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 waterpotential (l), whole-plant transpiration and tree canopy spectral reflectance indices were evaluated inrainfed and well-watered (control) mature olive (Olea europaea L., cv. Leccino) trees. The objective wasto evaluate whether photochemical reflectance index (PRI), water index (WI) and normalized differencevegetation 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 droughtperiod of the growing season. Rainfed trees were subjected to prolonged water deficit with soil watercontent ranging between ∼30% and 50% than that of control. Consequently, there were significant differ-ences in the diurnal trend of l, A, gsand sap flux density between treatments. Under severe drought, lranged between ∼−4.5 MPa (predawn) and ∼−6.4 MPa (midday), A ranged between maximum morningvalues of ∼6 mol m−2s−1and minimum late afternoon values of 2.5 mol m−2s−1, gswas lower than∼0.03 mol m−2s−1for most of the daily courses, whereas stem sap flux density reached maximum peaksof 2.1 g m−2s−1in rainfed plants. The diurnal trends of all these parameters fully recovered to the con-trol level after autumn rains. PRI, NDVI, and WI of olive tree canopy assessed significantly the effects ofdrought 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 growingseason. In contrast, NDVI showed a stronger relationship with l(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 gsand lincreased (r2= 0.597and r2= 0.576, respectively) and, even more interestingly, a good correlation was found between WI andwhole-plant transpiration (r2= 0.668) and between WI and A (r2= 0.640). Overall PRI and WI ranked betterthan NDVI for tracking photosynthesis, whereas WI was the most accurate predictive index of plant waterstatus and whole-plant transpiration. This study, which is the first to our knowledge that combines diurnaland seasonal trends of leaf gas-exchange, whole-plant transpiration and reflectance indices, clearly showsthat PRI and WI measured at the tree canopy can be used for fast, nonintrusive detection of water stress.