Dec. 5, 2020
Nov. 27, 2020
In an attempt to improve food production at a time when water resources are getting scarce, scientists are focusing on novel methods to manipulate the physiology of crops. Many deficit irrigation strategies that reduce water use are being tried. One of them is the new partial rootzone irrigation, which can improve water use efficiency, crop health, and yield parameters.
Deficit Irrigation (DI) is an optimization strategy, where irrigation is only applied during the drought-sensitive growth stages of a crop and the crop is rainfed for the remaining crop-growing phases. There are many such DI strategies to improve crop water productivity. While crops will face a certain degree of drought stress and yield loss, water use efficiency is improved. Through typical application of these techniques, it is only possible to stabilize production, not maximize it.
A new method of deficit irrigation that seeks to prevent these negative effects is called partial rootzone irrigation, also known as partial rootzone drying (PRD). It is touted to improve the water use efficiency of plants, without causing drought stress and reduction in crop yield.
In this method, the rootzone is divided into two parts. At each irrigation, only one part (half of the rootzone) is watered, while the other half remains dry. At every irrigation event, the two halves are alternately wetted and kept dry. Since soil evaporation causes 20% of water loss during an irrigation event, PRD reduces evaporation loss by 50%.
The drier part of the root system also sends chemical signals to the shoot, leading to a reduction in transpiration and an increase in water uptake by roots in the wet zone. Thus, PRD provides the required amount of water to the shoots to maintain plant-water status but also triggers stress response to increase water use efficiency.
One of the chemical signals sent by the roots due to dryness is abscisic acid (ABA), which is sent via the xylem to the shoots. Increased ABA concentration can reduce vegetative growth and water use but doesn’t affect fruit growth, so yield remains high.
In an effort to control various shoot responses (like stomatal closure, increase in root biomass, reduction in the canopy area, etc.), scientists are studying ABA signaling during PRD to increase the ABA.
Moreover, PRD effects go beyond ABA signaling. These effects include an increase in soil aeration, soil microbe activities, better mineralization of organic carbon and nitrogen in the soil and use by plants, and higher photosynthesis. Changes in root biomass are accompanied by altered root growth and distribution in the fields because increased uptake in the wet zone areas triggers root distribution.
However, PRD doesn’t produce these results uniformly in all plants. Scientists suspect that the species could be lying on a continuum of PRD response and can also vary in the type of response.
Furthermore, soil types, environmental conditions, and plant genotype can influence the PRD response.
The effect of PRD in grape cultivation has not been fully understood, as reports suggest varying degrees of efficiency.
Hence, scientists Romero et al. from Spain decided to compare conventional regulated deficit irrigation (RDI) with PRD using the same amount of low to moderate irrigation regimes. They decided to focus on root growth and distribution, water uptake, ABA in xylem, and shoot vigor in a four-year experiment in a semi-arid Mediterranean area in Marcia.
Figure 1: “Images of representative grapevines from each treatment showing root system development, trunk and cordons after 4 years of applying irrigation treatments (December 2012), and the parameters derived from analysis of the images. PRD (partial rootzone drying) and RDI (regulated deficit irrigation) effects on RL(total root length) and RV (total root volume) are shown,” Romero et al. 2014. (Image credits: doi:10.1071/fp13276)
Thirteen-year-old red wine grapes (Vitis vinifera L. cv.Monastrell) were chosen for the experiment. RDI was used as the conventional irrigation system and was compared with PRD.
In one treatment (RDI-1 and PRD-1), moderate irrigation ranging from 13-30% of crop evapotranspiration (ETc) was given at different phases from bud formation to harvest. The second treatment (RDI-2 and PRD-2) provided only 0-20% of the ETc. 40% ETc was given after fruit harvest to all plants in both treatments.
