RESPONSE OF TRANSPIRATION AND PHOTOSYNTHESIS TO A TRANSIENT CHANGE INILLUMINATED FOLIAGE AREA FOR A PINUS-RADIATA D DON TREE

Sudden but transient changes in the fraction of illuminated foliage ar ea in a well-watered 7-year-old Pinus radiata D. Don tree were imposed by completely covering either the upper 22% or the lower 78% of the f oliage for periods of up to 36 h. Measurements of transpiration flux d ensity (E), tree conductance (g(t)), stomatal conductance (g(s)) and n et photosynthesis (A) were made to test the hypothesis that compensato ry responses would occur in the remaining illuminated foliage when the cover was installed, When the lower foliage was covered there was an immediate decrease in g(t). However, when tree conductance was normali zed with respect to the illuminated leaf area (g(t)'), it increased be tween 50 and 75%, depending on the value of air saturation deficit (D) , The effect was also apparent from concurrent measurements of increas es in g(s) and A up to 59 and 24%, respectively, for needles in the to p third of the crown, When the cover was removed these effects were re versed, The changes in the lower foliage when the upper foliage was co vered were much smaller. Changes in bulk needle water potential were s mall. It is suggested that the observed responses occurred because of a perturbation to the hydraulic pathway in the xylem that could have t riggered the action of a chemical signal to regulate stomatal conducta nce and photosynthesis.