Pinus pumila photosynthesis is suppressed by water stress in a wind-exposed mountain site

Wind exposure is known to have stressful effects on plant growth, particularly at high altitudes. We studied how environmental factors affected carbon assimilation in Pinus pumila needles at a wind-exposed site. Needle gas exchange rates were determined for detached shoots in the laboratory where the needles were free from field environmental stresses, and also determined for attached shoots in the field under in situ environment. There was no difference in gas exchange characteristics determined in the laboratory between shoots from the wind-exposed and the wind-protected sites, suggesting that wind exposure did not affect the photosynthetic potential. In the field, however, the photosynthetic rate of one-year-old needles (in situ A_area) was significantly lower at the wind-exposed site than that at the wind-protected site. There was a positive correlation between the in situ A_area and the xylem pressure potential, suggesting that water deficit caused photosynthetic suppression at the wind-exposed site. The in situ A _area was lower at the wind-exposed site, even with the same electron transport rate and the same stomatal conductance. These results suggest that CO₂ assimilation is suppressed by lower mesophyll CO₂ conductance at the wind-exposed site. We conclude that the carbon gain is limited by water stress in wind-exposed regions.