Virginia Hernandez-Santana

Most stomatal closure in woody species under moderate drought can be explained by stomatal responses to leaf turgor

Reduced stomatal conductance (gs ) during soil drought in angiosperms may result from effects of leaf turgor on stomata and/or factors that do not directly depend on leaf turgor, including root-derived abscisic acid (ABA) signals. To quantify the roles of leaf turgor-mediated and leaf turgor-independent mechanisms in gs decline during drought, we measured drought responses of gs and water relations in three woody species (almond, grapevine and olive) under a range of conditions designed to generate independent variation in leaf and root turgor, including diurnal variation in evaporative demand and changes in plant hydraulic conductance and leaf osmotic pressure. We then applied these data to a process-based gs model and used a novel method to partition observed declines in gs during drought into contributions from each parameter in the model. Soil drought reduced gs by 63-84% across species, and the model reproduced these changes well (r(2)  = 0.91, P < 0.0001, n = 44) despite having only a single fitted parameter. Our analysis concluded that responses mediated by leaf turgor could explain over 87% of the observed decline in gs across species, adding to a growing body of evidence that challenges the root ABA-centric model of stomatal responses to drought.

Role of hydraulic and chemical signals in leaves, stems and roots in the stomatal behaviour of olive trees under water stress and recovery conditions.

The control of plant transpiration by stomata under water stress and recovery conditions is of paramount importance for plant performance and survival. Although both chemical and hydraulic signals emitted within a plant are considered to play a major role in controlling stomatal dynamics, they have rarely been assessed together. The aims of this study were to evaluate (i) the dynamics of chemical and hydraulic signals at leaf, stem and root level, and (ii) their effect on the regulation of stomatal conductance (gs) during water stress and recovery. Measurements of gs, water potential, abscisic acid (ABA) content and loss of hydraulic functioning at leaf, stem and root level were conducted during a water stress and recovery period imposed on 1-year-old olive plants (Olea europaea L.). Results showed a strong hydraulic segmentation in olive plants, with higher hydraulic functioning losses in roots and leaves than in stems. The dynamics of hydraulic conductance of roots and leaves observed as water stress developed could explain both a protection of the hydraulic functionality of larger organs of the plant (i.e., branches, etc.) and a role in the down-regulation of gs. On the other hand, ABA also increased, showing a similar pattern to gs dynamics, and thus its effect on gs in response to water stress cannot be ruled out. However, neither hydraulic nor non-hydraulic factors were able to explain the delay in the full recovery of gs after soil water availability was restored.

Role of leaf hydraulic conductance in the regulation of stomatal conductance in almond and olive in response to water stress

The decrease of stomatal conductance (gs) is one of the prime responses to water shortage and the main determinant of yield limitation in fruit trees. Understanding the mechanisms related to stomatal closure in response to imposed water stress is crucial for correct irrigation management. The loss of leaf hydraulic functioning is considered as one of the major factors triggering stomatal closure. Thus, we conducted an experiment to quantify the dehydration response of leaf hydraulic conductance (Kleaf) and its impact on gs in two Mediterranean fruit tree species, one deciduous (almond) and one evergreen (olive). Our hypothesis was that a higher Kleaf would be associated with a higher gs and that the reduction in Kleaf would predict the reduction in gs in both species. We measured Kleaf in olive and almond during a cycle of irrigation withholding. We also compared the results of two methods to measure Kleaf: dynamic rehydration kinetics and evaporative flux methods. In addition, determined gs, leaf water potential (Ψleaf), vein density, photosynthetic capacity and turgor loss point. Results showed that gs was higher in almond than in olive and so was Kleaf (Kmax = 4.70 and 3.42 mmol s(-1) MPa(-1) m(-2), in almond and olive, respectively) for Ψleaf > -1.2 MPa. At greater water stress levels than -1.2 MPa, however, Kleaf decreased exponentially, being similar for both species, while gs was still higher in almond than in olive. We conclude that although the Kleaf decrease with increasing water stress does not drive unequivocally the gs response to water stress, Kleaf is the variable most strongly related to the gs response to water stress, especially in olive. Other variables such as the increase in abscisic acid (ABA) may be playing an important role in gs regulation, although in our study the gs-ABA relationship did not show a clear pattern.

