasing root mass) compared to trees setting bud in autumn. Occurrence of bud set coincided with modified physiology of the existing canopy where photosynthesis-related traits were enhanced relative to trees still actively growing. These physiological changes were unaltered by occurrence of lammas. This suggests that bud set prompts a significant, coordinated mechanism of higher carbon gain physiology and belowground biomass accumulation in plants within a “post-bud set” phase.

     

Elevated [CO2] and nutrient status modified leaf phenology and growth rhythm of young Populus trichocarpa trees in a 3-year field study

Young individuals of a single clone of black cottonwood, in Iceland, were exposed for 3 years to elevated atmospheric CO₂ concentrations [CO₂] in whole-tree chambers at natural and high nutrient availability. No treatment effects were found at bud break or the start of shoot extension in spring. Autumn phenology was, however, affected both by elevated [CO₂] and changes in nutrient status. The time of annual growth cessation was linearly related to leaf nitrogen concentration, irrespective of CO₂ treatment. At low (natural) nutrient availability, elevated [CO₂] accelerated growth cessation and bud set, which reduced the period of active growth. An earlier and more pronounced leaf senescence and corresponding loss of photosynthetic capacity further decreased carbon acquisition in elevated [CO₂]. The negative [CO₂] effect on duration of shoot extension and leaf senescence existed, but was not as pronounced, when trees grew at higher nutrient availability. Improved nutrient availability extended the shoot extension period and delayed leaf senescence. It is suggested that trees grown in elevated [CO₂] altered their autumn phenology as an effect of a signal similar to that in trees growing at low nutrient availability, i.e. an imbalance between carbon and nitrogen sources. These alterations in autumn phenology may be important when predicting how trees will grow in a future CO₂ environment.     

Contrasting patterns of cytokinins between years in senescing aspen leaves

Cytokinins are plant hormones that typically block or delay leaf senescence. We profiled 34 different cytokinins/cytokinin metabolites (including precursors, conjugates and degradation products) in leaves of a free‐growing mature aspen (Populus tremula) before and after the initiation of autumnal senescence over three consecutive years. The levels and profiles of individual cytokinin species, or classes/groups, varied greatly between years, despite the fact that the onset of autumn senescence was at the same time each year, and senescence was not associated with depletion of either active or total cytokinin levels. Levels of aromatic cytokinins (topolins) were low and changed little over the autumn period. Diurnal variations and weather‐dependent variations in cytokinin content were relatively limited. We also followed the expression patterns of all aspen genes implicated as having roles in cytokinin metabolism or signalling, but neither the pattern of regulation of any group of genes nor the expression of any particular gene supported the notion that decreased cytokinin signalling could explain the onset of senescence. Based on the results from this tree, we therefore suggest that cytokinin depletion is unlikely to explain the onset of autumn leaf senescence in aspen.     

Low night temperature and inhibition of gibberellin biosynthesis override phytochrome action and induce bud set and cold acclimation, but not dormancy in PHYA overexpressors and wild-type of hybrid aspen

Juvenile trees of temperate and boreal regions cease growth and set buds in autumn in response to short day-lengths (SD) detected by phytochrome. Growth cessation and bud set are prerequisites for the development of winter dormancy and full cold hardiness. In this study we show that the SD-requirement for bud set and cold hardening can be overcome in hybrid aspen (Populus tremula L. × tremuloides Michx.) by low night temperature and inhibition of gibberellin (GA) biosynthesis. Bud set and increased cold hardiness were observed under normally non-inductive long day-length (LD) in wild-type plants, when exposed to low night temperature and paclobutrazol. In addition, the effect of PHYA overexpression could be overcome in transgenic plants, producing bud set and cold acclimation by treatment with: SD, low night temperature and paclobutrazol. After cold acclimation, the degree of bud dormancy was lower for wild-type plants prior treated with LD and transgenic plants (overexpressing PHYA), than SD-treated, wild-type plants. Thus, low night temperature in combination with reduced GA content induced bud set and promoted cold hardiness under normally non-inductive photoperiods in hybrid aspen, but was unable to affect development of dormancy. This might suggest separate signalling pathways from phytochrome regulating the induction of cold/cold hardiness and bud dormancy in hybrid aspen or alternatively, there might be one pathway that fails to complete its action in the transgenic and paclobutrazol treated plants.     

