Oct. 20, 2021
Oct. 7, 2021
Today we take a look at two studies of lead toxicity in plants, and the environment at large. Lead toxicity is increasing and it is impacting the performance of many crops. Leaf parameters, such as leaf area and chlorophyll levels, are reduced due to heavy metal lead pollution. This pollutant ends up in soils, reducing crop growth and productivity and also raising health concerns.
A study looking for treatments found that lead affects leaf area and stomatal conductance by regulating levels of hormones, which together reduce yield, in cowpea. However, exogenous applications of a known phytohormone could be the solution. In a separate experiment, examining the amelioration action of a bioactive brassinosteroid, results were also used to suggest potential solutions for the problems caused by lead toxicity.
Figure 1: “Mercury cycle schematic diagram,” Busairi & Syahir, 2018. (Image credits: doi.org/10.23937/2572-4061.1510010)
Heavy metal pollution caused by anthropogenic activities is emerging as a serious threat to agricultural production. Lead (Pb) is one of the most common heavy metal pollutants. It is released when vehicles and factories burn fossil fuels during mining and smelting and as effluents from industries involving the production of lead, battery, fertilizers, and pesticides. Produced either as a gaseous or liquid pollutant, lead can end up affecting land and water resources. The lead fumes mix with rainwater and enter the soil, as shown in Figure 1.
In turn, lead is absorbed by plants through the roots and leaves even though it is not needed as a nutrient. Once inside the plant, the compound can upset enzyme action and hormonal status to the detriment of nutrition and water uptake. The plant will develop symptoms such as stunted growth, chlorosis, and blackening of roots. Like any other form of stress, lead toxicity in plants will affect photosynthetic rate and, in the end, the productivity of crops. Moreover, eating lead-tainted food can cause severe health problems for people.
The farmers have no way of preventing the pollutants from reaching their land. Hence, scientists have been trying to figure out ways to modify the response of plants to help them cope with lead pollution.
Phytohormones regulate the response of plants to various stresses. Nitric oxide (NO), one such phytohormone, is known to act as a signaling molecule, which controls essential biochemical and physiological directions in plants. Given its importance in regulating root growth and stomatal opening, it is crucial during biotic and abiotic stresses. So far, nitric oxide has been found to be involved in protection against stress caused by drought, salt, chilling, and even some heavy metals. Until recently, nitric oxide’s role in response to lead toxicity had never been studied.
Sadeghipour, an Iranian agricultural scientist, recently conducted research to find out if nitric oxide could offer plants any kind of protection against lead toxicity. He and his team also studied the effect of lead on the action of phytohormones and the changes that occurred in cowpea.
Cowpea (Vigna unguiculata (L.) Walp) is a tropical crop suitable for drought-prone regions. The legume is a valuable fodder, green manure, and food crop.
The scientists chose sodium nitroprusside (SNP) nitric oxide donor. Healthy cowpea seeds were soaked in three different levels of SNP—0, 0.5, and 1 mM—for 20 hours before being sown in pots. Lead was added as lead nitrate in concentrations of 0 and 200 mg per kilo of soil.
There were four treatments: one set acted as a control, the second treatment had lead stressed plants and no SNP treatment, the third had lead stress and 0.5 mM SNP, and the last set had lead stress and 1 mM SNP.
During the flowering stage, the plant hormones’ concentrations were estimated by using leaves. The scientist extracted four hormones: Indole-3-acetic acid (IAA), Gibberellic acid (GA3), Abscisic acid (ABA), and Cytokinin. Stomatal conductance was measured in the mornings using a leaf porometer.
To calculate the leaf area, the scientist used the CI-202 Portable Laser Area Meter. The leaf is placed on the palette, and as the high-resolution scanner sweeps over the leaf lamina, precise measurements of leaf length, width, perimeter, and shape factor are recorded. Using preloaded formulae, the leaf area is then calculated. The light and portable leaf area device manufactured by CID Bio-Science Inc. gives rapid results. The leaf measurement tool can also store several thousand readings that can be later transferred to a computer via USB.
When the crop was physiologically mature, the cowpea was harvested and separated into root, stem, leaf, and seed components, which were then oven-dried and ground into a powder. Extracts from the powder were used to measure lead in plants through atomic absorption spectrometry.
