REACTIONS OF BIRCH LEAVES TO CHANGES IN LIGHT DURING EARLY ONTOGENY -COMPARISON BETWEEN IN-VIVO AND IN-VITRO TECHNIQUES TO MEASURE CARBON UPTAKE

Trends in several photosynthetic parameters and their response to chan ged growth light were followed for 15 d in leaves of young birch sapli ngs using a rapid-response gas exchange measuring equipment. These in vivo measurements were compared to biochemical assays that were made f rom the same leaves after the gas exchange studies. The measurements w ere made on leaves that were selected prior to the study and were at t hat time of similar age. For the first 7 d the photosynthetic paramete rs were followed from the growth conditions of moderate light (200 mu mol m(-2) s(-1); referred to as controls later in the text). On day 7 some of the saplings were transferred to grow either under high (450 m u mol m(-2) s(-1); referred to as high light plants) or low (75 mu mol m(-2) s(-1); referred to as low light plants) light and the capabilit y of the preselected leaves far acclimation was followed for 6 d. For comparison, at the end of the experiment the measurements were made on both controls and on young leaves that had developed under high and l ow light. Generally the in vivo measured rate of CO2 uptake (gross pho tosynthesis) both at 310 ppm CO2 and 2000 ppm CO2 corresponded very we ll to the biochemically determined CO2 fixation capacity in vitro afte r rapid extraction (measured as the initial and total activity of Rubi sco, respectively). However, if the flux of CO2 into the chloroplasts was limited by the closure of the stomata, as was the case of the high light plants, then the in vitro measured Rubisco activity was greater than the in vivo measured CO2 uptake. V-max, calculated from the meso phyll conductance at 1% O,, exceeded the initial activity of Rubisco ( assayed at saturating RuBP and CO2) constantly by 60%. The catalytic a ctivity of Rubisco in birch leaves was overall very low, even when cal culated from the total activity of Rubisco (k(cat) 0.63-1.18 s(-1)), w hen compared to herbaceous C-3 species. Signs of light acclimation wer e not observed in most of the photosynthetic parameters and in chlorop last structure when mature birch leaves were subjected to changes in g rowth light for 6 d. However, the change of the growth light either to high or low light caused day-to-day fluctuations in most of the measu red photosynthetic parameters and in the case of the high light plants signs of photoinhibition and photodestruction were also observed (dec rease in the amount of chlorophyll and increase in chlorophyll a/b rat io). As a result of these fluctuations these plants achieved a new and lower steady-state condition between the light and dark reactions, as judged from the molar ratio of RuBP to Rubisco binding site.