Changes in respiration and chemical content during autumnal senescence of Populus tremuloides and Quercus rubra leaves

Abstract

Changes in respiration rate, chemical content and chemical concentration were measured in leaves of field-grown Populus tremuloides Michx. and Quercus rubra L. trees throughout the growing season and autumnal senescence. Chlorophyll, soluble sugar, N, P, K and Mg contents and concentrations all declined during leaf senescence, whereas Ca content and concentration increased. Leaf dry mass per area declined 24 and 35% in P. tremuloides and Q. rubra, respectively, during senescence. In leaves of both species, respiration rates peaked during leaf expansion in the spring and then declined, as a result of reduced cytochrome-mediated respiration, to reach relatively constant rates by midsummer. In senescing P. tremuloides leaves, respiration rates remained relatively constant until mid-October and then declined rapidly. In senescing Q. rubra leaves, respiration rates increased in late September, as a result of the appearance of residual respiration that could not be reduced by respiratory inhibitors, and then declined quickly in early November. No changes in alternative pathway respiratory activity were observed in leaves of either species during senescence until late autumn when rates declined. Because respiration rates were correlated with both leaf sugar and nitrogen content during leaf senescence, we conclude that respiration rates were maintained or increased during leaf senescence to supply energy for degradation and mobilization of chemical constituents.

Resorption Protection. Anthocyanins Facilitate Nutrient Recovery in Autumn by Shielding Leaves from Potentially Damaging Light Levels

“…The reductions in LMA observed in WT plants within each treatment are comparable with other studies where decreases in LMA ranged from 16% to 42% (Oland, 1963;Adams et al, 1990;Collier and Thibodeau, 1995;Polle et al, 2001). Senescencerelated reductions in LMA may be expected because senescence activity normally results in an increasingly carbon-starved environment within the leaf (Buchanan et al, 2000).…”

“…In contrast, the resorption efficiencies of these same plants would indicate better resorption performance by WT (77%) versus mutant plants (73%), due to the 55% greater presenescent leaf N content of WT plants. In general, leaf N content slowly declines from peak values in early to mid summer until the onset of senescence, when nutrients are exported more rapidly (Chapin and Moilanen, 1991;Collier and Thibodeau, 1995;Kö niger et al, 2000). A source of error within the presenescent N content measurements is variation in timing of presenescent nutrient resorption between plant types, leading to disparities in presenescent values between plants that had exported different amounts of N by the time of sampling.…”

Seasonal controls on net branch CO2 assimilation in sub-Arctic Mountain Birch (Betula pubescens ssp. czerepanovii (Orlova) Hamet-Ahti)

“…At Kevo, we observed disproportionately high values of the respiration parameter in light response curves for Mountain Birch during the early growing season, consistent with the observed seasonality of foliar respiration in other deciduous trees (Collier and Thibodeau, 1995, Ow et al, 2009and Xu and Baldocchi, 2003. The temperature response of whole-branch respiration measured at Abisko also varied seasonally, indicating higher temperature sensitivity during the early season.…”

Water use strategies and ecosystem-atmosphere exchange of CO<sub>2</sub> in two highly seasonal environments

“…It is unclear whether the spot radiation measurements that serve as indicator for changes in canopy LAI were sufficiently precise to represent the entire forest; there may well have been some shedding of leaves that was not picked up by the radiation sensors. However, autumnal leaves when still attached contain proteases and proteins that are associated with respiration and the breakdown of molecules (Bhalerao et al, 2003), and broad-leaf trees generally appear to either maintain or increase leaf respiration during senescence (Collier and Thibodeau, 1995). The remobilisation of leaf nutrients therefore is likely the cause for the observed autumnal increase in Betula respiration before the onset of leaf fall.…”