Pilar M. Gil

Electrical signaling, stomatal conductance, ABA and Ethylene content in avocado trees in response to root hypoxia

Avocado (Persea americana Mill.) trees are among the most sensitive of fruit tree species to root hypoxia as a result of flooded or poorly drained soil. Similar to drought stress, an early physiological response to root hypoxia in avocado is a reduction of stomatal conductance. It has been previously determined in avocado trees that an extracellular electrical signal between the base of stem and leaves is produced and related to reductions in stomatal conductance in response to drought stress. The current study was designed to determine if changes in the extracellular electrical potential between the base of the stem and leaves in avocado trees could also be detected in response to short-term (min) or long-term (days) root hypoxia, and if these signals could be related to stomatal conductance (gs), root and leaf ABA and ACC concentrations, ethylene emission from leaves and leaf abscission. In contrast to previous observations for drought-stressed trees, short-term or long-term root hypoxia did not stimulate an electrical potential difference between the base of the stem and leaves. Short-term hypoxia did not result in a significant decrease in gs compared with plants in the control treatment, and no differences in ABA concentration were found between plants subjected to hypoxia and control plants. Long-term hypoxia in the root zone resulted in a significant decrease in gs, increased leaf ethylene and increased leaf abscission. The results indicate that for avocado trees exposed to root hypoxia, electrical signals do not appear to be the primary root-to-shoot communication mechanism involved in signaling for stomatal closure as a result of hypoxia in the root zone.

Effect of soil water-to-air ratio on biomass and mineral nutrition of avocado trees

Abstract In Chile, expansion of avocado production has resulted in many orchards established in marginal soils that are poorly drained and have high soil water-to-air ratios when soil moisture is at field capacity. However, avocado trees are sensitive to poor soil aeration . A study was conducted to determine the effects of different soil water-to-air ratios (W/A) on biomass and nutrient content of avocado trees. Two-year-old avocado trees were grown for 2 seasons in containers in soils, with different W/A, collected from different avocado growing regions of Chile. There were five treatments corresponding to each of the five soils. At field capacity, the two-season average W/A was 1.7, 1.3, 0.6, 0.4 or 0.3 for treatments T1, T2, T3, T4, or T5, respectively. The same amount of fertilizer was applied to each soil. Mineral ele-ment concentrations and total mineral element contents in leaves, shoots, wood and roots were determined for each tree in each treatment at the end of the experimental period. Shoot and root fresh and dry weights, leaf area and leaf retention were also determined. Although all treatments showed non-limiting soil oxygen conditions for avocado root growth, trees in soils with lower W/A had greater shoot and root dry weights and longer autumn leaf retention. Macro- and micronutrient concentrations in any plant tissue were not related to soil W/A. However, total tissue contents of N, P, K, Ca, Mg, C, N and B in roots and whole plants were highest in treatments with lower soil W/A. The results indicate that soil W/A significantly affects growth and mineral nutrition of avocado trees and should be considered for avocado site selec-tion and management.

The electrical response of fruit trees to soil water availability and diurnal light-dark cycles

Recent studies have associated the effect of water stress, irrigation, and light cycles with electrical signaling in fruit tree species including avocado, blueberry, lemon and olive. In those studies, changes in the electrical potential (EP) difference were detected between the base of the stem and leaf in response to drought, irrigation, and diurnal changes in light and dark. In avocado, the changes in EP between the base of the stem and leaf petiole (ΔVL-S) observed in response to decreased soil water content have been associated with a decrease in stomatal conductance, indicating that stomatal closure might be associated with an electrical signal. New experiments were conducted to determine the effects of short- and long-term drought on root to leaf electrical signaling in avocado confirming that in both situations significant changes in EP differences can be detected and that an extra-cellular electrical signal appears to be involved in root to leaf communication initiating stomatal closure. Day-to-night fluctuations in EP observed in fruit trees appeared to correspond to the time of day and were therefore presumably affected by diurnal changes in ambient light and vapor pressure deficit. Addendum to: Gil PM, Gurovich L, Schaffer B, Alcayaga J, Rey S, Iturriaga R. Root to leaf electrical signaling in avocado in response to light and soil water content. J Plant Physiol 2008; 165:1070-8.

Stomata regulation by tissue-specific expression of the Citrus sinensis MYB61 transcription factor improves water-use efficiency in Arabidopsis

Water-use efficiency (WUE) is a quantitative measurement of biomass produced per volume of water transpired by a plant. WUE is an important physiological trait for drought response to mitigate the water deficiency. In this work, a cisgenic construction from Citrus sinensis was developed and its function in the improvement of WUE was evaluated in Arabidopsis. Sequences of the CsMYB61 coding region, a transcription factor implicated in the closure of stomata, together with a putative stomata-specific promoter from CsMYB15, were identified and cloned. The protein encoded in the CsMYB61 locus harbors domains and motifs characteristic of MYB61 proteins. In addition, a 1.2 kb promoter region of the gene CsMYB15 (pCsMYB15) containing regulatory elements for expression in guard cells and in response to Abscisic Acid (ABA) and light was isolated. In Arabidopsis, pCsMYB15 directs the expression of the reporter gene GUS in stomata in the presence of light. In addition, transgenic lines expressing the CsMYB61 coding region under transcriptional control of pCsMYB15 have a normal phenotype under in vitro and greenhouse conditions. These transgenic lines exhibited a smaller opening of the stomata pore, lower stomatal conductance and respiration rate, enhanced sensitivity to exogenous ABA, and high drought stress tolerance. Our results indicate that stomata-specific expression of CsMYB61 enhances water use efficiency under drought conditions in Arabidospis.

Sensitivity and Variability of Two Plant Water Stress Indicators: Exploring Criteria for Choosing a plant Monitoring Method for Avocado Irrigation Management

El palto (Persea americana Mill.) es una especie frutal altamente sensible al estres hidrico, haciendo el manejo del riego una tarea dificil para los productores. Cuando el riego es inadecuado, los arboles reducen el crecimiento, pierden fruta y sufren dano radical. Este estudio aborda el tema de como evaluar el estres hidrico en palto y las consideraciones para elegir un indicador de estres hidrico de la planta. Este trabajo analizo la sensibilidad y variabilidad de dos indicadores de estres en respuesta al deficit hidrico: potencial hidrico xilematico (SWP) y contraccion maxima diaria del tronco (MDTS). Durante un periodo de alta demanda hidrica, un grupo de arboles de palto establecidos en suelo franco-arcilloso fue sometido a estres hidrico mediante suspension del riego y comparado con arboles control regados segun la evapotranspiracion maxima de cultivo. Durante el estudio, los arboles alcanzaron un SWP minimo de -0.9 MPa y un MDTS maximo de ~ 285 μm. Para entender mejor la respuesta de los arboles al estres hidrico tambien se midio concentracion foliar de acido abscisico, conductancia estomatica, humedad del suelo y deficit de presion de vapor. Los indicadores de estres hidrico mostraron diferencias entre los arboles tratados y control despues de 3 d de tratamiento. Fue posible observar que MDTS fue un indicador mas sensible que SWP, intensidad de la senal de 4.5 vs. 1.2 respectivamente; sin embargo, la mayor variabilidad de MDTS disminuyo su capacidad como indicador de estres, coeficiente de variacion de 32% vs. 9% respectivamente. Este estudio confirma que monitorear el estres hidrico es una herramienta importante para el manejo de riego en palto y que la eleccion del metodo debiera considerar tanto la sensibilidad como la variabilidad del indicador.