The lack of water has a negative effect on the whole set of vital plant processes such as photosynthesis, respiration, absorption of nutrients, and assimilate translocation. It also affects growth, reproduction, and the development of the seed. From the agronomical point of view, the main interest is focused on the effect of water deficit on yield. The effect of water stress on plant development and productivity has been the object of several studies [32-35] and is dealt with in Section 5.5.
The most visible effects of water deficit are decreased size of the plant and of its foliar surface, as well as decreased yield. Other agronomic consequences of water deficit are modifications in the chemical composition of agricultural products (such as sugar, oil, and protein) and in other quality features of yield (such as diameter and color of fruits). Negative impacts brought about by stress essentially will depend on the stage of the cycle during which it takes place, on the intensity of the deficit involved, and on its duration.
To illustrate some aspects of water stress on yield and yield components, an experiment with soybeans is referenced . The treatments were established by fixing six different levels of ETc restitution through irrigation: 20%, 40%, 60%, 80%, 100%, and 120%. Deficits or excess water applications were kept constant throughout the crop cycle. The seasonal depths of irrigation water applied were, respectively, 84, 171, 253, 324, 435, and 504 mm. In addition, all plots received 87 mm of effective rainfall.
The yield obtained was shown to have been clearly affected by the water treatment received, with values ranging from 1135 to 4660 kg/ha of grain. Among the components of the yield, the one that seemed to be the most sensitive to the different depths applied was the number of grains per plant. The number of plants per unit surface and the number of pods per plant were affected, but the only significant difference was between those treatments receiving less water and the remaining treatments. The quality of the grain was assessed through oil and protein content. The oil content grew with the seasonal quantity of water (from 24.4% to 27%), whereas the protein content decreased (45.5% to 35.1%). The treatments with the greatest water supply, which kept the largest active leaf area over the longest time, achieved the largest quantity of oil. However, the yield of both oil and protein tended to increase with the depth of water applied.
In areas with a shortage of water, a water-saving irrigation strategy thus increasingly is being implemented, is known as controlled-deficit irrigation (CDI). This is based on reducing water applications during those phenological periods in which controlled water deficit does not significantly affect the production and quality of the crop involved, while satisfying crop requirements during the remainder of the crop cycle. CDI may produce economic returns higher than those obtained when irrigating for maximum production .
Research carried out in several regions in the world [38-42] into the response of CDI in fruit trees, such as peach, pear, almond, and citrus, shows that this technique can lead to up to 50% water savings with a small decrease in yield, while keeping, or even improving, the quality of the production. The main problem is to determine the phenological periods when the impact of water deficit does not significantly affect the production and/or quality of the crop. Existing literature on the subject does not provide homogeneous information; hence, appropriate data validation needs to be performed on local conditions before planning CDI.
Planning for deficit irrigation entails a greater challenge than for full irrigation, because it requires the knowledge of the appropriate MAD and how this deficit will affect the yield. Deficit irrigation may call for changes in cropping practices, such as moderation of plant population density, decreased application of fertilizers and agrochemicals, flexible sowing dates, and the selection of shorter-cycle varieties.
The relationship between water deficit and yield has to be well known when planning deficit irrigation. To determine when irrigation is to take place (and the amount of water to be applied), suitable water-stress indicators should be used. These indicators may refer to the depletion of soil water, soil water potential, plant water potential, or canopy temperature. The last indicator seems to be the most suitable, but the most widely used, for practical reasons, concerns soil water content and soil water potential. However, the spatial variability of the soil and of the spatial irrigation give rise to uneven soil water storage, which causes problems in analyzing indicator information when these originate from point measurements. Indicators resulting from areal observation or measurement, such as the CWSI , are the most appropriate.
There are different ways to carry out deficit-irrigation management. The irrigator can reduce the irrigation depths, refilling only a part of the soil reserve capacity in the root zone. Or the irrigator may reduce the frequency of irrigations, but pay attention to the timing and depth of a limited number of irrigation sessions. In surface irrigation, deficit irrigation can be carried out by moistening furrows alternately or digging them farther apart .
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