José E. Fernández

Regulation of photosynthesis and stomatal and mesophyll conductance under water stress and recovery in olive trees: correlation with gene expression of carbonic anhydrase and aquaporins

Summary In plants with sclerophyll leaves, the response of stomatal and mesophyll conductance to CO2 to water stress and recovery is correlated with the expression of aquaporins and carbonic anhydrase.

Vulnerability to cavitation in Olea europaea current-year shoots: further evidence of an open-vessel artifact associated with centrifuge and air-injection techniques

Different methods have been devised to analyze vulnerability to cavitation of plants. Although a good agreement between them is usually found, some discrepancies have been reported when measuring samples from long-vesseled species. The aim of this study was to evaluate possible artifacts derived from different methods and sample sizes. Current-year shoot segments of mature olive trees (Olea europaea), a long-vesseled species, were used to generate vulnerability curves (VCs) by bench dehydration, pressure collar and both static- and flow-centrifuge methods. For the latter, two different rotors were used to test possible effects of the rotor design on the curves. Indeed, high-resolution computed tomography (HRCT) images were used to evaluate the functional status of xylem at different water potentials. Measurements of native embolism were used to validate the methods used. The pressure collar and the two centrifugal methods showed greater vulnerability to cavitation than the dehydration method. The shift in vulnerability thresholds in centrifuge methods was more pronounced in shorter samples, supporting the open-vessel artifact hypothesis as a higher proportion of vessels were open in short samples. The two different rotor designs used for the flow-centrifuge method revealed similar vulnerability to cavitation. Only the bench dehydration or HRCT methods produced VCs that agreed with native levels of embolism and water potential values measured in the field.

A regulated deficit irrigation strategy for hedgerow olive orchards with high plant density

Background & AimsThere is not a consensus on the best irrigation approach for super-high density (SHD) olive orchards. Our aim was to design and test a regulated deficit irrigation (RDI) strategy for a sustainable balance between water saving, tree vigour and oil production.MethodsWe tested our RDI strategy for 3xa0years in an ‘Arbequina’ orchard with 1,667 trees ha−1. Two levels of irrigation reduction were applied, 60RDI and 30RDI, scaled to replacing 60xa0% and 30xa0%, respectively, of the of irrigation needs (IN). We also had a full irrigation (FI) treatment as control, with IN totalling 4,701xa0m3xa0ha−1ResultsThe 30RDI treatment showed the best balance between water saving, tree vigour and oil production. With a yearly irrigation amount (IA) of 1,366xa0m3xa0ha−1, which meant 72xa0% water saving as compared to FI, the reduction in oil yield was 26xa0% only.ConclusionsOur results, together with recent knowledge on the effect of water stress on fruit development, allowed us to suggest a potentially improved RDI strategy for which a total IA of ca. 2,100xa0m3xa0ha−1 was calculated. Both some management details and the benefits of this suggested RDI strategy are still to be tested.

Potential and limitations of improving olive orchard design and management through modelling

Abstract Data from Spanish and Italian olive orchards with different cultivars (“Manzanilla”, “Frantoio”, “Leccino”, “Razzola” and “Taggiasca”) growing in different environments (southern Spain; north, centre and south of Italy) were used to illustrate how models on water use and photosynthetic behaviour of the olive tree can be useful tools for choosing pruning intensity and canopy shape for optimum water use and carbon assimilation. Both original and published data on olive physiological behaviour were used to illustrate limitations of model performance due to both inadequate input variables and poor description of the processes involved. We observed differences on leaf water status between cultivars and locations, likely due to differences in soil characteristics related to soil matric potential and soil hydraulic conductivity. This suggests that the volumetric soil water content may not be the best variable for characterizing the soil water status when modelling, and highlights the importance of considering specific environmental conditions before extrapolating data from the literature to specific orchards. We evaluated the importance of considering the reduction in olive photosynthetic capacity under stress conditions, driven by a reduction in leaf mass per area and leaf nitrogen content. A modelling exercise was then made for a “Manzanilla” olive tree at the Spanish orchard, by combining a gas exchange model for olive leaves with a model able to simulate the spatial distribution of radiation and leaf – gas exchanges within the olive canopy as a function of canopy structure, canopy microclimate, and physical and physiological leaf properties. Simulated values for two different canopy shapes showed that a top-open spherical canopy, typical in the area, improved water use efficiency, as compared to a spherical canopy closed at the top. The model also showed the value of leaf area density that must be left after pruning for optimizing carbon assimilation relative to water consumption.