Leaf Conductance during the Final Season of a Senescing Aspen Branch

Leaf conductance, transpiration, and environmental conditions were measured on two aspen (Populus tremuloides Michx.) branches in a natural stand, using an automatic cuvette system. Fortuitously, leaves on one branch senesced about 10 days early, allowing comparison between a senescing branch and a normal branch. Terminal bud development was retarded on the senescent branch, and a portion of the branch eventually abscised about 20 centimeters from the end. Roughly 1% to 2% of the other branches on the study tree and adjacent trees of that clone also senesced and were dead the following spring.

Although no visual symptoms of senescence were observed until September, stomatal behavior was atypical shortly after leaves were fully expanded. During July and August, leaf conductances under full sunlight were higher on the branch which senesced than on the branch which was normal, reaching values greater than 1.0 centimeters per second, and conductance was highly variable.

     

Within-stand variation in silver birch (Betula pendula Roth) phenology

Key message

Within a local population genotypes differ in the timing of bud burst, but genotypes with early bud burst unfold their leaves slower, resulting in an equal period of carbon gain.

Abstract

The ability of local populations to cope with disturbances like adverse weather events or a changing climate depends on the genotypic richness of such populations, emphasising the importance of differences between genotypes in traits related to growth and survival at this scale. Due to their longevity, these differences are of special importance in trees, yet for trees, differences between genotypes within local populations remain unexplored. The phenological cycle is important in this respect, since a correct timing of phenological events is critical for growth and survival of trees, especially in environments with strong seasonality and changes in the timing of phenological events has consequences for, among others, net ecosystem productivity and the climate system as a whole. In this light accounting for differences in the timing of phenological events within species is currently identified as a research challenge. This study contributes to the knowledge of differences between genotypes on the small spatial scale of a local population. We examined the timing of phenological events of 15 micropropagated silver birch (Betula pendula Roth) genotypes representing a natural population. Measurements covered bud burst (7 years) and leaf unfolding in spring and chlorophyll degradation in autumn (2 years for both). These data were used to estimate the period of carbon gain. Differences between genotypes in the temperature sum required for bud burst were present, with genotypes showing ‘early’ (i.e. a low temperature sum requirement for bud burst) and ‘late’ bud burst across the 7-year study period. Differences were small in most years (i.e. 3 days), but differences of 16 days were recorded within the 7-year study period as well. Genotypes with ‘early’ bud burst were less sensitive to variations in environmental conditions in spring compared to genotypes with ‘late’ bud burst. Differences in bud burst were not carried over to the estimated period of carbon gain. Due to faster leaf expansion in genotypes with ‘late’ bud burst and the lack of differences between genotypes in autumn senescence the estimated period of carbon gain was similar among genotypes.     

Timing of photoperiodic competency causes phenological mismatch in balsam poplar (Populus balsamifera L.)

Plant phenology is expected to be sensitive to climate warming. In boreal trees, spring flush is primarily temperature driven, whereas height growth cessation and autumn leaf senescence are predominantly controlled by photoperiod. Cuttings of 525 genotypes from the full range of balsam poplar were planted into two common gardens (Vancouver and Indian Head, Canada) at similar latitudes, but with differing winter temperatures and growing seasons. There was clinal variation in spring and, particularly, summer and fall phenology. Bud flush and, despite milder climate, bud set and leaf drop were earlier at Vancouver than at Indian Head by 44, 28 and 7 d, respectively. Although newly flushed growth is insensitive to photoperiod, many genotypes at both sites became competent before the summer solstice. At Vancouver, high‐latitude genotypes set dormant terminal buds in mid‐spring. Most other genotypes grew until midsummer or set bud temporarily and then experienced a second flush. In both gardens and in a growth chamber experiment, earlier bud set was associated with reduced height growth and higher root/shoot ratios. Shoots attained competency ～5 weeks after flushing, which would normally prevent dormancy induction before the solstice, but may be insufficient if spring advances by more than a few weeks.     

Nucleotide polymorphism and phenotypic associations within and around the phytochrome B2 Locus in European aspen (Populus tremula, Salicaceae)

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.     

