Leaf Eh and pH: A Novel Indicator of Plant Stress. Spatial, Temporal and Genotypic Variability in Rice (Oryza sativa L.)

Olivier Husson,Jean-Pierre Sarthou,Philippe Menozzi,Stéphane Boulakia,Jaroslav Benada,Stephen Joseph,Alain Audebert,Firmin Tano,Koichi Futakuchi,Ibnou Dieng,Brigitte Soglonou,Lydia Bousset

doi:10.3390/agronomy8100209

Abstract

A wealth of knowledge has been published in the last decade on redox regulations in plants. However, these works remained largely at cellular and organelle levels. Simple indicators of oxidative stress at the plant level are still missing. We developed a method for direct measurement of leaf Eh and pH, which revealed spatial, temporal, and genotypic variations in rice. Eh (redox potential) and Eh@pH7 (redox potential corrected to pH 7) of the last fully expanded leaf decreased after sunrise. Leaf Eh was high in the youngest leaf and in the oldest leaves, and minimum for the last fully expanded leaf. Leaf pH decreased from youngest to oldest leaves. The same gradients in Eh-pH were measured for various varieties, hydric conditions, and cropping seasons. Rice varieties differed in Eh, pH, and/or Eh@pH7. Leaf Eh increases and leaf pH decreases with plant age. These patterns and dynamics in leaf Eh-pH are in accordance with the pattern and dynamics of disease infections. Leaf Eh-pH can bring new insight on redox processes at plant level and is proposed as a novel indicator of plant stress/health. It could be used by agronomists, breeders, and pathologists to accelerate the development of crop cultivation methods leading to agroecological crop protection.

Highlights

PH has been recognized for long as a master variable in plant physiology, from subcellular to cellular and at the whole plant levels [1]
As it could be expected, the spatial and temporal variations measured in leaf Eh, pH and Eh to pH7 (Eh@pH7) are in accordance with the well-known variations in photosynthetic activity: Eh, pH and Eh@pH7 were maximum in the top, youngest leaf, in which photosynthetic activity had not fully started yet, and Eh was maximum in the lowest, oldest leaves, in which photosynthesis was probably not fully functional any longer
This study provides essential information to define the conditions of measurement for these parameters to be used in various disciplines like agronomy, plant breeding, or pathology

Summary

Introduction

PH has been recognized for long as a master variable in plant physiology, from subcellular to cellular and at the whole plant levels [1]. Reactive oxygen species (ROS), which largely determine redox status in the plant, are no longer seen as toxic by-products of aerobic pathways but as beneficial companions of plants’ developmental processes [2]. The consensus view is that redox signaling is intrinsic to many developmental processes and responses to the environment [3,4,5] and that reactive oxygen species (ROS) have a profound influence on almost every aspect of plant biology [6]. ROS are further recognized as important molecules that affect vegetative and pathogenic processes in pathogenic fungi. They are secreted during the interaction of host and pathogen and are involved in tightly controlled intracellular processes [7]. It is expected that a multitude of redox enzymes play an important role in plant immunity regulation [10,11]

Methods

Results

Discussion

Conclusion