J.E. Fernández

Stomatal control of water use in olive tree leaves

Little is known about the strategies used by olive trees to overcome the long dry periods typical of the areas where they are cropped. This makes it difficult to optimize the water supply in orchards. To study the control of water consumption by olive trees, measurements of leaf water potential (Ψ) and stomatal conductance to H2O (g) were made on 26-year-old t Manzanillo olive trees under three irrigation treatments. The first treatment provided enough water to cover the crop water demand, the next treatment supplied one third of that rate, and the final treatment was no irrigation at all, typical of dry-farming conditions. Under conditions of high vapour pressure deficit of the air (Da), the olive trees prevented excessive water loss by closing their stomata. Leaves of the current year showed better stomatal control than did the 1-year-old leaves. The upper-bound functional relationships between t g and t Da and photon flux density (IP) were obtained by boundary-line analysis, based on a technique of non-linear least squares. Maximum values of t g were observed at relatively low levels of t IP, from about 500 μmol m-2 s-1, and a proportional decrease in t g with increasing t Da was also found, at least for values of up to approximately 3.5 kPa. Higher values of t g were observed in the morning than in the afternoon, for similar levels of t IP and t Da. Unirrigated olive trees recovered quickly after the dry season, showing values of Ψ and t g similar to those of irrigated trees after just two days.

Soil physical properties, water depletion and crop development under traditional and conservation tillage in southern Spain

Tillage methods affect soil physical properties and, thus, have a direct influence on the replenishment and depletion of soil water storage and crop performance. This study was conducted to determine the effects of traditional and conservation tillage on soil physical properties, soil water replenishment and depletion, and crop development and yield under southern Spanish conditions. The experiments were carried out from 1992 to 1995 in a sandy clay loam soil (Xerofluvent). The traditional tillage (TT) method consisted mainly of the use of mouldboard ploughing, and the conservation tillage (CT) was characterized by not using mouldboard ploughing, by reduction of the number of tillage operations and leaving the crop residues on the surface as mulch. In both tillage treatments a wheat (Triticum aestivum, L.)-sunflower (Helianthus annuus, L.) crop rotation was established. In each treatment, systematic measurements of bulk density, resistance to penetration, infiltration rate and hydraulic conductivity (using tension disc infiltrometers) in the soil top layer were carried out. Changes in water profiles through the experimental period were also followed using a neutron probe. Crop development and yield were determined. The soil bulk density in the 0 to 20 cm layer was significantly higher in the CT than in the TT treatment, mainly after tillage operations (between 10% and 24% higher in CT than in TT). After 3 years of continuous tillage treatments, the soil bulk density did not increase. The resistance to penetration at any time was higher in the CT than in the TT treatment, but not always significantly different. Infiltration rates were significantly higher in the TT than in the CT treatment (about 35% higher in TT than in CT). After 3 years of the tillage treatments the hydraulic conductivity of the soil surface layer, at a pressure head of 0 mm, was significantly higher in the CT (124 mm h−1) than in the TT (66 mm h−1). This is related to the existence of preferential paths created by an increase of the earthworm population in the CT treatment. Soil water profiles showed higher replenishment of soil water storage in the CT than in the TT treatment, particularly from October 1994 to June 1995 when the lowest precipitation of the experimental period was recorded. Plant height, leaf area index (LAI) and root length density (RLD) of the first sunflower crop were significantly higher in the TT than in the CT treatment. Nevertheless, the seed yield was slightly higher in the CT than in the TT treatment. In the second sunflower crop, plant height, LAI, RLD were significantly higher in the CT than in the TT treatment except early in the season, and yield was significantly (and extremely) higher in the CT (1521 kg ha−1) than in the TT (473 kg ha−1) treatment. During the wheat crop season, plant height and RLD were higher in the TT than in the CT treatment, but grain yield was again slightly higher in the CT than in the TT treatment. The conservation tillage applied seems to be highly effective in enhancing soil water recharge and water conservation, particularly in years with much lower than average precipitation.

