Mitigation of Water Stress

The adverse effects of water stress on crop growth can be mitigated by the application of chemicals such as nutrients, anti-transpirants and Plant Growth Regulators (PGRs), which induce the plants to become adaptive to water stress situations for a specified period and the water requirement for such periods can be minimized or saved.

1. Nutritional Management

Among the major nutrients, potassium and magnesium are found to be highly deficient due to water deficit conditions. Therefore, application of potassium enhances the water uptake as well as the water relations in the plant tissues by osmoregulation processes, by acting as a potent osmoregulator (osmolyte), thereby the solute potential is reduced. Besides, potassium nutrition also helps in the favourable stomatal regulatory mechanisms, which regulate the water balance of the plants. This has also resulted in the increased WUE of the plants. Similarly, magnesium is component of chlorophyll, its content and uptake is drastically reduced due to the water stress effect. This is most prominent in Mg-loving crops like cotton.

 Besides macronutrients, deficiencies of micronutrients also appear under water deficit situations due to the following reasons:

    
Therefore, foliar application of the following nutrients depending upon the occurrence of their efficiencies will mitigate the water-stress induced nutritional imbalance in crops.

Use of Antitranspirants

In India, about 90% of the land is under rainfed farming; therefore, it is very essential to manage every drop of water received through rains. Though various measures are adopted to conserve he rain water, yet rainfed farming is often subjected to drought. Transpiration is said to be unavoidable evil but it has several functions to attend in the crop cycle. For producing one tone of food, the crop plant requires varied amount of water as furnished below:

Cereals and legumes: 400 – 500 litres of water / kg of grains
Fruits and vegetable: 1000 litres / kg of food

Water transpired by crops (season / plant):

Maize                                 : 200 litres
Sunhemp                           :  27 litres
Cotton                               : 8 – 10 litres / day
Citrus                                :  100 – 200 litres / day
Trees (9 – 10 m height)      : 300 – 800litres / day
Forest trees of 400 – 600 trees:  20,000 barrels / day (1 barrel = 500 litres)

Similarly, the WUE of crops is also different and ranges from 0.24 to 1.75 kg / mm of water / ha. The WUE of sorghum is higher but that of cotton is the lowest. This difference lies with the maturity period and nutritive value of the crop. Cotton grows for six to seven months while sorghum grows for four moths.

Drought reduces the yield by 0 – 100% depending upon the severity. Prolonged drought can drastically reduce the yield to zero level. But, intermittent drought for 10 – 15 days at early or late stage is common under rainfed conditions. Drought during the critical phenological phase like flowering and grain development is highly detrimental. However, the crop productivity is dependant on how fast a plant can recover after a stress of 6-10 days.

The severity of intermittent drought of 6-10 days during critical stages of the crop can reasonably be avoided by the use of antitranspirants and thus crops can be saved. Antitranspirants can effectively be used to the crop under water stress with adverse rainfall.

Classification of Antitiranspirants (ATs) and field responses

The ATs are categorically classified on mode of action in the following four types:

I. Materials causing stomatal closure

1. Herbicides like 2, 4 – D, Phosphon D and Atrazine
2. Fungicides like Phenyl Mercuric Acetate (PMA)
3. Metabolic inhibitors like hydroxy sulfonates, potassium metabisulphite etc.
4. Growth hormones like ABA, Ethrel, TIBA, succinic acid, ascorbic acid and
   Cycocel (CCC)

II. Reflectant Types

1. Kaoline
2. China Clay
3. Calcium bicarbonate
4. Lime water

III. Thin-forming chemicals

1. Hexadecanol (Higher alcohols)
2. Cetyl alcohol
3. Methanol
4. Paclobutrazol
5. Brassinolide
6. Resorcinol

IV. Polyethylene materials forming thick films

1. Mobileaf
2. Folicot
3. Waxol
4. S- 800
5. Hico-110R
(All the above chemicals are trade names given by the companies)

The purpose of ATs is to maintain the growth and productivity under stress conditions and it is never recommended for high productivity / unit area. It saves the crop and helps to get marginal yield when the expectations are zero.

Role of ATs in Irrigation Water Saving

Some of the ATs can also be used through drip (as Fertigation) to save the frequency of irrigation. In this context, the crop productivity could also be increased by 26.2, 23.6 and 15.4 % over unsprayed control with the sprays of Hico-100 R, paclobutrazol and 8– Hydro Quinine respectively under 6 limited irrigations as against 9 irrigations and thus considerably saved irrigation water.

Thus, assured benefits of ATs to the crops can be summarized as below:

USE OF PLANT GROWTH REGULATORS (PGRS)

The plants possessing moderate canopy development (moderate values for LAI), less reduction in photosynthesis, deeper root system, higher root / shoot ratio and delayed senescence will perform better under water stress conditions.

Toward this, application of some of the PGRs will prove beneficial for better crop growth and development when grown under water deficit situations. Some of the PGRs and their effects on crops in order to suit to the water stress conditions are:

Cycocel & Mepiquat chloride:

For promoting root growth (for more water absorption) and suppressing leaf area development (for reducing transpiration loss of water) and delaying on set of leaf senescence.

Cytokinins and Salicylic acid: 

They delay the leaf senescence processes and also favour stem reserve utilization by the developing grains especially during the water defict situations.

Brassinolides:

These PGRs increase the photosynthetic activity of the plants

Ascorbic acid:

Ascorbic acid acts as an anti-oxidant agent for scavenging Reactive Oxygen Species (ROS) accumulating under stress and thus avoiding membrane damage.

Pre-sowing Hardening of Seeds / Plants:

Hardening of seeds / plants to required temperature / chemicals enables the plants to overcome the specific stresses. This process actually hardens the protoplasm (by osmoregulation), which enables the seeds to absorb more water under favourable situations to maintain its viability under unfavourable conditions.

Chemicals used for seed hardening process especially under rainfed conditions:
           

1. 1% KCl      2. 1% KH2PO4    3. 100 ppm Succinic acid        4. 0.5% NaCl        5. 100 ppm ZnSO4 
                 

6. 100 MnSO4         7. 100 ppm Ascorbic acid        8. 250 ppm Cycocel               9. 0.5% MgSO4                

Thus, these chemicals / PGRs could serve as boon to the frustrated farmers of rainfed areas, if rightly adopted with perspective vision to have food security. Adoption of the agrotechniques is the only solution for farmers of dryland and water stressed scenario to save millions of world population in millennium especially in the developing countries, like India.

Agro-techniques for mitigating Water Stress