Regulated Deficit Irrigation (RDI): An improved and water-efficient irrigation practice
All living organisms require water for their survival, growth, and reproduction. Let’s start with the role of water in the plants. Water is required for all the life processes like basic cellular, biochemical and physiological functions, turgor maintenance for plants shape and size, transpiration, photosynthesis, uptake of nutrients, growth, and development like vegetative, flowering, and fruit development, metabolites production like sugar, anthocyanin, flavonoid, and phenolics compounds.
What is a water deficit?
When the moisture level in soil or any media goes below the demand of plants, it is known as water deficit. The deficit can be in the range of a slight deficit to a severe deficit where it touches the wilting point.
What is the effect of water deficit on crops?
The above-mentioned functions will be disrupted due to the water deficit in the plants. Water deficit induces the ABA production and causes the stomatal closing which reduces transpiration, photosynthesis and at later, growth and development get restricted. Water deficit induces reactive oxygen species and causes several damages to the cells and tissues. Water deficit has different effects on different stages of crop.
Some positive aspects of water deficit
We now know that water deficit during different stages of crops is lethal and cause yield reduction. But at the same time, the regulated water deficit can be useful as well. It is being practiced in different horticultural crops and vegetables and flowers. It is highly practiced in the wine grape vineyards where the regulated water deficit during the veraison period facilitates the berry ripening and release of different metabolites from the plants. Plants produce different metabolites during water deficit. And these metabolites can make fruit tastier and attractive. Like regulated water, deficit grape produces more anthocyanins, phenolics and brings more aroma and quantity to the wine. This practice is very important in the vineyard and it determines the ultimate wine quality for the vineyard growers. Similar in other fruit crops like apple, pear, peach, pomegranate, and many other tropical, subtropical and temperate fruits.
What is Regulated Deficit Irrigation?
Regulated Deficit Irrigation (RDI) is primarily about restricting irrigation after fruit setting or in excess canopy growth to control shoot growth with the aim of influencing fruit quality by partitioning more photosynthates and resources from shoot growth to fruit development and manipulation of quality aspects of fruits. RDI is usually applied during the period of slow fruit growth when shoot growth is rapid. Overall, it helps in the improvement of fruit quality, reduction of water consumption, and control of canopy growth. It also helps to improve control of vegetative vigor in high-density orchards to optimize fruit size, fruitfulness and fruit quality. RDI can generate considerable water savings upto 50 percent.
Thus, it is useful for reducing excessive vegetative growth, minimizing irrigation and nutrient loss through leaching. It also reduces the possible shading and associated barrenness.
Determining “optimum water level” and “deficit water level” is very important to understand and practice the RDI. It is possible through technology like sensors or sometimes in personal observations. For effective apply of regulated deficit irrigation management, the understanding of the effects of irrigation at different stages of crop growth is required.
What are the effects of water deficit and Regulated Deficit Irrigation on different stages of crops?
Transplanting/Seedling/Pruning Stage: During this stage, the roots and shoots are very small so water deficit restricts further growth and mortality increases. In the case of grapes, water deficit causes buds to dry and restricts the root and shoot growth which will have long term impact in the whole season causing compact bunches. High chances of compact bunches which will be more prone to diseases and pests.
Vegetative phase: At this stage plants are in continuous need of water and nutrients supply due to exponential vegetative growth from seedlings to flowering stage. Water deficit at early vegetative will reduce the growth and poor canopy will result in low yield. Sometimes excess canopy growth occurs which can be optimized through pruning or thinning of leaves and branches or with RDI application. RDI application will be more economical for high-density crops and large scale farming for controlling the canopy growth. The water deficit intensity at this stage will result in impacts in further growing stages. RDI can be applied in vineyards during this stage when there is excessive canopy growth.
Inflorescence development: In this stage, there is the development of flower buds or reproductive parts. It starts during the vegetative stage. water deficit during this stage can disrupt the development process which leads to bud abortion and fewer flowers. This stage is very critical for irrigation. The optimum moisture level should be maintained in the root zone.
Flowering to fruit setting: Adequate moisture status is very critical during flowering. Proper irrigation and fertigation required. The different parts of flowering develop and mature here. The flower opening causes pollination in many of the crops while in some crops, there is closed flower pollination. The flowering determines the number of fruit set. Moisture stress and high temperature are very lethal to the flowering which can significantly reduce the fruit set and yield. Although we can’t control the atmospheric temperature, the optimum moisture will help to reduce the effects and maintain proper flowering. Flower abortion due to moisture stress can reduce the yield up to 70% and even more in severe stress. This stage is short but very critical for management.
Fruit development and Maturity (Veraison): Fruit development and maturity requires proper nutrients and photosynthates. The early stage of fruit development is critical to the water deficit. The RDI offers great potential to reduce excessive shoot growth and increase the qualitative aspects of fruits. When plants are in moisture stress, they release many metabolites to combat the stress which can be attributed to quality for fruits. For example, water stress before veraison reduces the vegetative growth and berry size, directs more nutrients and photosynthates towards berries and increases the soluble solids and metabolites like sugar, anthocyanin, flavonoid, and phenolics compounds (aroma factors). This leads to an increase in titratable acidity and pH Level which are very important factors for wine quality. The reduction in yield can be complemented by an increase in soluble solids and quality.
Again, determining “optimum water level” and “deficit water level” is very important for the RDI application. RDI application starts when the flowering and fruit set are complete. The stress application should be slow and gradual so plants do not feel sudden stress. The gradual stress applications help to complete the metabolites production cycles resulting in the higher release of metabolites. The frequent breaking of stress or short-duration stress will interrupt the metabolites formation cycle so that there won’t be any positive output of stress application.
After harvest: Postharvest moisture control is very important to take care of next season fruiting especially in fruit crops like grapes, mango, and pomo. The moisture level should be enough to foster the photosynthates storage for the coming season but not so much that vegetative growth is encouraged prior to leaf fall. This will help for proper bud development in next season.
Fasal RDI approaches
It is very necessary for adequate assessment of water deficit in any environment. With the help of different microclimatic sensors, Fasal assesses irrigation and plants water status by using microclimatic factors like Primary soil moisture, Secondary soil moisture, Temperature, Humidity, Rainfall, Vapor pressure deficit, Evapotranspiration and Growing degree days. These factors help to plan the crop stages and intensity of the regulated deficit irrigation (RDI) period. It is commonly used in grapevine vineyards and pomegranates and other fruit orchards. Fasal is also developing possible RDI strategies for vegetable crops. Fasal is actively working on building the RDI model for different crops based on the soil types and agroclimatic zones.
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