Water balance and nitrate leaching in an irrigated maize crop in SW Spain

Abstract During 3 consecutive years (1991–1993) a field experiment was conducted in an intensively irrigated agricultural soil in SW Spain. The main objective of this study was to determine the water flow and nitrate (N0 3 ) leaching, below the root zone, under an irrigated maize crop and after the growing season (bare soil and rainy period). The experiment was carried out on a furrow-irrigated maize crop at two different nitrogen (N)-fertilization rates, one the highest traditionally used by farmers in the region (about 500 kg N ha −1 per year) and the other one-third of the former (170 kg N ha −1 per year). The aim was to obtain data that could be used to propose modifications in N-fertilization to maintain crop yield and to prevent the degradation of the environment. The terms for water balance (crop evapotranspiration, drainage and soil water storage) and nitrate leaching were determined by intensive field monitoring of the soil water content, soil water potential and extraction of the soil solution by a combination of neutron probe, tensiometers and ceramic suction cups. Nitrogen uptake by the plant and N0 3 -N produced by mineralization were also determined. The results showed that, in terms of water balance, crop evapotranspiration was similar at both N-fertilization rates used. During the irrigation period, drainage below the root zone was limited. Only in 1992 did the occurrence of rainfall during the early growing period, when the soil was wet from previous irrigation, cause considerable drainage. Nitrate leaching during the whole experimental period amounted to 150 and 43 kg ha −1 in the treatments with high and low N-fertilization, respectively. This occurred mainly during the bare soil and rainy periods, except in 1992 when considerable nitrate leaching was observed during the crop season due to the high drainage. Nitrate leaching was not so high during the bare soil period as might have been expected because of the brought during the experimental period. A reduction of N-fertilization thus strongly decreased nitrate leaching without decreasing yield.

Drip irrigation, soil characteristics and the root distribution and root activity of olive trees

A study was carried out on the root distribution and root activity of the olive tree (Olea Europaea, L., var. manzanillo) as influenced by drip irrigation and by several soil characteristics such as texture and depth. The experiments were conducted in two plots within a drip-irrigated grove of 20-year-old trees planted at 7×7 m spacing. One soil was a sandy loam, the other a clay-loam. Both cylinder and trench methods were used to determine root distribution. Labelling with 32P was used to determine root activity. Under dryland conditions the adult tree adapted its rooting system, following the installation of a drip system, by concentrating the roots within the wet soil zones near the drippers. The highest root densities occur in those zones, down to a 0.6 m depth, the most abundant being the <0.5 mm diameter roots. The most intensive root activity was also found in that zone. For a given irrigation system, wet soil bulbs are more extensive and therefore root distribution expands to a larger soil volume when the soil is more clayey and with a hard calcareous pan present at about 0.8 m depth which prevents deep drainage.

The use of sap flow measurements for scheduling irrigation in olive, apple and Asian pear trees and in grapevines

We evaluated three approaches for scheduling irrigation in wine grape vineyards and in olive, apple and Asian pear tree orchards, based on sap flow measurements and models of plant transpiration. In the first approach, we analysed how the shape of the sap-flow profile changed in response to root-zone soil water conditions and potential evaporative demand. The second approach was based on a transpiration ratio, as defined from the actual daily water use of a target plant divided by the potential daily water use of similar-sized plants under non-limiting soil water conditions (“well-irrigated” plants). Values of the actual plant water use were always determined from measured sap flow. Two independent methods were assessed for the calculation of potential plant water use; either sap flow was measured in well-irrigated plants or we used a leaf-area based model of plant transpiration. On some occasions water stress was found to modify the shape of the sap velocity profile. However, most of the time the velocity profile was found to be an insensitive indicator for triggering irrigation. The transpiration ratio method, using measured sap flow in well-irrigated plants, was more useful for irrigation scheduling, at least for the two species (i.e. olive and grape) that were investigated here. Nonetheless, realization of such an approach in a commercial orchard may not be practical due to problems associated with irrigation management e.g. excessive vegetative growth may occur on the reference plants over time. Besides, irrigating the orchard to maintain non-limiting soil water conditions is not always the best option for water and nutrient management. The alternative transpiration ratio method based on a leaf-area based model of plant water use, yielded the best results. Modelled transpiration rates always provided reliable information not only for well-irrigated plants, but also for deficit-irrigated plants. This result lends support to the use of the method for irrigation scheduling of vineyard and orchard trees. However, the use of models does require detailed microclimate data as well as a user-friendly technique to quantify plant leaf area. From a practical viewpoint the method should encompass the spatial variability of the soil and plants within the orchard. Accurate quantification of these factors is a cornerstone of precision horticulture and such information would help to minimise risks associated with insufficient as well as excessive irrigation applications.