The phloem–xylem consortium: until death do them part

It is difficult to find an organ in the plant where the phloem and xylem do not run in parallel, effectively ‘hugging’ each other. Over the last two decades, there has been a growing body of evidence confirming the interaction and coordination between these two vascular tissues (De Schepper et al. 2013, Mencuccini et al. 2013). And so it must be, since both need each other to fully develop their functions, like two symbionts in a mutually beneficial relationship. Xylem transports water and nutrients to the leaves where they are converted in sugars. Increased turgor pressure in the phloem drives food to sink organs below the canopy through phloem sieve tubes. Nature had to coordinate the regulation of both tissues to enable them to function as smoothly as a Swiss watch, since the performance success of the plant in a changing environment rests on the fine tuning of their activity. Our knowledge of phloem transport in plants under natural conditions and the regulation of this transport are, however, not well understood (De Schepper et al. 2013, Mencuccini et al. 2013). Experimental techniques aimed at measuring phloem transport rates have proven unsuccessful due to the fragility of this vascular tissue (Helfter et al. 2007). This is despite recent efforts to noninvasively determine phloem and xylem sap flow velocities using nuclear magnetic resonance, indicator-dilution techniques, pulselaser-based approach and plant positron emission tomography (Helfter et al. 2007, Hubeau and Steppe 2015). Interestingly, the application of stem diameter variations measured with dendrometers together with biophysical modeling could enhance our understanding of phloem function, which in turn would shed light on whole-plant functioning and source–sink relationships (Mencuccini et al. 2013, De Swaef et al. 2015). Although some models have been developed, including one that demonstrates the hydraulic connection between xylem and phloem in plants (Steppe et al. 2006, De Pauw et al. 2008, Mencuccini et al. 2013), and another that shows simultaneous water and sugar transport (Daudet et al. 2002, Hölttä et al. 2006, Lacointe and Minchin 2008, De Schepper and Steppe 2010, Nikinmaa et al. 2012), they have not been able to link the coordination of phloem and xylem with stomata regulation. Hölttä et al. (2017) have developed a whole-tree model in which xylem and phloem fluxes are used to link leaf gas exchange, sugar utilization and soil water uptake. The central hypothesis stated by the authors is that the plant attempts to maximize the photosynthesis rate within the constraints imposed by meteorological conditions, soil water availability and sugar transport by the phloem and its subsequent use by sinks. Their theoretical framework allows for a simultaneous limitation of photosynthesis by sources and sinks. Hölttä et al. (2017) have shown, as a cornerstone for this novel coordination between the two vascular tissues, the role played by stomata. Stomatal conductance is at the crossroads of both water and CO2 fluxes in the plant, whereupon the regulation of stomatal conductance might involve phloem fluxes as well as the maintenance of a favorable carbon balance. Nowadays, xylem and phloem are not considered to be passive transport elements, and over the past 20 years or so it has been demonstrated that these two vascular tissues are involved in communicating and processing information (Holbrook and Zwieniecki 2005). Xylem and phloem lie side-by-side in the plant, sharing water potential, which is largely determined by xylem tension induced by transpiration. This, obviously, affects turgor pressure within sieve elements, which influences the phloem transport of photoassimilates. In turn, the osmotic potential in the sieve elements gives rise to the final equilibrium in water potential between xylem and phloem. At the

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.

Precision Irrigation in Olive (Olea europaea L.) Tree Orchards

Abstract The olive tree is well adapted to water stress but, at the same time, it shows a remarkable response to irrigation. Full irrigation is rarely the best option, both because olive is usually grown in areas where water for irrigation is scarce and because of its remarkable response to low irrigation supplies. We address in this work new approaches for the management of deficit irrigation in olive orchards, in a context of a rational use of water in agriculture. Special attention is paid to precise irrigation and, more precisely, to irrigation strategies suitable for olive orchards, and to new methods allowing for the continuous and automatic assessment of tree water stress, with a potential for irrigation scheduling. We focus on methods based on measurements related to sap flow, trunk diameter variations, and leaf turgor pressure, since evidence shows the great potential of plant-based methods as compared to those using measurements of soil water status or atmospheric demand. This review includes the potential of those methods for precise irrigation in olive orchards, with particular attention to hedgerow olive orchards with high plant densities. Such potential is achieved after combining plant-based measurements with remote imagery and user-friendly applications for smartphones, tablets, or computers. After considering these new advances in the management of irrigation in olive orchards, we review the effects of irrigation on the production of both fruit and oil, with special attention to those aspects for which the impact of irrigation is still unclear.