Author: Jyoti Chauhan
Abstract: In recent years, deficit irrigation has been widely investigated as a valuable and sustainable production strategy especially for rainfed areas. The adverse effects of water stress on crop growth can be mitigated by the application of materials or chemicals such as nutrients, anti-transpirants and Plant Growth Regulators (PGRs), which induce the plants to become adaptive to water stress situations for a particualr period so the water requirement for such periods can be minimized or saved. 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 the rain water, yet rainfed farming is often subjected to drought. Transpiration is said to be a necessary evil which cause nearly 99 per cent of the water loss absorbed by the plant but it has several functions to attend in the crop cycle. There are some materials or chemicals which decrease the water loss from plant leaves by reducing the size and number of stomata, by improving water uptake, osmoregulation and by modifying the plant architecture.
Introduction
Environmental factors such as high light intensity, high temperature, salinity and
drought stress affect yield of crops. In general field crops are highly dependent on current photosynthesis for growth and final yield. Therefore it is unlikely that currently available chemicals or materials would increase yield of an annual crop unless crop suffers stressed from inadequate water and or a very high evaporative demand, particularly during a moisture sensitive stage of development. For producing one tone of food, the crop plant requires varied amount of water (Cereals and legumes: 400–500 litres of water/kg of grains; Fruits and vegetable: 1000 litres/kg of food). Also, Water transpired by crops (season/plant) very considerably (Maize: 200 litres; Cotton: 8 – 10 litres/day ( http://agritech.tnau.ac.in/ agriculture /agri_drought_management.html). Similarly, the WUE of crops is also differ considerably (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 months).. Prolonged drought can drastically reduce the yield to zero level depending upon severity. 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. The severity of intermittent drought of 6-10 days during critical stages of the crop can reasonably be avoided by the use of such materials and thus crops can be saved.
Transpiration is the loss of water in the form of vapour from the stem and leaves of the living plants. The process of gaseous diffusion is responsible for the loss of water from plant to atmosphere through the leaf stomata. This gaseous diffusion of water is directly controlled by the regulation of stomatal apprature. The stomatal movement is controlled by plant and atmospheric factors (Kramer, 1969). Antitranspirants have shown to reduce transpiration (Davenport and Hagan, 1973). Among the major nutrients, some are found to be highly deficient due to water deficit conditions. Therefore, application of these enhances the water uptake as well as the water relations in the plant tissues by osmoregulation processes, due to their role as a potent osmoregulator (osmolyte), resulting reduced the solute potential. Some plants characteristics such as 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.
Role of antitranspirants affecting plant life
Antitranspirants are chemicals capable of reducing the transpiration rate when applied to plant foliage. Since water loss normally occurs through the stomatal pores in the leaves, artificial control of transpiration have been developed and tested for a decade. The major type of materials used were, film forming substances which works by blocking water vapour diffusion from the leaf (Slatyer and Bierhuizen 1964); chemicals that inducing stomatal closure and increase resistance to the diffusion of water vapour (Waggoner and Zelitch 1965); and reflected materials that decrease the evaporative materials that decrease the evaporative energy to the plant (Aboukhaled 1970). There are four types of antitranspirants:
1. Stomatal closing type: Most of the transpiration occur through the stomata on the leaf surface. Some fungicides like phenyl mercuric acetate (PMA) and herbicides like Atrazine in low concentration serve as antitranspirants by inducing stomatal closing. These might reduce the photosynthesis. PMA was found to decrease transpiration than photosynthesis. (other eg.- Herbicides like 2,4–D, Phosphon D etc., Metabolic inhibitors like hydroxy sulfonates, potassium metabisulphite etc., Growth hormones like ABA, ethrel, TIBA, succinic acid, ascorbic acid and Cycocel (CCC)
2. Film forming type: Plastic and waxy material which form a thin film on the leaf surface and result into physical barrier. For example ethyl alcohol. It reduces photosynthesis eg. Tag 9, S - 789 foliate (other eg.- Hexadecanol (Higher alcohols), Mobileaf etc.).
3. Reflectance type: They are white materials which form a coating on the leaves and increase the leaf reflectance (albedo). By reflecting the radiation, vapour pressure gradient and thus reduce transpiration. Application of 5 per cent kaolin spray reduces transpiration losses. eg. Diatomaceous earth product (Celite), hydrated lime, calcium carbonate, magnesium carbonate, zincs sulphate etc.
4. Growth retardant: These chemicals reduce shoot growth and increase root growth and thus enable the plants to resist drought. They may also induce stomatal closure. Cycocel is useful for improving water status of the plant. The ATs are categorically classified on mode of action in the following four types:
The assumptions behind the use of antitranspirants are that an increase in resistance at the leaf surface will decrease transpiration more than it will decrease CO2 uptake (Kramer, 1983). Antitranspirants have shown to reduce transpiration (Davenport and Hagan, 1973). Antitranspirant affect water loss more than CO2 exchange in leaves (Fuehring and Finkner, 1983). Wendt (1973) obtained slight increase in yield of potatoes by use of antitraspirants. Since stomata serve as portals for both the loss of water vapour and for the intake of carbon dioxide (which is necessary for photosynthesis), an antitranspirant barrier against water loss also may reduce plant growth. When Reflecting materials are applied to the upper surfaces of leaves they do not cause blockage of stomatal pores due to occurrence of stomata exclusively on the lower surfaces. However, such coatings may curtail photosynthesis on overcast days when light is limited. Film-forming antitranspirants can cause large reductions in transpiration rates, but since the H2O:CO2, permeability ratios tend to exceed unity for currently available film materials, the transpiration ratios may not be reduced (Davenport et al, 1969). As, growth can be retarded by natural stomatal closure when an untreated plant wilts, due to low soil water potentials and/or high evaporative demand. Antitranspirants will help by slowing down the rate at which water is lost, to prevent or at least delay wilting. Therefore, treatment with an antitranspirant must be made as a preventive measure before the onset of wilting.
