Abstract
Gas exchange parameters were monitored during the first growing season on Pinus pinaster young seedlings belonging to six provenances and submitted to two water supply regimes in the open air under cover. Significant differences were found between water supply regimes and measurement dates; sometimes also between provenances. Gas exchange rate responses to needle water potential were similar for all the provenances, and rate changes were only detected as water potential went down to less than $-1.3$ MPa. The Iberian provenances, in contrast to the Landes, showed a tendency to save water at the end of Spring, which indicates an adaptation to locations with Summer drought. The growth differences between provenances were not expressed in terms of differences in the instantaneous net photosynthetic rate, since this will depend on other factors, such as seedling water status and the time that the measurement was made. However, provenance growth differences may be partially explained by the differences in water use efficiency and nitrogen productivity.
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