Establishment of a shortened annual cycle system; a tool for the analysis of annual re-translocation of phosphorus in the deciduous woody plant (Populus alba L.)

The supply of phosphorus, the essential element for plant growth and development, is often limited in natural environments. Plants employ multiple physiological strategies to minimize the impact of phosphate deficiency. In deciduous trees, phosphorus is remobilized from senescing leaves in autumn and stored in other tissues for reuse in the following spring. We previously monitored the annual changes in leaf phosphate content of white poplar (Populus alba) growing under natural conditions and found that about 75 % of inorganic and 60 % of organic leaf phosphates observed in May were remobilized by November. In order to analyze this process (such annual events), we have established a model system, in which an annual cycle of phosphate re-translocation in trees can be simulated under laboratory conditions by controlling temperature and photoperiod (=‘shortened annual cycle’). This system evidently allowed us to monitor the annual changes in leaf color, phosphate remobilization from senescent leaves, and bud break in the next spring within five months. This will greatly facilitate the analysis of cellular and molecular mechanisms of annual phosphate re-translocation in deciduous trees.     

Imprints of Natural Selection Along Environmental Gradients in Phenology-Related Genes of Quercus petraea

We explored single nucleotide polymorphism (SNP) variation in candidate genes for bud burst from Quercus petraea populations sampled along gradients of latitude and altitude in Western Europe. SNP diversity was monitored for 106 candidate genes, in 758 individuals from 32 natural populations. We investigated whether SNP variation reflected the clinal pattern of bud burst observed in common garden experiments. We used different methods to detect imprints of natural selection (F_ST outlier, clinal variation at allelic frequencies, association tests) and compared the results obtained for the two gradients. F_ST outlier SNPs were found in 15 genes, 5 of which were common to both gradients. The type of selection differed between the two gradients (directional or balancing) for 3 of these 5. Clinal variations were observed for six SNPs, and one cline was conserved across both gradients. Association tests between the phenotypic or breeding values of trees and SNP genotypes identified 14 significant associations, involving 12 genes. The results of outlier detection on the basis of population differentiation or clinal variation were not very consistent with the results of association tests. The discrepancies between these approaches may reflect the different hierarchical levels of selection considered (inter- and intrapopulation selection). Finally, we obtained evidence for convergent selection (similar for gradients) and clinal variation for a few genes, suggesting that comparisons between parallel gradients could be used to screen for major candidate genes responding to natural selection in trees.     

Genetic basis of climatic adaptation in scots pine by bayesian quantitative trait locus analysis

We examined the genetic basis of large adaptive differences in timing of bud set and frost hardiness between natural populations of Scots pine. As a mapping population, we considered an "open-pollinated backcross" progeny by collecting seeds of a single F(1) tree (cross between trees from southern and northern Finland) growing in southern Finland. Due to the special features of the design (no marker information available on grandparents or the father), we applied a Bayesian quantitative trait locus (QTL) mapping method developed previously for outcrossed offspring. We found four potential QTL for timing of bud set and seven for frost hardiness. Bayesian analyses detected more QTL than ANOVA for frost hardiness, but the opposite was true for bud set. These QTL included alleles with rather large effects, and additionally smaller QTL were supported. The largest QTL for bud set date accounted for about a fourth of the mean difference between populations. Thus, natural selection during adaptation has resulted in selection of at least some alleles of rather large effect.     

Simulated herbivory advances autumn phenology in Acer rubrum

To determine the degree to which herbivory contributes to phenotypic variation in autumn phenology for deciduous trees, red maple (Acer rubrum) branches were subjected to low and high levels of simulated herbivory and surveyed at the end of the season to assess abscission and degree of autumn coloration. Overall, branches with simulated herbivory abscised ～7 % more leaves at each autumn survey date than did control branches within trees. While branches subjected to high levels of damage showed advanced phenology, abscission rates did not differ from those of undamaged branches within trees because heavy damage induced earlier leaf loss on adjacent branch nodes in this treatment. Damaged branches had greater proportions of leaf area colored than undamaged branches within trees, having twice the amount of leaf area colored at the onset of autumn and having ～16 % greater leaf area colored in late October when nearly all leaves were colored. When senescence was scored as the percent of all leaves abscised and/or colored, branches in both treatments reached peak senescence earlier than did control branches within trees: dates of 50 % senescence occurred 2.5 days earlier for low herbivory branches and 9.7 days earlier for branches with high levels of simulated damage. These advanced rates are of the same time length as reported delays in autumn senescence and advances in spring onset due to climate warming. Thus, results suggest that should insect damage increase as a consequence of climate change, it may offset a lengthening of leaf life spans in some tree species.     

