Oct. 15, 2020
Oct. 13, 2020
With irrigation being used in many spheres of life and food production methods getting diversified, it is not surprising that research on this topic is getting more specialized. Procedures for analysis, which were restricted earlier to big institutions, need to be widely available so that their reach is extended and science will benefit from more in-depth studies. An increasing number of conventional analytic techniques are being miniaturized to fit into small devices so that they are both portable and affordable.
Irrigation is not limited to largescale open farms. Greenhouses and indoor vertical farms are increasingly providing vegetables, fruits, and flowers year-round. Moreover, irrigation is used in sports, for maintaining golf courses, and buildings, as passive cooling systems for green walls or roofs.
There were 38,940 golf courses around the world in 2019, according to the Golf Monthly, of which 16,752 were in the USA.
The average size of a golf course ranges between 100 to 190 acres, and approximately eighty percent of the turf grass is irrigated. The golf courses in the US alone use 2.08 billion gallons of water per day. So, golf courses account for 0.5% of all the water use in the USA, with the amount of irrigation depending on the species of grass, the local climatic conditions, and the season.
There is a scarcity of water. People need to save as much of this precious resource as possible to grow food and for other vital human functions. At present, forty-three percent of groundwater is used for irrigation. Hence, scientists are looking at ways in which irrigation can also be optimized in greenhouses, golf courses, and passive cooling systems.
A slew of modern portable instruments, which collect and analyze data simultaneously, are helping the scientists in their job. The devices have miniaturized high technology, which provide accurate and rapid results. Most often, they require little sample preparations, and the measurements are non-destructive.
In terms of water usage, greenhouses are more sustainable than open fields. There is less water loss because evapotranspiration, row spacing, and crop cycle lengths are less. Possibilities to use drip irrigation and collect and recycle unused water are some other advantages. Evapotranspiration that influences irrigation can depend on shading and ventilation provided.
In Turkey, scientists wanted to find the lowest quantities of water that could be supplied to grow spinach in pots in greenhouses. They also tested if water deficiency could be offset by supplying organic matter as nutritional amendments. The spinach was watered to maintain the soil water content at a hundred, eighty, sixty, and forty percent of field capacity. Organic amendments at two levels were given as seed treatments and as foliar sprays.
The scientists used leaf area and root area as indicators of water-use along with chlorophyll content, stomatal conductance, leaf relative water content, and membrane permeability. One of the innovative tools used in the experiment to measure leaf area was the CI-202 Portable Laser Leaf Area Meter, produced by CID Biosciences Inc.
The CI-202 is used for non-destructive measurement of leaves of various sizes and thickness. The leaf-blade is flattened and scanned by the device to record the length, width, and outline of the leaf. Area and leaf shape factor are calculated by the incorporated software within a matter of seconds.
The Turkish scientists found that adding organic amendments increased both leaf and root area. However, no level of amendments could compensate for a marked decrease in irrigation. This shows that irrigation has a higher importance than nutrient supply in the greenhouse cultivation of spinach.
Growing on rooftops is becoming popular for many reasons, including producing vegetables, creating a specific aesthetic, or for cooling the building. This has several other advantages in urban areas that suffer from a shortage of trees. Carbon sequestration and production of clean air are additional benefits that residents of the building get from green roofs. However, this places extra demand on water for irrigation.
Figure 1: Euonymus japonicus, variety Chollipo. (Image credits: https://landscapeplants.oregonstate.edu/plants/euonymus-japonicus-variegated-selections
China is trying to encourage more green roofs in a bid to clean its air and cool temperatures, especially in Beijing. Euonymus japonicus, or evergreen spindle, is a flowering shrub native to China that is often used for landscapes and rooftop gardens. It is a hardy plant that tolerates poor soil and extreme heat. There are several attractive varieties available on the market (See Figure 1).
Scientists wanted to find out how irrigation affected the performance of the E. japonicus on rooftops. They watered the plants so that plants were fully irrigated and maintained at 90-100% of field capacity (FC). Other plants were subjected to varying levels of stress by reducing irrigation. Plants were at low water stress when they were maintained at 75%-85% FC, medium stress at 65%-75% FC, and serious water stress at 50%-60% FC.
The parameters measured were photosynthetic rate, stomatal conductance, transpiration rate, leaf area, chlorophyll content, and water use efficiency. The CI-340 Handheld Photosynthesis System was one of the instruments used in this experiment.
