| Summary: | Deciduous trees integrate environmental signals to determine the onset and rate of
autumnal foliar senescence. In this thesis I studied that process by characterizing
environmentally-induced foliar senescence in western larch (Larix occidentalis Nutt.). I
obtained measurements from seedlings senescing outdoors and from those induced to senesce
in environmentally controlled chambers. The data were examined using graphs and regressions.
Findings from outdoor growing seedlings were similar to results from seedlings that senesced
in growth chambers, suggesting that both experimental designs allow one to study the
senescence process effectively.
I quantified the timing and process of senescence as it varied with leaf age and
environmental conditions. In most experiments, senescing needles displayed a decline in
pigment levels first, followed by carbon assimilation and Rubisco amount, and finally,
chlorophyll a/b ratio and quantum yield. Warmer air temperature delayed the onset of
chlorophyll and photosynthetic decline. Extended photoperiod delayed the onset of chlorophyll
decline but did not affect the timing of photosynthetic decline. Neither air temperature or
photoperiod directly affected the onset of decline in chlorophyll a/b ratio. Instead, the initial
decline in pigment and assimilation rates may have caused the subsequent decline in chlorophyll
a/b ratio. Finally, increased leaf age accelerated the onset of decline of all measures of
senescence.
Environmental conditions affected not only the onset but also the rate of senescence.
While extended photoperiods delayed the onset of pigment loss, the rate of photosynthetic
decline, once it began, was unaffected by extended photoperiod. In contrast, seedlings grown
in 8°C above ambient warmer soil had higher photosynthetic rates per unit chlorophyll than
control seedlings while seedlings grown in 3°C warmer soil did not display different
photosynthetic rates from control seedlings.
Using new methods developed in this thesis, I provide evidence that leaves integrate
temperature and photoperiod signals differently. I show that it is possible to quantify the relative
importance of these conditions in stimulating autumn chloroplast senescence. Furthermore, if
differences in the weather and age of the leaf are accounted for, it is possible to predict the
timing of pigment loss and rate of photosynthetic decline during autumn.
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