Leaf gas exchange characteristics of a Canarian laurel forest tree species [Persea indica (L.) K. Spreng.] under natural conditions

Persea indica (L.) K. Spreng. is a tree species of the Canarian laurel forest. Diurnal courses of gas exchange were measured during the period of one year in fully expanded current-year leaves in the uppermost canopy (sun leaves, 18 m above ground) and in the lower canopy (shade leaves, 12 m above ground) in order to understand the general behaviour of this tree species in relation with the environmental conditions of its habitat. Persea indica showed a great plasticity in gas exchange characteristics with large differences between sun and shade leaves (50 percnt; of maximum carbon assimilation rate (Amax) reduction in the shade) that were modulated by the strong light attenuation and high leaf area index (LAI) of the stand. This species presented low Amax, about 11 μmol m−2 s−1, high maximum transpiration (E, above 8 mmol m−2 s−1) and high stomatal conductance (gs, 800 mmol m−2 s−1) and therefore very low instantaneous water use efficiency (WUE, mean maximum 1.5 mmol mol−1) and A/gs (mean maximum between 20-35 μmol mol−1) and high mean minimum Ci value (around 237 μmol mol−1), measured in sun leaves. All these gas exchange characteristics fit well with a non-conservative strategy of water use efficiency, which is appropriate for species that are subject to mild or short duration droughts. Overall, this species is more similar to subtropical plants of humid regions than to sclerophyllous plants of Mediterranean-type climate. However, under strong sunlight and dry conditions, sun leaves of P. indica show midday stomatal closure and downregulation of photochemical efficiency, effective ways to avoid excess water loss and photodamage of photosynthetic apparatus.