Incidence of Venturia shoot blight in aspen (Populus tremuloides Michx.) varies with tree chemistry and genotype

Quaking aspen (Populus tremuloides) is a foundation tree species in North American forests, as well as a valuable source of wood and paper products. Quaking aspen exhibits substantial genetic variation within and among natural populations in phytochemical compounds that influence both interactions with herbivores and ecosystem dynamics. The potential association of these phytochemicals with disease resistance, however, is unknown. Here we present the results of a “natural experiment” in a common garden of quaking aspen genotypes infected with shoot blight (Venturia moreletii). We found that the incidence of shoot blight varied by 10-fold among aspen genotypes, and was strongly and negatively correlated with constitutive foliar concentrations of condensed tannins. Selection factors that shape the genetic and phytochemical architecture of aspen populations may thus simultaneously influence aspen resistance to pathogen attack, with consequences for individual tree fitness as well as community organization, via “extended phenotype” effects.     

Seasonality and phenology alter functional leaf traits

In plant ecophysiology, functional leaf traits are generally not assessed in relation to phenological phase of the canopy. Leaf traits measured in deciduous perennial species are known to vary between spring and summer seasons, but there is a knowledge gap relating to the late-summer phase marked by growth cessation and bud set occurring well before fall leaf senescence. The effects of phenology on canopy physiology were tested using a common garden of over 2,000 black cottonwood (Populus trichocarpa) individuals originating from a wide geographical range (44–60ºN). Annual phenological events and 12 leaf-based functional trait measurements were collected spanning the entire summer season prior to, and following, bud set. Patterns of seasonal trait change emerged by synchronizing trees using their date of bud set. In particular, photosynthetic, mass, and N-based traits increased substantially following bud set. Most traits were significantly different between pre-bud set and post-bud set phase trees, with many traits showing at least 25 % alteration in mean value. Post-bud set, both the significance and direction of trait–trait relationships could be modified, with many relating directly to changes in leaf mass. In Populus, these dynamics in leaf traits throughout the summer season reflected a shift in whole plant physiology, but occurred long before the onset of leaf senescence. The marked shifts in measured trait values following bud set underscores the necessity to include phenology in trait-based ecological studies or large-scale phenotyping efforts, both at the local level and larger geographical scale.     

Genetic and environmental influences on leaf phenology and cold hardiness of native and introduced riparian trees

To explore the roles of plasticity and genetic variation in the response to spatial and temporal climate variation, we established a common garden consisting of paired collections of native and introduced riparian trees sampled along a latitudinal gradient. The garden in Fort Collins, Colorado (latitude 40.6°N), included 681 native plains cottonwood (Populus deltoides subsp. monilifera) and introduced saltcedar (Tamarix ramosissima, T. chinensis and hybrids) collected from 15 sites at 29.2–47.6°N in the central United States. In the common garden both species showed latitudinal variation in fall, but not spring, leaf phenology, suggesting that the latitudinal gradient in fall phenology observed in the field results at least in part from inherited variation in the critical photoperiod, while the latitudinal gradient in spring phenology observed in the field is largely a plastic response to the temperature gradient. Populations from higher latitudes exhibited earlier bud set and leaf senescence. Cold hardiness varied latitudinally in both fall and spring for both species. For cottonwood, cold hardiness began earlier and ended later in northern than in southern populations. For saltcedar northern populations were hardier throughout the cold season than southern populations. Although cottonwood was hardier than saltcedar in midwinter, the reverse was true in late fall and early spring. The latitudinal variation in fall phenology and cold hardiness of saltcedar appears to have developed as a result of multiple introductions of genetically distinct populations, hybridization and natural selection in the 150 years since introduction.