Water use and yield of maize with two levels of nitrogen fertilization in SW Spain

Abstract Maize (cv. Prisma ), managed with furrow irrigation was cropped consecutively for three years under Mediterranean management practices in the Guadalquivir river valley of SW Spain. Two N fertilization rates were used in different subplots: Subplot A had 510 kg N ha −1 yr −1 , a rate widely used in the area. Subplot B at 170 kg N ha −1 yr −1 , was one third of this. The water balance components and crop response to the irrigation regime were quantified to determine the efficiency of furrow irrigation practices in the area. Furthermore we sought to establish whether a reduced N fertilization rate would have any influence on the crop yield. No consistent differences were observed between subplots in any of the water balance components. Measured crop evapotranspiration ( ET c ) averaged 6250 m 3 ha −1 Water losses by drainage were usually below 20% of ( ET c ) Soil evaporation under the canopy ( E ) was measured by using a specially-designed microlysimeter. Root density, soil water content and soil bulk density inside the microlysimeters were similar to those measured in the surrounding soil. Average values of E between two irrigations when the crop was fully mature amounted for 18°-20% of ET c . A maximum value of 1.6 for the crop coefficient ( K c ) was calculated after using the Penman-Monteith combination equation for the calculation of the reference evapotranspiration ( ET r ). A standard formulation was used for the resistance term, which might account for the large value of ( K c ) Measurements of crop development parameters (height, leaf area index and phenological stage) showed some differences between subplots during the growing period, but no final differences were detected. No differences in yield were observed during the course of the study. Monitoring of physiological parameters (leaf water potential, leaf conductance and net photosynthesis rate) showed that water relationships of maize were not affected by the reduced N fertilization.

Leaf patch clamp pressure probe measurements on olive leaves in a nearly turgorless state.

The non-invasive leaf patch clamp pressure (LPCP) probe measures the attenuated pressure of a leaf patch, P(p) , in response to an externally applied magnetic force. P(p) is inversely coupled with leaf turgor pressure, P(c) , i.e. at high P(c) values the P(p) values are small and at low P(c) values the P(p) values are high. This relationship between P(c) and P(p) could also be verified for 2-m tall olive trees under laboratory conditions using the cell turgor pressure probe. When the laboratory plants were subjected to severe water stress (P(c) dropped below ca. 50 kPa), P(p) curves show reverse diurnal changes, i.e. during the light regime (high transpiration) a minimum P(p) value, and during darkness a peak P(p) value is recorded. This reversal of the P(p) curves was completely reversible. Upon watering, the original diurnal P(p) changes were re-established within 2-3 days. Olive trees in the field showed a similar turnover of the shape of the P(p) curves upon drought, despite pronounced fluctuations in microclimate. The reversal of the P(p) curves is most likely due to accumulation of air in the leaves. This assumption was supported with cross-sections through leaves subjected to prolonged drought. In contrast to well-watered leaves, microscopic inspection of leaves exhibiting inverse diurnal P(p) curves revealed large air-filled areas in parenchyma tissue. Significantly larger amounts of air could also be extracted from water-stressed leaves than from well-watered leaves using the cell turgor pressure probe. Furthermore, theoretical analysis of the experimental P(p) curves shows that the propagation of pressure through the nearly turgorless leaf must be exclusively dictated by air. Equations are derived that provide valuable information about the water status of olive leaves close to zero P(c) .