The CI-340 is a versatile instrument that is portable and can be operated with a single hand. It measures transpiration, stomatal conductance, photosynthesis, photosynthetically active radiation, chlorophyll fluorescence, and internal plant carbon dioxide levels.
The CI-340 is connected to a leaf chamber available in ten sizes to fit different species. Accompanying control modules regulate the amount of water vapor and carbon dioxide, temperature, and light intensity in the leaf chamber. It is basically a gas exchange analyzer that uses electrochemical and infra-red technology. The measurements are non-destructive and rapid, and the device can be used on the field for time-series study of the same plant or leaf.
The scientists found that photosynthetic rate, stomatal conductance, transpiration rate, and leaf area were at a maximum under low water stress conditions. They surpassed the performance of plants even at full irrigation. Though leaf area, chlorophyll content, oxygen production, and carbon sequestration were the most with no water stress, the difference with plants under low stress was not significant. Thus, the scientists were able to recommend the growth of E. japonicus at 75%-85% FC levels of irrigation, so that future growers can save nearly a quarter of the water used in irrigation.
Among the turf grass species used on golf courses, creeping bentgrass (CBG), or Agrostis stolonifera, and annual bluegrass (ABG), or Poa annua, are sensitive to drought and need more irrigation. ABG is considered a weed that golf managers want to get rid of; however, if it developed desirable green, it could also become an attractive option.
Figure 2: Golf course in Michigan, where the experiment on irrigation was conducted, Laskowski et al. 2019. (Image credits: https://www.gcmonline.com/course/environment/news/soil-surfactant-greens)
Both species suffer during drought. The slow-growing CBG can experience a decrease in growth rate, tissue water content, and chlorophyll content. The fast-growing annual bluegrass, on the other hand, shows reduced viability due to drought.
Drought is not the only stress that golf courses have to suffer. Since golfing continues even in dry seasons, the foot traffic affects the turf grass by subjecting it to the additional stress of trampling and soil compaction. In these conditions, the phytohormone ethylene, which causes senescence, is expected to rise in the plant.
A two-year-long research study was conducted in the USA to see the combined effect of the two stresses on the turf grass species. Scientists wanted to know how this influenced the spread of the weedy AGB at the cost of CBG.
Irrigation was automatic and controlled by soil sensors that monitored the soil water content and temperatures. Standard fertilizer and pesticide protocols for maintaining golf turf grasses were applied to all plots. Some of the strip plots were subjected to traffic, ranging from none to moderate to heavy (See Figure 2). Irrigation was supplied during summer from June to September, to maintain the soil at eight, twelve, and sixteen percent volumetric water content (VWC).
The researchers measured leaf water content and electrolyte leakage, chlorophyll levels, ethylene production, normalized difference vegetative index (NDVI), and root biomass. They used the CI-600 In-Situ Root Imager, from CID Biosciences Inc., to measure root length and biomass.
The root imager is a minirhizotron that has a meter long probe with a camera that takes high-resolution pictures of the roots. The probe is used with transparent root tubes that are installed permanently in a field allowing the roots of the crop to grow around it.
The imager can be rotated 360 degrees around in the tube to take scans of the whole root system. An accompanying software measures the root length, diameter, area, volume, and branching area. With these data, scientists can calculate the total biomass of a plant. Many root tubes can be installed in a field, and the measurements are quick and need only a few seconds, so several root tubes can be scanned in a single day to give average root biomass for a field.
Scientists found that VWC of twelve percent was ideal, eight percent VWC created stress, and sixteen percent VWC resulted in excess of leaf leachates. Moreover, at the low and high levels of irrigation, ethylene production due to foot traffic was high in CBG. Irrigation produced healthy CBG that was able to out-compete the weed AGB, which didn’t benefit as much from watering.
It was only under moderate foot traffic that AGB had an advantage over CBG. AGB suffers more from drought, as it has a shallower root system than CGB.
By conducting short experiments, scientists have found that more water is not always conducive for plant health. In fact, levels of soil moisture below field capacity produce the plants with the best health and growth, since there is more root respiration. Recommendations based on analysis by sophisticated but small instruments can save significant quantities of water used in irrigations. Since water is a scarce natural resource, using less amounts makes irrigation more sustainable.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
Feature photo courtesy of NOAA's National Ocean Service
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