The control of autumn senescence in European aspen

Abstract

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.

Figure 1.

Relationships between mean temperatures during scores 3 to 7, date of appearance of yellow color (score 3), and time to complete senescence (score 7) in aspens growing naturally. The number of petals on the flower plots indicates the number of overlapping points at the indicated positions.

Figure 2.

Relationships between origins of the clones (ranked from south to north) and date of initiation of autumn senescence in the SwAsp collection grown in greenhouses under natural (white circles) and controlled (black circles) photoperiods. The dotted line indicates when the additional light was turned off in the controlled-photoperiod experiment (on September 21). Values are means ± sd of six to 10 clones per location. h, Repeatability within populations; R², squared Pearson correlation coefficients for the relationship between trait and latitude of origin. ***, Statistically significant at P < 0.001.

Figure 3.

Relationships between the date of onset and the rate of senescence in greenhouses under natural (A) and controlled (B) photoperiods. NS, Not significant; R², squared Pearson correlation coefficients for the relationship between trait and latitude of origin. ***, Statistically significant at P < 0.001.

Figure 4.

Relationships between clone origins (ranked from south to north) and date of bud set in the SwAsp collection grown in greenhouses under natural (white circles) and controlled (black circles) photoperiods. The dotted line indicates when the additional light was turned off in the controlled-photoperiod experiment (on September 21). Values are means ± sd of six to 10 clones per location. h, Repeatability within populations; R², squared Pearson correlation coefficients for the relationship between trait and latitude of origin. ***, Statistically significant at P < 0.001.

Figure 5.

Relationship between latitude of origin and lag phase between bud set and start of senescence in the SwAsp collection grown in greenhouses under natural (A) and controlled (B) photoperiods. NS, Not significant; R², squared Pearson correlation coefficients for the relationship between trait and latitude of origin. ***, Statistically significant at P < 0.001.

Figure 6.

Timetable of autumn senescence-related events in aspen based on data presented by Keskitalo et al. (2005), Ruttink et al. (2007), and this study. Dates above the line indicate typical dates for the event in trees naturally grown in Umeå, and numbers below the line indicate the variation (days; bottom horizontal arrows) in the length of certain processes. ABA, Abscisic acid.