RELATIONSHIP BETWEEN NET PHOTOSYNTHESIS AND NITROGEN IN SCOTS PINE - SEASONAL-VARIATION IN SEEDLINGS AND SHOOTS

The relationship between light saturated net photosynthesis (A(max)) and nitrogen concentration (N) was studied in needles of both Scots pine seedlings, grown at three relative growth rates (2, 6 and 8 %) controlled by nutrient addition rate, and Scots pine shoots collected from four sites with different fertility. In the seedlings, A(max) was measured on 14 different dates starting at the beginning of the second growing season and ending when growth of the new shoot and the secondary needles had finished. In shoots from the natural stands A(max) of the previous-year shoots was measured on 6 dates throughout the growing season. Both in seedlings and shoots, the correlation between A(max) and N was poor, when data from all sampling dates were taken together. However, A(max) was correlated with N in most instances when the age of the needles was considered and the data were examined either at weekly intervals (seedlings) or separately for each sampling date (shoots). The slope of the A(max) vs N relationship varied greatly between sampling dates. In the seedlings the correlation between A(max) and N was strongest by the time when the new needles were developing. In the shoots the correlation was significant from mid June until mid August, while no correlation was found in the beginning and at the end of the growing season. Our data indicate that in pine needles the photosynthesis-nitrogen relationship is more complex than in broadleaved species. Contrary to the broadleaved species, where the correlation is independent of sampling time, in this conifer the time of the year affects the correlation and there are phases during the growing season when the correlation is poor or nonexistent.