Effects of water supply on gas exchange in Pinus pinaster Ait. provenancesduring their first growing season

Gas exchange parameters were monitored during the first growing season on P inus pinaster young seedlings belonging to six provenances and submitted to two water supply regimes in the open air under cover. Significant differen ces were found between water supply regimes and measurement dates; sometime s also between provenances. Gas exchange rate responses to needle water pot ential were similar for all the provenances, and rate changes were only det ected as water potential went down to less than -1.3 MPa. The Iberian prove nances, in contrast to the Landes, showed a tendency to save water at the e nd of Spring, which indicates an adaptation to locations with Summer drough t. The growth differences between provenances were not expressed in terms o f differences in the instantaneous net photosynthetic rate, since this will depend on other factors, such as seedling water status and the time that t he measurement was made. However, provenance growth differences may be part ially explained by the differences in water use efficiency and nitrogen pro ductivity.