Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

8. Plant Adaptation and Resistance to Unfavorable Environmental Factors
8.6 Resistance to Oxygen Deficiency

Conditions of oxygen deficiency arise during temporary or permanent waterlogging, soil marshing, plant waterlogging, ice crust formation on winter crops, As a result of urban asphalt paving, in irrigated agriculture, during agricultural produce storage, etc.

Despite the low oxygen content in waterlogged or flooded soils, many plant species have adapted to these conditions. Adaptation to oxygen deprivation aims to maintain a near-steady oxygen level within Plant Tissues and to adjust functioning under reduced environmental oxygen concentrations.

In plants whose roots permanently lack oxygen, long-term evolution has driven diverse morphological and anatomical changes in tissue Structure: expansion of the stem base, formation of an additional superficial ROOT system, intercellular ventilation systems, and aerenchyma. Leaves play a significant role in supplying The Root System with oxygen, while lenticels, twigs, and trunks do so in woody plants. In some plants, the fraction of oxygen transported from the aerial parts to the roots accounts for 8-25 % .

The utilization of an external oxygen source and its delivery to roots subjected to Hypoxia or anoxia (shortage or absence of oxygen) cannot fully meet the plant's demands; therefore, PHYSIOLOGICAL AND BIOCHEMICAL (metabolic) adaptations emerge to enable plant survival under reduced oxygen concentrations. These involve reducing the overall Respiration rate and utilizing respiratory substrates, even though substrate reserves are sufficient.

Another type of adaptation involves the reorganization of respiratory pathways, primarily required to sustain ATP synthesis sufficient for plant viability. Under hypoxia, an increased activity of the Pentose Phosphate Pathway can serve an adaptive role, provided There is a mechanism to oxidize the NADPH+ generated within it. As anaerobiosis intensifies, The Glycolytic Pathway of Glucose Catabolism becomes predominant. Despite the low energy efficiency of Glycolysis, its significance for short-term survival under anaerobic conditions is immense, especially when detoxification systems for anaerobic degradation products (ethyl alcohol, lactic acid) are operational. Detoxification involves both the removal of these substances and their re-entry into metabolic pathways.

In the absence of oxygen as a final electron acceptor under anaerobic conditions, processes known as anoxic endogenous oxidation prove adaptive, whereby electrons are transferred to substances such as nitrates and double bonds of unsaturated compounds (Fatty acids, carotenoids). Electron transfer to such acceptors acts as an "emergency exit" and helps prolong plant survival in these environments.

Nowadays, the issue of enhancing plant tolerance to oxygen deficiency is only just beginning to be explored. One agrotechnical measure that increases Plant resistance to excessive moisture is treating plants and soaking seeds in chlormequat chloride solutions. Alleviated growth inhibition and increased yields under barley flooding are also observed following pre-sowing seed soaking in a 0.001-0.000 1 % nicotinic acid solution.