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).

Nature of Water logging 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.

Plant Water Relations in Flooding Stress

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.

Levels of Endogenous PGRs under Flooding Stress

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.

Mitigation of water logging

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

please see paper.

There are many ways that flooding can damage plants. Excessive moisture in soil decreases oxygen levels. This impedes respiration (where energy is released from sugars) in the roots leading to the build-up of carbon dioxide, methane and nitrogen gases. Ultimately, the roots can suffocate and die. Toxic compounds such as ethanol and hydrogen sulphide can also build up in the soil and damage plants. If leaves and stems are submerged, photosynthesis(absorption of energy from the sun to produce sugars) can be inhibited and plant growth can slow or even stop

Plants that are suffering from excessive-water stress are more prone to infection by disease-causing organisms such as fungi or insects. Also, excessively wet soil tends to favour the growth of soil-microbes such as Fusarium spp., Phytophora spp. and Rhizoctonia solani, which can infect plant roots leading to diseases such as root and crown rot.

The deposition of soil and rocks onto plants during flooding can damage plants, as can the exposure of roots to the air by the washing away of surrounding soil. Also, even after the flood waters recede the damaged plants can be more vulnerable to other stresses. For example, trees with substantial root damage are more likely to be uprooted in windy weather.

"It's absolutely vital that one doesn't walk on the soil when it's this wet because you compact it and destroy its structure. Don't dig it or disturb it but leave it to its own devices until the tide goes out," says Guy Barter, head of RHS advisory service.

Plants likely to be worst affected include those from dry climates such as lavender and rosemary, while lawns can also suffer as a result of excessively wet weather, he notes. Don't mow the lawn in wet weather or even walk on it, as the pressure can cause structural damage, especially to those grown from seed in spring - most established lawns can cope with excessive rainfall.

Kindly mention your crop/plant in which you are planning to work. After that you will fined the appropriate answer of your. Questions arise in your mind. Suppose if you are working with pigeon pea or corn/ maize. These above mentioned crops develop airenchima formation in Root's are root regenration abilities after withdrawal of stress. Kindly contact my well known plant physiologist my research supervisor Prof. J. P. Srivasta for detail advise. The mail id I'd jpsbhu25 @yahoo.com. Thanks for sharing the idea of most time needed problem related to global demand.

Thank you for all the answers and I appreciate your time and wisdom, I am still taking some academic subjects, such as plant stress physiology, water relations in plants and vegetable physiology, this semester which will enhance my understanding towards my planned thesis.

By the way, my proposed crop would be onion (my province is the highest producer) which is a highly susceptible to flood/high moisture especially when "unexpected typhoon" landed during planting season which is a result of climate change in the Philippines. My ideal work plan would be most likely to combat the harmful effects of flood stress such as use of antioxidants/chemicals that may alleviate the stress on the plant. Also, the level of flood stress could be variable depending on the water content/submergence in the soil as well as the amount of time of flood stress condition. Most likely it would be a pot experiment first under greenhouse condition and if the outcome is favorable it may undergo field trial.

These are some of my reading articles:

Waterlogging tolerance of Welsh onion (Allium fistulosum L.) enhanced by exogenous spermidine and spermine

Use of spermidine reduced the oxidative damage in onion seedlings under salinity by modulating antioxidants

It may be a challenged for this crop, also it still a long run (about 5 months) for me to write my proposed thesis, I will still gather more information, related researches and discussion with my adviser. I am gladly to contact all of you to seek for an advice.