Improving xylem hydraulic conductivity measurements by correcting the error caused by passive water uptake.

Xylem hydraulic conductivity (K) is typically defined as K = F/(P/L), where F is the flow rate through a xylem segment associated with an applied pressure gradient (P/L) along the segment. This definition assumes a linear flow-pressure relationship with a flow intercept (F(0)) of zero. While linearity is typically the case, there is often a non-zero F(0) that persists in the absence of leaks or evaporation and is caused by passive uptake of water by the sample. In this study, we determined the consequences of failing to account for non-zero F(0) for both K measurements and the use of K to estimate the vulnerability to xylem cavitation. We generated vulnerability curves for olive root samples (Olea europaea) by the centrifuge technique, measuring a maximally accurate reference K(ref) as the slope of a four-point F vs P/L relationship. The K(ref) was compared with three more rapid ways of estimating K. When F(0) was assumed to be zero, K was significantly under-estimated (average of -81.4 ± 4.7%), especially when K(ref) was low. Vulnerability curves derived from these under-estimated K values overestimated the vulnerability to cavitation. When non-zero F(0) was taken into account, whether it was measured or estimated, more accurate K values (relative to K(ref)) were obtained, and vulnerability curves indicated greater resistance to cavitation. We recommend accounting for non-zero F(0) for obtaining accurate estimates of K and cavitation resistance in hydraulic studies.

The effect of strobilurins on leaf gas exchange, water use efficiency and ABA content in grapevine under field conditions

Strobilurins are one of the most important classes of agricultural fungicides. In addition to their anti-fungal effect, strobilurins have been reported to produce simultaneous effects in plant physiology. This study investigated whether the use of strobilurin fungicide improved water use efficiency in leaves of grapevines grown under field conditions in a Mediterranean climate in southern Spain. Fungicide was applied three times in the vineyard and measurements of leaf gas exchange, plant water status, abscisic acid concentration in sap ([ABA]), and carbon isotope composition in leaves were performed before and after applications. No clear effect on stomatal conductance, leaf water potential and intrinsic water use efficiency was found after three fungicide applications. ABA concentration was observed to increase after fungicide application on the first day, vanishing three days later. Despite this transient effect, evolution of [ABA] matched well with the evolution of leaf carbon isotope ratio, which can be used as a surrogate for plant water use efficiency. Morning stomatal conductance was negatively correlated to [ABA]. Yield was enhanced in strobilurin treated plants, whereas fruit quality remained unaltered.

Deficit irrigation and fertigation practices in olive growing: Convergences and divergences in two case studies

Abstract The response of mature olive (Olea europaea L.) trees to both regulated deficit irrigation and fertigation was studied for some consecutive years in sub-humid (Italy) and semi-arid conditions (Spain). All cultivars tested in these experiments were preferentially for table olive consumption (pickling and double-aptitude). Data from these long-term studies on field-grown, mature trees are summarized for assessing common effects of water and nutrient supply on plant behaviour and fruit yield. Seasonal yield variations were observed, and showed that the effect of irrigation treatments was somewhat more gradual in the “on” than in the “off” year. Differences amongst cultivars were always marked. Dry matter accumulation in pulp and pit was positively affected by progressively increasing the amount of water supplied to plants, regardless of years and cultivars. The ratio between pulp and pit fresh weight was negatively affected by irrigation, while fruit size was positively influenced by the gradual increase of irrigation water. Average nitrogen levels in leaves were generally above the deficiency threshold at both experimental plantations. Crop yield was not consistently increased by addition of soil nutrients. The effect of fertigation treatments was relatively uninfluential on oil accumulation in fruits. However, polyphenols in oil samples decreased progressively as the addition of nutrients increased. Critical aspects and strategic decisions in modern olive growing are the selection of cultivar, the supply of water, the option of fertilization, together with other orchard management practices. In this paper we outline various aspects of long-term studies in experimental olive plantations with the aim of highlighting inter-regulatory processes, involving plant growth and crop yield, which must be taken into account when planning future field research and modelling exercises.

Virgin olive oil quality of hedgerow 'Arbequina' olive trees under deficit irrigation.

