The effects of temperature, photoperiod, and leaf age on foliar senescence in western larch (larix occidentalis nutt)

• Main Author: Rosenthal, Selma Iduna
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/8960

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.