Gas exchange characteristics of Pinus canariensis needles in a forest stand on Tenerife, Canary Islands

Gas exchange characteristics and chlorophyll fluorescence of the Canarian endemic pine (Pinus canariensis) were measured during the day for a year in a field stand on Tenerife, Canary Islands, Spain. Diurnal tendencies of gas exchange were variable depending on ambient conditions. In general they paralleled photosynthetic photon flux density with only one peak at midday, except on summer days with high air vapour pressure deficit (VPD), when needles exhibited a severe midday depression of CO2 assimilation rate (A), in parallel with a reduction of stomatal conductance (g(s)). The internal CO2 concentration tendencies during the day suggest that stomatal closure was the main cause of the midday depression of photosynthesis. Chlorophyll fluorescence data corroborate this assertion, with the parameter F-v/F-m reaching high values throughout day and year. P. canariensis living in the sub-tropic exhibited high values of A (maximal A value of 17 mumol m(-2) s(-1)) and high optimal needle temperature for photosynthesis (25degreesC) which were at the upper limit of the values given for conifers and similar only to data obtained for some pine species adapted to habitats at similar latitudes. g(s) was reduced to half when VPD attained 40 mbar, allowing this pine to have high A/g(s) values during high evaporative demand conditions.