Root growth analysis was done by installing root tubes that went 60 cm deep in the soil. Root scans were taken three years after the experiment began with a Minirhizotron, the CI-600 In-Situ Root Imager, produced by CID Bioscience Inc. The CI-600 has a rotating scan head, which takes high-resolution images of the root growing around the root-tubes. Root growth parameters like length, area, and volume were calculated from the images.
At the end of the experiment, the entire plant, including the root system of representative vines from each treatment, was excavated carefully to calculate root and shoot biomass. The root system was also photographed.
Volumetric soil water content was measured each week with a soil probe, as was the stem water potential. Also, the rate of photosynthesis, transpiration, and stomatal conductance was measured every 7-14 days. ABA levels in the sap were measured at the beginning, middle, and end for PRD treated vines. Trunk growth and diameter, vine vigor, leaf area, nutrient content in leaves, and fresh and dry weight of main shoots were also recorded.
PRD-1 vines had more water in the soil than RDI-1, so they had a larger root system and more fine-root growth to explore soil for water. The rootstock 1103P was able to go deeper than other rootstocks. This helped PRD-1 vines in more water uptake during irrigation events and they could use water from the entire soil profile better than RDI-1 vines. The lack of water on the dry side was compensated by more uptake from the wet side.
Even though PRD-1 and RDI-1 suffered similar levels of drought stress, PRD-1 plants maintained higher rates of photosynthesis and plant water use efficiency.
PRD-1 vines had higher stomatal conductance, so the water use efficiency at the leaf-level was lower. There was also more ABA in leaf xylem sap following reirrigation.
The situation was different in the second treatment with less irrigation. Since irrigation was less, the root system could not be properly developed and the water uptake could not be increased; see Figure 1. As a result, gas exchanges were slower, including photosynthesis, though there was no damage to the leaves or internal tissues.
Since the vines were subject to higher levels of drought, the ABA levels were higher in RDI-2 but not PRD-2.
PRD-1 showed better trunk growth, as well as both fresh and dry shoot weight compared to RDI-1 vines, indicating that the plant water status was more easily maintained in PRD-1 than in RDI-1 vines, allowing for more carbon assimilation.
RDI-2 plants showed better vegetative growth than the PRD-2 vines, suggesting that RDI-2 vines could use more water than PRD-2 vines. The PRD-2 vines had less fine root growth development and biomass accumulation. Due to a lack of water in the sap, the ABA levels were also decreased during the end of the PRD cycle.
The scientists were able to establish that under moderate irrigation, PRD was better for promoting vine root and shoot growth, while under low irrigation, RDI was the better choice.
Figure 2: “Representative images of fine root distribution in the soil profile (0–50 cm), taken with minirhizotrons (located close to the drip, at 10–15 cm perpendicularly from the drip head) in August 2011, after 6 years of application,” Romero et al. 2016. (Image credits: https://doi.org/10.1007/s40626-016-0061-y)
Besides its effect on the growth of roots and shoots, irrigation strategies can also influence yield.
Therefore, after the previous experiment that proved the usefulness of PRD in promoting vegetative growth, the same scientists studied its effect on yield and berry and wine quality. This study tested both RDI and PRD. RDI maintains soil water status within a narrow tolerance range to reduce stress effects on plants. So, both techniques have the potential to improve yield parameters in comparison to vineyards that use no irrigation and sustained deficit irrigation (SDI) practices.
In the second study, the scientists focused on comparing the effects of different irrigation techniques on root and shoot development, soil and plant-water relationships, yield, and berry quality in four watering strategies.
Two different treatments of RDI and PRD - one with 30-60% and 20-28% ETc (RDI-1 and PRD-1) and the other with 15-60% and 20% ETc (RDI-1 and PRD-1)–at various phases, using drip irrigation, were tested for seven years and compared to the sustained deficit at 60–40% ETc (SDI) throughout the season.
Root tubes 60 cm deep were installed, and roots were allowed to grow for six years. In the seventh year, the roots were scanned by the minirhizotron, CI-600 In-Situ Root Imager, produced by CID Bioscience Inc. The root length, area, and volume were analyzed and used to calculate the root length and surface densities.
