Summary -Maritime pine seedlings were grown in 4 L pots filled with coarse sand in a greenhouse. Seedlings were supplied with a nutrient solution with three different concentrations of phosphorus (0, 0.125 and 0.5 mM). After 1 year of growth, gas exchange measurements were performed on mature needles. From these measurements, the main parameters of CO 2 assimilation (the carboxylation efficiency, the apparent quantum efficiency and the maximal rate of electron transport) were estimated using the biochemical model of photosynthesis as described by Farquhar et al (1980)
There are different biomechanical properties between the main and the 3rd tiller. The osmotic potential of the main tiller is lower than that of the 3rd tiller. Accordingly, the main tiller can maintain a positive turgor for longer periods. The turgor threshold of the 3rd tiller is lower and requires a lower turgor to maintain the same relative growth rate as the main tiller. These properties could lead to growth compensation between tillers.The 3rd tiller population which declines during the fast growing phase cannot be sorted at an early stage with the growth parameters we studied, as these parameters change when the decline is obvious. At that time, a decrease in sugar content and a relative increase in nitrate content are measured in declining tillers.
Summary — Nodulated soybeans, Glycine max (L Merr), were grown under 6 photon flux densities (PFD) from 0.35-1.03 mmol photons m -2 ·s -1 , in order to determine the optimal light intensity for symbiotic nitrogen fixation in controlled environment. The nitrogenase activity (C 2 H 2 reduction) was highest at 0.68 mmol photons m -2 ·s -1 which also corresponded to the highest growth of plants and the biggest mass of nodules. The specific acetylene reducing activity was not affected by PFD, (fig 1). L'activité nitrogénase (réduction de l'acé-tylène) est la plus élevée à 0,68 mmol(photons)·m -2 ·s -1 (fig 2), densité de flux de photons qui correspond également aux croissances de plante et aux masses modulaires les plus élevées (fig 1). L'activité spécifique de réduction de l'acétylène par les nodosités n'est pas affectée par la DFP, excepté à 1,03 mmol(photons)·m -2 ·s -1 pour laquelle elle est plus faible (fig 2). La diminution de la DFP n'a pas d'effet sur l'activité nitrogénase pendant au moins 24 h (fig 3). Au contraire une augmentation de la DFP provoque rapidement une diminution significative de l'activité nitrogénase (fig 3). Pour une même DFP, un éclairement périodique comparé à un éclairement stable induit une croissance et une activité nitrogénase plus faibles (fig 2). La photosynthèse détermine donc la masse nodulaire qui en retour détermine la vitesse de fixation de l'azote. Cependant, l'activité nitrogénase nodulaire n'est pas directement régulée par la photosynthèse simultanée, probablement du fait de l'existence de réserves de photosynthats et d'autres mécanismes de ré-gulation. Un excès de lumière inhibe spécifiquement la fixation d'azote.nitrogénase / nodule / photosynthèse / Rhizobium / Glycine max = soja Abbreviations : ARA, acetylene reducing activity; sARA, specific, ie per g nodule acetylene reducing activity; DW, dry weight; PFD, photon flux density.
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