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.

Fungal endophytes are found in most seed plants, but their ecological function mainly remains elusive, except in pooid (or clavicipitalean) systems. The diversity and dynamics of endophytes in non clavicipitalean plants make studies of their ecological function challenging. This paper describes the advantage of using molecular techniques to survey the ecological function of endophytes in Populus tremula clones. About 1,000 endophyte isolates were distinguished using traditional methods; these isolates represented approximately 100 morphologically distinct groups. We generated 73 DNA-sequences (18S and ITS rDNA) from these groups and determined 33 distinct taxa. They represented the Basidiomycota and Ascomycota, including diverse Sordariomycetes and Dothideomycetes, and three sequences that were identified, by their ITS sequences, as a species of Epicoccum (a genus of supposedly unknown relation within the Ascomycota) were placed within the Pleosporales, in the 18S phylogeny. Primer pairs were designed for eleven of the fungi. Of these, three primers produced bands for a subset of Aspen samples. The primer pairs allowed endophytes in field samples to be readily identified, with a detection limit of 0.15 percent fungal DNA. The presence of fungi in Aspen clones was related to field damage by herbivores and the pathogen Venturia tremula. A negative association was found in two separate surveys between Aureobasidium sp. and herbivore damage, but we found no evidence that endophyte presence was related to a history of Venturia symptoms. This approach promises to enhance greatly the scope for qualitative and quantitative detection of endophyte communities, and to improve our ability to elucidate the ecological function of non clavicipitalean endophytes.

Autumn senescence in aspen (Populus tremula) is precisely timed every year to relocate nutrients from leaves to storage organs before winter. Here we demonstrate how stem girdling, which leads to the accumulation of photosynthates in the crown, influences senescence. Girdling resulted in an early onset of senescence, but the chlorophyll degradation was slower and nitrogen more efficiently resorbed than during normal autumn senescence. Girdled stems accumulated or retained anthocyanins potentially providing photoprotection in senescing leaves. Girdling of one stem in a clonal stand sharing the same root stock did not affect senescence in the others, showing that the stems were autonomous in this respect. One girdled stem with unusually high chlorophyll and nitrogen contents maintained low carbon-to-nitrogen (C/N) ratio and did not show early senescence or depleted chlorophyll level unlike the other girdled stems suggesting that the responses depended on the genotype or its carbon and nitrogen status. Metabolite analysis highlighted that the tricarboxylic acid (TCA) cycle, salicylic acid pathway, and redox homeostasis are involved in the regulation of girdling-induced senescence. We propose that disrupted sink-source relation and C/N status can provide cues through the TCA cycle and phytohormone signaling to override the phenological control of autumn senescence in the girdled stems.

Fertilization with nitrogen (N)‐rich compounds leads to increased growth but may compromise phenology and winter survival of trees in boreal regions. During autumn, N is remobilized from senescing leaves and stored in other parts of the tree to be used in the next growing season. However, the mechanism behind the N fertilization effect on winter survival is not well understood, and it is unclear how N levels or forms modulate autumn senescence. We performed fertilization experiments and showed that treating Populus saplings with inorganic nitrogen resulted in a delay in senescence. In addition, by using precise delivery of solutes into the xylem stream of Populus trees in their natural environment, we found that delay of autumn senescence was dependent on the form of N administered: inorganic N (NO3−) delayed senescence, but amino acids (Arg, Glu, Gln, and Leu) did not. Metabolite profiling of leaves showed that the levels of tricarboxylic acids, arginine catabolites (ammonium, ornithine), glycine, glycine‐serine ratio and overall carbon‐to‐nitrogen (C/N) ratio were affected differently by the way of applying NO3− and Arg treatments. In addition, the onset of senescence did not coincide with soluble sugar accumulation in control trees or in any of the treatments. We propose that different regulation of C and N status through direct molecular signaling of NO3− and/or different allocation of N between tree parts depending on N forms could account for the contrasting effects of NO3− and tested here amino acids (Arg, Glu, Gln, and Leu) on autumn senescence.

Fertilization with nitrogen (N)-rich compounds leads to increased growth, but may compromise phenology and winter survival of trees in boreal regions. During autumn, N is remobilized from senescing leaves and stored in other parts of the tree to be used in the next growing season. However, the mechanism behind the N fertilization effect on winter survival is not well understood and it is unclear how N levels or forms modulate autumn senescence. We performed fertilization experiments and showed that treating Populus saplings with high or low levels of inorganic nitrogen resulted in a delay in senescence. In addition, by using precise delivery of solutes into the xylem stream of Populus trees in their natural environment, we found that delay of autumn senescence was dependent on the form of N administered: inorganic N (NO3−1) delayed senescence but amino acids (Arg, Glu, Gln, and Leu) did not. Metabolite profiling of leaves showed that the levels of tricarboxylic acids (TCA), arginine catabolites (ammonium, ornithine), glycine, glycine-serine ratio and overall carbon-to-nitrogen (C/N) ratio were affected differently by the way of applying NO3−1 and Arg treatments. In addition, the onset of senescence did not coincide with soluble sugar accumulation in any of the treatments. Taken together, metabolomic rearrangement under different N forms or experimental setups could modulate senescence process, but not initiation and progression in Populus. We propose that the different regulation of C and N status through direct molecular signaling of NO3−1 could account for the contrasting effects of NO3−1 and Arg on senescence.One sentence summaryNitrate, administered by precision fertilization through injection into the trunk, may delay autumn senescence and change metabolism in Populus leaves, while the same amount of amino acids does not have the same effect.