Figure 2: “Effect of Pb (200 mg kg-1 soil) and SNP (0.5 and 1 mM as NO donor) on Pb concentration of different plant parts of cowpea. Different letters in each plant part indicate significant differences,” Sadeghipour, 2017. (Image credits: http://dx.doi.org/10.17576/jsm-2017-4602-02)
Lead added to the pots did, indeed find its way into the plants. In fact, compared to the control group, between 12.7 and 16.9 times more lead was observed in plants without nitric oxide treatments, as can be seen in Figure 2. About 50% of lead absorbed by the roots remained there and thus, there was more lead in the roots than in other parts of the plants. The roots contained 58% lead, which was 3.4 and 11.9 times more than the lead in the stem, leaves, and seeds, respectively. So, the plant prevents the movement of lead into other parts of the plants; see Figure 2.
Treatment by the nitric oxide source SNP in 0.5 and 1 mM concentrations reduced the amount of lead in plants by 50% and 26%, respectively.
As levels of lead in plants increased, a 45%, 33%, and 38% decrease was observed in the amounts of phytohormones IAA, GA3, and cytokinin, respectively; however, ABA levels also increased by 54%, inducing stomatal closure. Treating the plants with the nitric oxide source SNP reversed this negative trend. SNP levels of 0.5 and 1 mM increased IAA levels by 31% and 11%, cytokinin levels increased by 24% and 9%, and GA3 levels increased by 18% and 7%, respectively. Moreover, ABA levels decreased by 24% and 17%.
The nitric oxide-induced increase in IAA could help in root growth, while cytokinin boosts levels of pigments like anthocyanins and betacyanins.
Compared to the control group, lead stressed plants had 39% less stomatal conductance. Once again, 0.5 and 1 mM of SNP alleviate stomatal conductance by 64% and 34%, respectively, compared to plants without treatment.
As mentioned, the hormone ABA is known to induce stomatal closure, and as the levels of ABA increase, stomatal conductance reduces. When the stomata remain closed, there is less exchange of gases and transpiration; the former reduces photosynthesis and the latter reduces nutrient absorption. As a result, plant growth is decreased overall. When nitric oxide reduces ABA levels, the stomata open more and the leaves can resume their plant physiological activities.
Figure 3: “Effect of Pb (200 mg kg-1 soil) and SNP (0.5 and 1 mM as NO donor) on leaf area of cowpea. Different letters indicate significant differences,” Sadeghipour, 2017. (Image credits: http://dx.doi.org/10.17576/jsm-2017-4602-02)
Leaf area declined by 61% due to lead toxicity. Treating plants with nitric oxide reduced the decline. Plants treated with 0.5 and 1 mM concentrations of SNP had 101% and 51% more leaf area than lead stressed plants without treatment, as shown in Figure 3.
A similar reduction in seed yield of 56% in lead stressed plants compared to healthy plants was alleviated by 79% and 44% by SNP treatment by 0.5 and 1 mM of SNP, respectively.
Ideally, field trials should be conducted to confirm these novel results, as many factors in the natural ecosystems, like soil structure or chemistry, and soil microflora could influence the results.
This study was able to show how lead toxicity affected cowpea plants and reduced yield. Lead toxicity was deleterious, but its effects could be moderated by treatment with nitric oxide. The experiment was also successful in identifying the helpful amount. The lower dose of 0.5 mM of SNP was useful in reducing the uptake lead from soils and its subsequent translocation to other organs of the plants. By partially restoring the hormonal balance, nitric oxide could increase leaf area and stomatal conductance to reduce the loss in seed yield. This study has the potential for improving cowpea yields in lead polluted soils. Moreover, nitric oxide as a treatment for lead pollution could be studied for other crops, extending the potential benefits beyond cowpea production alone.
As covered in the previous study, lead, released due to burning fossil fuels and industrial activities, is one of the most common toxic heavy metal pollutants affecting agricultural soils. This is especially true for places close to urban areas, where lead accumulates in the soil.
Lead affects the growth of plants and impacts yield by reducing biomass accumulation.
It is a well-established fact that plant growth regulators help crops in dealing with heavy metal toxicity. Many applications seek to improve the concentrations of phytohormones to reduce the impact of lead on crop plants. Instead of using other chemicals to improve hormone levels, direct use of growth regulators as external applications is another solution. After all, external phytohormone applications are common in the food supply and are often used in the field and post-harvest.
It was precisely this line of inquiry that was investigated by two botanists, Rao and Raghu, for rose-scented geranium [Pelargonium graveolens (L.) Herit]. The scientists decided to use brassinosteroids for external applications, as they are pleiotropic in effect.
Pleiotropic effects are those where one agent affects diverse organs, and the many disorders caused tend to be associated. Brassinosteroids influence germination, growth, flowering, stress response, abscission, and senescence. See Figure 4 for more details on brassinosteroids’ effects.