BACKGROUNDnRegulated deficit irrigation (RDI) is used in hedgerow olive orchards to achieve a sustainable balance between water savings, tree vigor and oil production. Its effects on the presence of compounds responsible for the taste of the oil and its nutritional value are controversial. The present 3-year study was conducted in an Arbequina orchard (1667 trees ha-1 ) under a full irrigation (FI) treatment (470.1 mm year-1 of water) and two RDI treatments scaled to replace 60% and 30%, respectively, of FI. The quality parameters, antioxidant contents and volatiles of the extracted virgin olive oil (VOO) were analyzed.nnnRESULTSnIn general, oils from the 30% RDI treatment had higher contents of pigments and phenolic compounds, a higher oleic/linoleic ratio and the highest oxidative stability, despite their lower tocopherol content. FI oils showed higher (E)-2-hexenal, 1-penten-3-one, ocimene, E-2-pentenal and pentene dimer contents than 30RDI oils, but lower contents of (E)-2-pentenol and volatile esters.nnnCONCLUSIONnThe results of the present study suggest that a RDI strategy supplying 30% of the total irrigation needs induces an increase in natural antioxidants in VOO. Neither yield, nor the rest of the quality parameters were affected by the reduced irrigation. However, abundant autumn precipitation can over-ride these effects of 30% RDI treatment on oil quality.

Classification models for automatic identification of daily states from leaf turgor related measurements in olive

Abstract The leaf patch clamp pressure (LPCP) probe is being used to remotely assess leaf turgor pressure. Recently, different shapes of the LPCP daily curves have been suggested as potential water stress indicators for irrigation scheduling. These curves shapes, called states, have been studied and related to different water stress levels for olives. To our knowledge, the only way to differentiate these curves shapes or states is through the visual observation of the dynamics of the LPCP records during the day, which is highly time-consuming and reduces its potential to automatically schedule irrigation. The aims of this study were: (i) to obtain a random forest model to automatically identify the states from daily LPCP curves recorded in olive trees, by using visually identified states to train the model; (ii) to improve the identification of state II through a second random forest model, relating this state to the midday stem water potential, and; (iii) to obtain a random forest model to identify the states based on ranges of stem water potential. We used LPCP daily curves collected in a commercial olive orchard from 2011 to 2015. The states were visually identified for the days on which concomitant measurements of stem water potential and leaf stomatal conductance were made. We had a data set of 307 LPCP daily curves, being 157 curves in state I, 78 in state II and 71 in state III. The two biggest inflection points of the LPCP curves were used to adjust the models through the use of the R package “randomForest”, using the Leave-p-Out Cross-Validation method. With the first model, which was obtained from the whole dataset, its data regarding the inflection points and the visually identified states, we obtained an overall accuracy of 94.37%. With the second model, obtained with the use of the data regarding curves visually identified as state II only, the overall accuracy was of 88.64%. This model was adjusted to be used after the first model, to narrow the stem water potential range of state II curves. Finally, the third model was obtained using the whole dataset and the states established from ranges of stem water potential. This last model did not consider the visual identification, and yielded an overall accuracy of 88.08%. Our results facilitate the use of LPCP probes, since it allows for the automatic identification of the states related to leaf turgor pressure, a key information to schedule irrigation.

Lowering the Fertilisation Rate for Corn Monocropping: Nutritional Parameters

Corn (Zea mays cv Prisma) was cropped consecutively for 5 years under the common Mediterranean management practice of plentiful fertilisation and irrigation. The nutritional status of the crop was studied over 5 years of mono-cropping at three different stages of growth: plantlets of 15 cm height, early tas-seling and mature plants. The fertilisation was applied at both one of the highest rates used in SW Spain, 1000 kg ha -1 of a 15(N)-15(P 2 0 5 )-15(K 2 O) fertiliser, plus two top-dressings of urea each of 400 kg ha -1 , and the same fertiliser but at one third of this rate. The nutritional status of the crop, and yield, decreased over the study. Concentrations of N and P declined along the years, this effect being most evident during early growth in the case of P and in mature plants in the case of N. Zinc concentration also declined over the first 4 years. DRIS approach also reflected a decrease in the nutritional status of the crop. The high rate of fertilisation did not alleviate the decline in nutritional status, which shows that its routine application is not justified under Mediterranean conditions since it hardly provokes any crop response and increases the risk of environmental pollution.