Modeling photosynthesis in olive leaves under drought conditions.

We quantified parameters for a model of leaf-level photosynthesis for olive, and tested the model against an independent dataset. Specific temperature-dependence parameters of the model for olive leaves were measured, as well as the relationship of the model parameters with area-based leaf nitrogen (N) content. The effect of soil water deficit on leaf photosynthesis was examined by applying two irrigation treatments to 29-year-old trees growing in a plantation: drip irrigation sufficient to meet the crop water requirements (I) and dry-farming (D). In both treatments, leaves had a higher photosynthetic capacity in April than in August. In August, photosynthetic capacity was lower in D trees than in I trees. Leaf photosynthetic capacity was linearly and positively related to leaf N content on an area basis (N(a)) and to leaf mass per unit area (LMA), and the regression slope varied with irrigation treatment. The seasonal reduction in N(a) was used in the model to predict photosynthesis under drought conditions. Olive leaves showed a clear limitation of photosynthesis by triose phosphate utilization (TPU) even at 40 degrees C, and the data suggest that olive invests fewer resources in TPU than other species. The seasonal decrease in photosynthetic capacity moderated the stomatal limitation to carbon dioxide (CO(2)) fixation as soil water deficit increased. Further, it enabled leaves to operate close to the transition point between photosynthetic limitation due to RuBP carboxylation capacity and that due to RuBP regeneration capacity, and resulted in a near constant value of internal CO(2) concentration from April to August. Under well watered conditions, N-use efficiency of the olive leaves was enhanced at the expense of reduced water-use efficiency.

Calibration of sap flow estimated by the compensation heat pulse method in olive, plum and orange trees: relationships with xylem anatomy.

The compensation heat pulse method is widely used to estimate sap flow in conducting organs of woody plants. Being an invasive technique, calibration is crucial to derive correction factors for accurately estimating the sap flow value from the measured heat pulse velocity. We compared the results of excision and perfusion calibration experiments made with mature olive (Olea europaea L. 'Manzanilla de Sevilla'), plum (Prunus domestica L. 'Songal') and orange (Citrus sinensis (L.) Osbeck. 'Cadenero') trees. The calibration experiments were designed according to current knowledge on the application of the technique and the analysis of measured heat pulse velocities. Data on xylem characteristics were obtained from the experimental trees and related to the results of the calibration experiments. The most accurate sap flow values were obtained by assuming a wound width of 2.0 mm for olive and 2.4 mm for plum and orange. Although the three possible methods of integrating the sap velocity profiles produced similar results for all three species, the best results were obtained by calculating sap flow as the weighted sum of the product of sap velocity and the associated sapwood area across the four sensors of the heat-pulse-velocity probes. Anatomical observations showed that the xylem of the studied species can be considered thermally homogeneous. Vessel lumen diameter in orange trees was about twice that in the olive and plum, but vessel density was less than half. Total vessel lumen area per transverse section of xylem tissue was greater in plum than in the other species. These and other anatomical and hydraulic differences may account for the different calibration results obtained for each species.