LATE SUMMER-EARLY FALL PHOTOSYNTHESIS IN COTTONWOOD

Abstract

Photosynthesis was investigated during late summer and early fall in a population of mature cottonwood (Populus deltoides) trees growing on the campus of MSU-Billings in southcentral Montana. Parameters related to photosynthesis were measured in situ with a Licor 6400XT Photosynthesis system. A diurnal fluctuation in assimilation was observed with a peak value of 17.0 uM m-2 s-1 CO2 fixed during mid-day. We examined the capacity for assimilation at a PAR of 4000 uM m-2 s-1 (approx. 200% full sunlight) and observed assimilation values as high as 17.6 uM CO2 m-2 s-1 with no indication of photoinhibition. P.deltoides also responded to high ambient CO2 (1600 umol M-1) where assimilation increased to 31.5 umol CO2 m-2 s-1 under 1000 uM m-2 s-1 PAR.  We used an ACi curve fitting utility to obtain values of 104 mmol m-2 s-1, 117 mmol m-2 s-1 and 8.6 mmol m-2 s-1 for rubisco Vcmax, electron flow rate and triose phosphate utilization, respectively. Transpiration was 0.1-6.1 mmol m-2 s-1 and correlated with assimilation. Assimilation declined 37% from the earliest measurements on 23Sep to those taken on 15Oct. We conclude that photosynthesis continues in leaves of P.deltoides well into autumn despite shorter days and cooler temperatures, but with an adaptive response resulting in less CO2 fixation. Leaves can photosynthetically fix carbon, presumably stored as reserve carbohydrates well into late fall before the onset of autumnal leaf senescence.