Design and testing of an automatic irrigation controller for fruit tree orchards, based on sap flow measurements

We designed and tested an automatic irrigation control system for fruit tree orchards, designated CRP. At the end of each day, the device calculates the irrigation dose (ID) from sap flow readings in the trunk of trees irrigated to replenish the crop water needs, relative to similar measurements made in over-irrigated trees. It then acts on the pump and electrovalve to supply an ID sufficient to keep the soil close to its field capacity during the irrigation period. Remote control of the system is possible from any computer or Smartphone connected to the Internet. We tested the CRP in an olive orchard in southern Spain. The device was robust and able to filter and amplify the output voltages of the heat-pulse velocity probes and to calculate reliable sap flow data. It calculated and supplied daily irrigation amounts to the orchard according to the specified irrigation protocol. The remote control facility proved to be useful for getting real-time information both on the CRP behaviour and the applied IDs, and for changing parameters of the irrigation protocol. For our conditions, olive trees with big root systems growing in a soil with a remarkable water-holding capacity, the approach mentioned above for calculating ID had not enough resolution to replace the daily crop water consumption. The device, however, was able to react when the soil water content fell below the threshold for soil water deficit. The threshold value was identified with simultaneous measurements of stem water potential in the instrumented trees. Our results suggest a change in the irrigation protocol that will allow the CRP to apply a recovery irrigation whenever that threshold is reached, making the device suitable for applying a deficit irrigation strategy in the orchard.

Simulating the fate of water in a soil-crop system of a semi-arid Mediterranean area with the WAVE 2.1 and the EURO-ACCESS-II models

In this paper. we evaluated the WAVE 2.1 and the EURO-ACCESS-II models for predicting crop water consumption, water losses by drainage and volumetric soil water content in a cropped soil under Mediterranean conditions. A detailed dataset was constructed for a furrow-irrigated experimental plot on a homogeneous sandy soil, cropped with maize during the seasons 1992 and 1993. The calibration and the validation results of the models were evaluated by using both simulation graphics and two specific modelling evaluation statistics: the root mean square error (RMSE) and the modelling efficiency. Results showed that both models are able to predict the fate of water in this coarse textured irrigated soil subjected to semi-arid Mediterranean environmental and agronomical conditions. Reasonably good predictions of the seasonal evolution both of the crop evapotranspiration and the water losses by drainage were obtained with both models. Soil moisture was also well predicted by the models when considering total soil moisture storage in the root zone of plants. However, the deviations between the predicted and observed moisture values increased when considering each soil layer separately. Sensitivity analysis showed that the input values of the crop coefficient have a significant influence on predictions made by the WAVE 2.1 model. The value of the hydraulic conductivity close to saturation had a significant influence on the predicted water losses by drainage in the EURO-ACCESS-II model. Our results showed that calibration for the specific environmental conditions of the field sites is required before using any of the two models in a purely prediction model. During this calibration, special attention must be given to the input values of those variables which are most affected by spatio-temporal variability within the field

Simulation of the impact of subsoil compaction on soil water balance and crop yield of irrigated maize on a loamy sand soil in SW Spain

Abstract Irrigation of crops in Mediterranean countries can produce some conditions that favour soil compaction processes. The SIMWASER model takes into account the effects of subsoil compaction on water balance and crop yield. The objectives of this paper were: (i) to test the mentioned model using the data set collected, during three years (1991–1993), from irrigation experiments with maize (Zea mays L., cv. Prisma) on a sandy soil (Cambisols (FAO, 1990) or Xerocrepts (USDA, 1998)) in SW Spain and (ii) to estimate the influence of subsoil compaction on soil water balance and crop yield assuming long lasting heavy subsoil compaction that may be developed under irrigation for the SW Spain conditions. The model was run to simulate soil water content, evapotranspiration, drainage below the root zone, and crop yield for the same period in which the experiment was carried out. Results of simulation were compared with the experimental results in order to know the agreement between them. The results obtained show a fairly good agreement between simulated and measured values for most of the parameters considered. For the scenario in which subsoil compaction is developed under irrigation, the results simulated by the model indicate a reduction of the rooting depth. However, the effects on water balance and crop yield in this sandy soil were not relevant under the SW Spain conditions.