Assessing gas exchange, sap flow and water relations using tree canopy spectral reflectance indices in irrigated and rainfed Olea europaea L.

Diurnal and seasonal trends of leaf photosynthesis (A), stomatal conductance to water (gs) and water potential (Ψl), whole-plant transpiration and tree canopy spectral reflectance indices were evaluated in rainfed and well-watered (control) mature olive (Olea europaea L., cv. Leccino) trees. The objective was to evaluate whether photochemical reflectance index (PRI), water index (WI) and normalized difference vegetation index (NDVI) could be used for detecting plant functioning in response to seasonal drought. The measurements were made from March to November, repeated every four weeks during the drought period of the growing season. Rainfed trees were subjected to prolonged water deficit with soil water content ranging between ∼30% and 50% than that of control. Consequently, there were significant differences in the diurnal trend of Ψl, A, gs and sap flux density between treatments. Under severe drought, Ψl ranged between ∼−4.5 MPa (predawn) and ∼−6.4 MPa (midday), A ranged between maximum morning values of ∼6 μmol m−2 s−1 and minimum late afternoon values of 2.5 μmol m−2 s−1, gs was lower than ∼0.03 mol m−2 s−1 for most of the daily courses, whereas stem sap flux density reached maximum peaks of 2.1 g m−2 s−1 in rainfed plants. The diurnal trends of all these parameters fully recovered to the control level after autumn rains. PRI, NDVI, and WI of olive tree canopy assessed significantly the effects of drought on rainfed trees and their subsequent recovery. PRI resulted better correlated with A (r2 = 0.587) than with the other measured parameters, pooling together values measured during the whole growing season. In contrast, NDVI showed a stronger relationship with Ψl (r2 = 0.668) and gs (r2 = 0.547) than with A (r2 = 0.435) and whole-plant transpiration (r2 = 0.416). WI scaled linearly as gs and Ψl increased (r2 = 0.597 and r2 = 0.576, respectively) and, even more interestingly, a good correlation was found between WI and whole-plant transpiration (r2 = 0.668) and between WI and A (r2 = 0.640). Overall PRI and WI ranked better than NDVI for tracking photosynthesis, whereas WI was the most accurate predictive index of plant water status and whole-plant transpiration. This study, which is the first to our knowledge that combines diurnal and seasonal trends of leaf gas-exchange, whole-plant transpiration and reflectance indices, clearly shows that PRI and WI measured at the tree canopy can be used for fast, nonintrusive detection of water stress.