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

Abstract

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.

Keywords: Persea americana; electrical signals; hypoxia signaling; root hypoxia; stomatal conductance.

Figure 1

(A) Experiment 1: Schematic diagram of the digital acquisition system for recording extra-cellular voltage difference between the leaf petiole and the base of the stem (ΔV_L–S) in ‘Mexicola’ avocado trees. (B) Experiment 2: Schematic diagram of the digital acquisition system for recording voltage differences between the base of the trunk and the leaf zone (ΔV_l-b) in ‘Hass’ avocado trees.

Figure 2

Experiment 1. (A) Voltage difference between the leaf petiole and the base of the stem (ΔV_L–S) of 5 control plants for 90 min. (B) ΔV_L–S of 5 plants in the root hypoxia treatment from 5 to 90 minutes. Data were collected at 2 Hz and plotted at 1 min intervals. The arrows indicate the beginning of the treatment.

Figure 3

Experiment 1. (A) Voltage difference (ΔV_L–S) at specific points in time regarding the initial voltage value, at 10 min intervals from treatments imposition. NS indicates no significant difference (p > 0.05; t-test) among treatments.

Figure 4

Experiment 1. Average (n = 5) voltage (ΔV_L–S) and oxygen partial pressure (KPa) in the control and root hypoxia treatments. Arrow indicates the beginning of the treatment.

Figure 5

Experiment 2. (A) Soil oxygen diffusion rate (ODR) during the experimental period. (B) Stomatal conductance (gs) of plants in the root hypoxia and control treatments measured at 7-day intervals during the experimental period. (C) Leaf ethylene concentration (µmol mol⁻¹). (D) The number of abscised leaves. Asterisks indicate a significant difference (t-test) between treatments at each time interval. One asterisk indicates significant difference at p < 0.1, two asterisks indicate significant difference at p < 0.05, and three asterisks indicate significant difference at p < 0.01.