Stress Management

Water logging refers to a condition when water is present in excess amount than its optimum requirement. It creates an anaerobic situation in the rhizosphere due to which the plant experiences the stress (O2 deficient stress).

In the water logged soils, water gets filled in the pores of the soil which are previously occupied by O2. Such soils suffer O2 deficiency.

This O2 deficiency depresses growth and survival of plants growing in it. Flood sensitive plants (eg. Tomato, soybean and sunflower) are killed in the water logged conditions, while the tolerant species (eg. Rice) withstand water logging for a considerable time. However, continuous submergence of rice for more than 10 days is also deleterious resulting in death and decay of the plants.

The flooding often induces stomatal closure mostly in C3 plants. This causes lower water flow in these plants. This also results in leaf dehydration because of reduced root permeability. Ultimately, wilting of leaves occurs due top the restricted water flow from the roots to the shoots.

Occurrence of these changes in leaves, shoots or roots is due to the transfer of toxic substances (acetaldehyde / alcohol) produced under anaerobic conditions in the roots as well as the levels of plant growth regulators (PGRs) transported from the roots to shoots via transpiration stream.

Endogenous levels of PGRs such as GA and cytokinins (CK) are reduced in the roots. This has enhanced levels of ABA and ethylene in the shoots causing stomatal closure and early onset of senescence respectively.

It is also reported that levels of auxins are reduced and that of Aminocyclopropane -1-Carboxylic Acid (ACC), precursor for the ethylene biosynthesis are increased under flooding stress.

Important roles played by these endogenous PGRs during high moisture (flooding) stress are summarized in the following table.

Table 1. Effect of flooding stress on the endogenous levels of PGRs and their effect on plants

Sl. No.	Level of PGR in plants	Effects on plants under water logging
1.	Reduced Auxins	Causes “Hypertrophy” (Swelling of stem base by collapse or enlargement of cells in cortex)
2.	Decreased GA	Causes reduction in cell enlargement and stem elongation
3.	Decreased CK	Results in early on-set of senescence and reduced rate of assimilate partitioning to the sinks
4.	Increased ABA	Cause stomatal closure with consequential decrease in the rate of gas exchanges during photosynthesis, respiration and transpiration; results in efflux of K+ from the guard cells; decreases ion transport due to lower rate of transpiration; decrease the starch formation in the guard cells resulting in stomatal closure
5.	Increased Ethylene	Causes “Epinasty” of leaves (uneven growth of leaves due to more cell elongation on upper side than the lower side of the leaf); induces senescence and Hypertrophy in plants.

Thus, the O2 stress in the roots under flooding produces signals, via transpiration stream, to the leaves affecting stomatal behaviour ultimately.

Providing adequate drainage for draining excessive stagnating water around the root system.
Spray of growth retardant of 500 ppm cycocel for arresting apical dominance and thereby promoting growth of laterals
Foliar spray of 2% DAP + 1% KCl (MOP)
Nipping terminal buds for arresting apical dominance and thus promoting growth sympodial branches (as in cotton) for increasing productivity
Spray of 40 ppm NAA for controlling excessive pre-mature fall of flowering/buds/young developing fruits and pods
Spray of 0.5 ppm brassinolide for increasing photosynthetic activity
Foliar spray of 100 ppm salicylic acid for increasing stem reserve utilization under high moisture stress
Foliar spray of 0.3 % Boric acid + 0.5 % ZnSO4 + 0.5 % FeSO4 + 1.0 % urea during critical stages of the stress

Dr. A. Senthil, Associate Professor (Crop Physiology), Dept. of Biotechnology, CPMB, TNAU, Coimbatore.