The purpose of antitranspirants (ATs) is to maintain the growth and productivity under stress conditions so these are not recommended for high productivity unit area. These are used to save crops and help to get marginal yield when the expectations are near zero.
Thus, assured benefits of ATs to the crops can be summarized as below:
- Assured better crop growth and yield when no yields are expected under severe drought
- Getting normal sized grains
- Improved seed quality (so that produce can be used for seed purpose)
- Saving of crops with marginal crop productivity under drought
- Reducing irrigation especially in post-rainy long duration crops like cotton and pigeon pea
- Minimizing irrigation frequency and saving water through drip irrigation (eg. Cetyl alcohol and/or Hexadecanol)
- Monitoring crop loss with limited inputs
- Monitoring / managing drought
- Arresting fast receding soil moisture for better growth and yield of rabi crops
- Very useful for farmers with minimum irrigation facilities
- Optimized yield levels under infrequent rainfall situations
- Saving large nurseries when water is scarce in summer months
Under the water deficit conditions some major nutrients like potassium and magnesium are found to be highly deficient. Application of potassium, therefore, enhances the water uptake as well as the water relations in the plant tissues by acting as a potent osmoregulator (osmolyte), through osmoregulation processes, thereby the solute potential is reduced. Potassium nutrition also helps in the favourable stomatal regulatory mechanisms, which regulate the water balance of the plants. Besides, this has also resulted in the increased WUE of the plants. Similarly, magnesium is works as a major 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.
Other than macronutrients, deficiencies of micronutrients also appear under water deficit situations due to the following reasons:
- Use of high-yielding varieties (HYVs), adoption of intensive systems of farming and cropping and use of heavy doses of fertilizers, increased proportionately the mining of micronutrients from the soil
- Continuous use of micronutrients free NPK fertilizers in dryland agriculture and diminishing the use of organic matter, FYM, compost and green / green leaf manures.
- Depletion due to erosion and leaching. In India, annual soil loss is estimated to be about 6000 Metric tons and obviously due to loss through run off water and soils
- Since increased crop production arising from the heavy demand of the nutrients in rapid depletion of macro and micro-nutrients unless regularly replenished. Consequently, the deficiencies of micro-nutrients in general and that of Zn, Fe and B in particular are widely spread under stress conditions.
Therefore, foliar application of the following nutrients depending upon the occurrence of their efficiencies will mitigate the water-stress induced nutritional imbalance in crops:
- 0.3 % Boric acid
- 0.5 – 1.0 % Ferrous sulphate + 1 % urea
- 0.5 % Zinc sulphate
- 2 % DAP
- 0.5 to 1 % potassium chloride (KCl)
Some plant characters such as 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. These characters can be utilised in water saving or improving water uptake by application of some of the PGRs. These may 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 onset 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 deficit 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.
Conclusion
The most obvious use of materials or chemicals which reduce the transpiration losses or helps in improving water uptake are realized in dry farming areas, in nursery and some high value materials. The purpose of such materials is to maintain the growth and productivity under stress conditions so these are not recommended for high productivity unit area. These are used to save crops and help to get marginal yield when the expectations are near zero. However, applications for this purpose would be justified only if water costs are sufficiently high and if possible water savings are sufficiently high to save.
References
Aboukhaled, A., Hagan, R.M. and Davenport, D.C. (1970). Effect of kaolinite as an effective antitranspirant on leaf temperature, transpiration, photosynthesis and water use efficiency. Water Resource Research 6: 280-289.
Davenport, D.C., Martin, P.E. and Hagan, R.M. (1973). Effect of an antitranspirant on water use by highway oleander (Nerium oleander L.) Plantings. J. Amer Soc. Hort. Sci. 98: 421-425.
Fuehring, H.D. and Frinkner, M.O. (1983). Effect of folicot antitranspirant application on field grain yield of moisture stressed corn. Agron J. 75: 579-582.
Http://agritech.tnau.ac.in/agriculture/agri_drought_management.html.
Kramer (1983). Water relations of plants: Academic press, New York, 489.
Kramer, P.J. (1969). Plant and soil water relationship: A modern synthesis, McGrow-Hill Book Co., New York, 482.
Slatyer, R.O. and Bierhuizen, J.I. (1964). The effect of seasonal foliar sprays on transpirational and water use efficiency of cotton plants. Agr. Meteorol 1: 42-52.
Waggoner, P.E. and Zelitch, I. (1965). Transpiration and the stomata of leaves. Science 150: 1413-1420.
Wendet, C.W. (1983). Effect of an antitranspirant on the yield of crops in high plains of Texas during 1969 and 1970. Progress report 3165. Texas Agric. Exp. Sra. Lubbock.
About Author / Additional Info:
I am currently pursuing Ph.D in Plant Physiology from Banaras Hindu University