Photosynthetic decline and pigment loss during autumn foliar senescence in western larch (Larix occidentalis)

We measured needle pigment content and photosynthetic rates of 1-year-old western larch (Larix occidentalis Nutt.) during autumn foliar senescence. Chlorophyll (Chl) and carotenoid (xanthophyll + beta-carotene) contents of needles declined 11 and 17%, respectively, before CO2 assimilation rate began to decline. Chlorophyll a/b ratio, Chl/carotenoid ratio, photochemical efficiency (Fv/Fm), and photochemical quenching did not begin to decline until late in senescence. Internal CO2/ambient CO2 did not change during needle yellowing. In seedlings in warmed soil (average 3 degrees C above natural conditions), the decline in needle chlorophyll content was delayed by 10 days and the decline in CO2 assimilation rate was delayed by 5 days, compared with seedlings in soil at ambient temperature. In seedlings exposed to an extended 16-h photoperiod, the decline in needle chlorophyll content was delayed by 32 days, and the decline in CO2 assimilation rate was delayed by 21 days, compared with seedlings exposed to natural day lengths. In addition to delaying the onset of needle senescence, the treatments affected the sequence of events during senescence. Differences among treatment groups provide evidence that the onset of pigment loss and photosynthetic decline and the sequence of events during needle senescence are affected by soil temperature and day length.