Feb. 23, 2021
Feb. 18, 2021
Turmeric is integrated as an under-canopy cash crop in agroforestry, as it is considered to be tolerant to light shade. The microclimate created by canopy trees differs, so it is important to find which species creates the optimum growing conditions for turmeric. Finding out the amount of light that is transmitted to the under-canopy is crucial, and measuring it accurately is the crux of these studies. Luckily, there are precise field instruments for this purpose, such as the CI-110 Plant Canopy Imager.
Many cash crops, including turmeric, are grown as under-canopy plants. The canopy trees decrease light filtering down to the ground to reduce temperature while increasing relative humidity. The shade of canopy trees also reduces the amount of photosynthetic active radiation (PAR) that reaches the under-canopy crops and can influence the rate of photosynthesis, transpiration, and water use efficiency.
Therefore, scientists in India—Singh, Sharma, Chauhan, and Arora—created a study to see how the microclimate was altered under three canopy trees and how it affected the crop performance of turmeric.
The scientists chose three trees with a very diverse phenology: a summer deciduous tree (Ailanthus excelsa), a winter deciduous tree (Gmelina arborea), and
an evergreen species (Eucalyptus tereticornis). Turmeric was grown in the plantations of the three trees for a year. The experiment was conducted during the entire crop cycle, after which the turmeric rhizomes were harvested.
There were three replicates of each kind of plantations, as well as a control, where turmeric was grown in the open without any shade.
The researchers used a digital thermo-hygrometer to measure air temperature, soil temperature, and relative humidity. The light intensity at the ground level was recorded by a lux meter.
The scientists needed to measure PAR and wanted a tool that gave accurate measurements and could be used under the under the canopy in the field.
PAR is the light spectrum between 400 to 700 nm. Photosynthetic photon flux density (PPFD, μmol photons m−2 s−1), or the number of photons hitting an area of one square meter per second, is one means of expressing the light that reaches the leaves.
Measuring PAR under canopies requires a different approach to measurements in a homogeneous controlled space. The canopy characteristics of each species differ and create heterogeneity in the light they allow to pass.
The instrument has to be able to account for these variations. There are a few methods and tools to measure light interception into the understorey, but there is a variation in accuracy and effort involved.
Single point and handheld light sensors can estimate light coming through a canopy, but require a reference measurement under open skies. This makes this method more time consuming and laborious. It is also only useful for environments with less than 30% canopy cover.
Though convex spherical densiometer is useful to compare illumination between sites, it is not useful to calculate solar radiation coming through a canopy. In this experiment, the scientists needed to measure the actual light reaching the leaves, and illumination alone does not provide this information so this method was also not useful.
Photometers are not advisable under the canopy, as the tools are sensitive to changes in solar radiation due to the season and time of day.
For this type of experiment, many scientists prefer linear multiple measurements with ceptometers, coupled with the proper sampling method.
The scientists settled on the CI-110 Plant Canopy Imager, as it has a ceptometer with 24 light sensors mounted on an arm around 80 cm long, to record current PAR, expressed as PPFD (µmolm-2s-1).
The scientists recorded environmental conditions, including PAR, once every two weeks for the three treatments, in three replicate plantations each, over an 11-month period.
PAR was recorded under the trees and also in the open during the experiment. The control or open spaces had more light of 2712.4 µmol m-2s -1. All the trees intercepted light but to varying degrees:
The effect of variations in PAR reaching the turmeric created varying microclimates and effects on crop yield.
There were three treatments (canopy trees) and three replicates each, so the PAR was recorded in nine sites on 22 occasions.
The CI-110 was able to make recordings in all light conditions, and each measurement took only a few seconds. The tool was versatile enough to be used in field conditions with 5 to 50° C
The whole device weighs only 1.5 kilos, so it is easy to carry around in the field.
Using the device, it was possible to take the large number of PAR data collections easily, with a high accuracy of 5 umol m-2s-1.
The tool connects to four different satellites and can be used anywhere in the world, so the location of each measurement is easily tagged with GPS data. With the GPS, the scientists could easily repeat measurements on the same spot over the 11-month period.
The data collected was stored on the tool and later transferred by USB to their computers, so they could dive immediately into statistical analysis.
As expected, the air temperatures were lower under trees by 4.9-5.6% than in the open, and the relative humidity was 6.9-11.3% higher.
In all three plantations, the reduced PAR resulted in the reduction of light intensity by 48.9 to 56.9%. As a result, plant height was significantly affected.
Turmeric was tallest under G. arborea, a winter deciduous tree, compared to the other trees and open areas. Plants that grow in shade are usually taller, as they try to reach open skies and more sunlight.
Maximum turmeric leaf area was recorded under Eucalyptus tereticornis, followed by Ailanthus excelsa, Gmelina arborea, and control.
Rhizome yield of turmeric was highest in open areas (24.3 t ha-1) and the Eucalyptus plantations (25.1 t ha-1), than in Ailanthus plantations (16.1 t ha-1) and Gmelina plantations (15.8 t ha-1).
According to this study, turmeric performs best in low shade conditions. The best yields were obtained under Eucalyptus, which has a narrow canopy and provides a low amount of shade to the turmeric, so there was increased photosynthesis and stronger root growth in its plantations due to increased transpiration.
Choosing the right canopy trees is not simple. Though turmeric is light-shade tolerant, the results of this experiment confirmed that—like other root crops like garlic, onion, and colocasia—increasing shade decreased photosynthesis and, therefore, yield. By comparing several shade tree species, the scientists were able to establish, without doubt, what amount of shade was good for turmeric yield. Handy tools like the CI-110 from CID Bio-Science Inc. assisted them in these extensive experiments, which made measurement and recording easy, leaving them time for other tasks.
CI-110 Plant Canopy Imager
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
Feature image courtesy of Sophie
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