Potentiel de photosynthèse hivernale du douglas (Pseudotsuga menziesii Mirb.) en relation avec le régime thermique

In the eastern France, near Nancy, at different days from 02.16th.1979 to 05.16th.1979, shoots of a 19 year old Douglas-fir stand were cut and their net-photosynthesis response to temperature determined in laboratory under light-saturation conditions.1. Until beginning of April : - The response of net-photosynthesis to temperature was characterized by a wide optimal zone centered on 16-18 °C rather than by a marked optimum. No significative transformation in the shape of that response curve was observed during the measurement period. - The photosynthetic capacity is submitted to important changes corresponding to a damage and reparation action on the photosynthetic function linked to the alternation of frost and thaw periods. The after-effects of the thermal regime act upon photosynthesis in a cumulative way and with a 10 day retentivity. 2. From beginning of April to bud-flushing (05.16th) - In a first time, a more and more important impairment of the CO2 assimilation rate of the shoot was observed at the higher temperatures. Simultaneously the thermal optimum underwent a slight down to 8-10 °C, without the above mentionned climatic determinism of the photosynthetic capacity being significantly affected. That transformation appeared to be due to an increase of the respiratory activity of the developing buds. - In a second time, a few days before bud flushing, the photosynthetic capacity decreased in its turn under the influence of physiological changes which seemed to affect the photosynthetic apparatus itself. The Douglas-fir is characterized by a great capacity of reparation after a cold stress which allows it to turn to account mild temperature winter periods in a climate like that of Nancy. In the temperate zone such an evergreen conifer winter photosynthesis pattern is intermediate between the total winter photosynthetic rest which haracterizes the continental and mountain cold winter situations and a pattern without any winter depression as has been observed in mild winter oceanic conditions.

Cet article est une contribution à l’étude des potentialités photosynthétiques hivernales des conifères en zone tempérée. Il met en évidence une action du régime thermique sur l’activité photosynthétique du douglas à deux niveaux d’échelle dans le temps. A l’échelle instantanée, la réponse photosynthétique à la température se caractérise par un large domaine optimal plutôt que par un optimum marqué. A l’échelle de la décade, la capacité photosynthétique oscille en permanence sous l’action d’un jeu d’endommagement - réparation de la fonction photosynthétique liée à l’alternance de périodes de gel et de dégel. A l’approche du débourrement, notre étude permet de relier les modifications intervenant dans les échanges gazeux de CO2 au niveau du rameau au développement des bourgeons.