After the experiment, representative vines were completely removed to get photos of the roots and to measure shoot and root biomass. Total leaf area and yield parameters, such as bunch numbers, weight, and number and weight of berries, were estimated each year, along with internal fruit qualities.
The source-sink ratios were calculated using the shoot and yield parameters. The soil-vine water relationships and gas exchanges, such as net photosynthesis, stomatal conductance, and the transpiration rate, were measured.
SDI vines that got the same amount of water throughout the years had water available throughout the soil profile. This resulted in a thicker and more extensive root system, as well as fine root growth. The larger root system was also able to improve nutrient capture and the stomatal opening was larger, allowing the vines to produce more photoassimilates. This led to better canopy size and leaf area. SDI vines were able to use the carbon accumulation to go deeper into the soil than the other two treatments.
The two DI vines had smaller canopy size, reduced leaf area, and less root development. The root to shoot ratio was higher, as more carbon was allocated in their development due to water stress. Nonetheless, the two DI vines had thinner roots, which is usually found in water stress plants as it allows them to grow faster to explore more soil for water. Among the DI treatments, PRD-1 vines had more water availability in the entire soil profile in comparison to RDI. They also had a more extensive root system, with roots that were coarser and finer than RDI-1 vines.
PRD-2 vines, which were more water-stressed, did not fare as well as RDI-2 vines in terms of root growth, photosynthesis, or stomatal opening. The additional investment in roots did not help them overcome the water stress on shoots and yield parameters.
Compared to the two DI techniques of PRD and RDI, the SDI vines produced more yield in comparison to vegetative development. In terms of quality, the SDI berries had more soluble sugars and acidity than the other treatments but less anthocyanins and polyphenols. With moderately water-stressed vines, RDI-1 and PRD-1 produced better quality berries because they had fewer leaves, and the open canopy increased the production of anthocyanin and the red color of the berries. RDI-2 and PRD-2 were too stressed to produce good yield in quantities or quality, as the water use efficiency of the whole plant was affected.
The DI vines were better at leaf-level water use, but this didn’t affect the water use efficiency of the whole plant.
Among the DI methods, PRD-1 gave the best yield and quality of grapes. However, SDI seems to be the best of all three treatments.
Many people do not use PRD, as they are unaware of the benefits of this relatively new irrigation method. PRD can be used with any of the irrigation methods, like furrow or drip, where it is possible to control the water flow direction. The use requires some practice and knowledge, as moderate irrigation through PRD methods is more effective than a stark reduction in water. However, the increase in yield quantity and quality of grapes through this method is encouraging. Moreover, it frees water for other agricultural production and improves crop water productivity.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
Feature image courtesy of Nenad Stojkovic
Barideh, R., Besharat, S., Morteza, M., & Rezaverdinejad, V. (2018). Effects of Partial Root-Zone Irrigation on the Water Use Efficiency and Root Water and Nitrate Uptake of Corn. Water, 10(4), 526. doi:10.3390/w10040526
Romero, P., Fernández-Fernández, J.I., Gil-Muñoz, R. et al. Vigour-yield-quality relationships in long-term deficit irrigated winegrapes grown under semiarid conditions. Theor. Exp. Plant Physiol. 28, 23–51 (2016). https://doi.org/10.1007/s40626-016-0061-y
Romero, P., Pérez-Pérez, J. G., Amor, F. M., Martinez-Cutillas, A., Dodd, I. C., & Botía, P. (2014). Partial rootzone drying exerts different physiological responses on field-grown grapevine (Vitis vinifera cv. Monastrell) in comparison to regulated deficit irrigation. Functional Plant Biology, 41(11), 1087. doi:10.1071/fp13276
Zhang, H., & Oweis, T. (1999). Water-yield relations and optimal irrigation scheduling of wheat in the Mediterranean region. Agric. Water Manage 38, 195-211.
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