Figure 4: “Schematic representation of pleiotropic influences of BRs on fruit crops,” Baghel et al. (2019). (Image credits: https://doi.org/10.1007/s10725-018-0471-8)
The botanists studied the amelioration action of a bioactive brassinosteroid, 24-epibrassinolide, on rose-scented geranium against lead toxicity by using
lead (II) nitrate [Pb (NO3)2].
First, cuttings of geranium from healthy plants were rooted, and after twenty days, the plants were transplanted into pots and transferred to a glasshouse. Four treatments were used in the study:
The hormone was applied twice as a foliar spray during the 10th and 25th day of the experiment, while lead was applied to the soil at five day intervals. The plants were allowed to grow for 60 days, and then the data was collected.
Growth parameters of plant height, fresh weight, and dry weight were recorded. To get the fresh and dry weight, the entire plant was harvested and used. Before that, leaf area was recorded using the CI-203 Handheld Laser Leaf Area Meter.
The leaf area meter, manufactured by CID Bio-Science Inc., is a small portable device that can easily be used in the field. The wand of the leaf area meter is swept down the leaf. A combination of an optical motion sensor and laser beam collect data to measure leaf length, width, area, and perimeter. The leaf measurement tool has a resolution of 0.01cm2 and a rapid scanning speed of 200mm/second, so it was easy for the scientists to quickly collect precise data. Moreover, the leaf area meter stores around 250 scans per battery charge on the USB card, which can be transferred later to a computer for analysis.
To check if pigment production in geraniums is affected by lead, chlorophyll was extracted by acetone and measured.
Table 1: “Effect of 24-epibrassinolide on foliage growth of geranium plants. The data presented above are Mean ± S.E. (n=5). EBL=24-epibrassinolide, Pb=Lead,” Rao and Raghu (2016). (Credits: Journal of Medicinal Plants Studies, 4(6): 06-08)
As expected, lead toxicity substantially reduced all growth parameters measured, such as height, weight, and leaf area. The effect of toxicity was the greatest on the leaf area. Even chlorophyll levels were reduced, and lead toxicity produced chlorosis or yellowing of leaves. However, the levels of carotenoids remained unaffected.
Treating plants with brassinosteroid sprays reduced these adverse effects of lead toxicity. The growth of treated plants improved even though they were experiencing stress. Even leaf parameters, which were reduced under stress, were largely restored; see Table 1.
The brassinosteroids were also able to counteract the reduction of chlorophyll levels due to lead toxicity. A beneficial effect of the phytohormone on chlorophyll concentrations was also seen in unstressed plants.
The botanists concluded that brassinosteroids were able to alleviate lead toxicity in plants—increasing pigment levels and leaf area, leading to growth and productivity improvements.
Phytoremediation by plants is being increasingly used to remove and reduce heavy metal pollution in soils. Since plants would accumulate heavy metals, it is not advisable to use food plants for phytoremediation. The botanists suggest using plants like rose-scented geranium for phytoremediation or cultivation in polluted soils, instead of food crops.
A farmer whose land is polluted would not have to abandon the land or suffer crop losses. Moreover, the potential health risk to consumers from food grown in polluted fields can be avoided by shifting to cultivation of aromatic plants. Rose-scented geranium is an important cash crop, as it is a source of geraniol and citronellol. Using brassinosteroids as sprays, farmers can continue to earn from their farms by growing this fragrant crop, while society benefits through the reduction of pollution.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
Baghel, M., Nagaraja, A., Srivastav, M. et al. (2019). Pleiotropic influences of brassinosteroids on fruit crops: a review. Plant Growth Regul 87, 375–388. https://doi.org/10.1007/s10725-018-0471-8
Busairi, N., & Syahir, A. (2018). Recent Advances in Mercury Detection: Towards Enabling a Sensitive and Rapid Point-of-Check Measurement. J Toxicol Risk Assess 4:010. doi.org/10.23937/2572-4061.1510010
Pleiotropy. Retrieved from https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/pleiotropy
Rao, S.S.R., & Raghu, K. (2016). Amelioration of lead toxicity by 24-epibrassinolide in rose-scented geranium [Pelargonium graveolens (L.) Herit], a potential agent for phytoremediation. Journal of Medicinal Plants Studies, 4(6): 06-08
Sadeghipour, O. (2017). Nitric Oxide Increases Pb Tolerance by Lowering Pb Uptake and Translocation as well as Phytohormonal Changes in Cowpea (Vigna unguiculata (L.) Walp.). Sains Malaysiana 46(2), 189–195. http://dx.doi.org/10.17576/jsm-2017-4602-02
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