The effect of preconditioning light intensity on photosynthesis of seedlings of two ecotypes of Douglas-fir

Graduation date: 1974Douglas-fir (Pseudotsuga menziesii (Mirb. ) Franco) seedlings\ud of a high elevation Rocky Mountain source and a low elevation coastal\ud source were pretreated under both high and low intensity artificial\ud light in a growth chamber and outside in a cold frame. Net photosynthesis\ud was then measured at three ages, 6, 10, and 18 weeks, by\ud determining net carbon dioxide exchange with an infrared gas analyzer.\ud The light intensity-photosynthesis relationships of the seedlings were\ud determined at a number of light intensities, and the data were fitted\ud by least squares to a photosynthesis-light response curve with the\ud formula [see pdf for formula]\ud in which P is equal to the net photosynthesis, E is equal to the\ud incident light energy, and B₁, B₂, and B₃ are empirical constants.\ud Seedlings from the coastal source were significantly more\ud efficient photochemically regardless of age or light pretreatment as\ud indicated by the steeper initial slope of the light response curve of\ud photosynthesis. The coastal seedlings also had higher average light-saturated\ud rates of photosynthesis, except at 6 weeks of age.\ud The seedlings' response to light preconditioning was similar\ud for both sources but varied with the age and development of the\ud seedlings. The cotyledons of the six-week-old seedlings had a lower\ud maximum photochemical efficiency and were less susceptible to\ud photoinhibition of the light reactions of photosynthesis by high preconditioning\ud light than were the young juvenile needles of the ten-week-\ud old seedlings. The older juvenile needles of the 18-week-old\ud seedlings were less susceptible to photoinhibition of the dark\ud reactions of photosynthesis than were the immature needles of the\ud ten-week-old seedlings. For seedlings of both sources, at any given\ud age, increased preconditioning light intensity resulted in decreased\ud photosynthetic efficiency