The world population is suffering an exponential increase that requires to be followed by an increase of 70% of the current agricultural production, according to FAO. This production growth has been assumed for a long time by the abuse of fertilizers and water resources. Today, the salinization of soils and aquifers, together with the salinity of soils close to the sea due to seawater penetration has caused that most agricultural soils to be affected by high salinity.
One of the main problems that saline stress causes on the plant is a physiological drought, due to the increase of the salt concentration in the environment which then limits the intake of water. In addition, the increase of Na+ and Cl– into the plant can reach toxic levels, nutritional imbalances and unfavorable nutrient ratios, that may seriously affect the development of the crop.
The second consequence of saline stress in plants is the increase in the production of reactive oxygen species (ROS) and the appearance of oxidative stress. As a defense mechanism against this stress, plants increase the synthesis of antioxidant enzymes such as catalase and ascorbate peroxidase. Knowing the levels of these two enzymes allows to know the stress degree in the plant.
Finally, saline stress causes an increase in the production of ethylene by the plant. This hormone is known to be responsible for the phenomenon of plant stress. So that, the excessive increase of this hormone during critical development moments triggers a premature senescence, stomatal closure and a decreased evotranspiration and therefore a decrease of the xylem transport and root absorption, resulting in a decline in photosynthetic performance and a physiological blockage of the plant, that can become irreversible.
To talk about the anti-stress effects of cobalt (Co) and molybdenum (Mo) based fertilizers, their physiological function in the plant should be known.
The best-known function of Co in agriculture is usually on the biological nitrogen fixation in legumes. But this experiment focuses on its anti-stress function as an inhibitor of the enzyme ACC oxidase, which catalyzes the last step in ethylene synthesis. This way, Co is capable of reducing ethylene generation and decreasing the unwanted effects that the excess of this hormone under physiological stress produces.
Mo is an essential element for plants, but it is not biologically active by itself, it is found in the composition of a large number of enzymes. One of the most relevant enzymes is nitrate reductase, which is involved in the assimilation of nitrate by the plant and its transformation to nitrite and ammonium. Maintaining a good level of Mo in plants, favors the activity of the enzyme and thus vegetative development, photosynthetic capacity and the amount of photoassimilates increases. Mo also participates in the synthesis of indoleacetic acid and abscisic acid through the enzyme aldehyde oxidase, contributing to a correct hormonal regulation. All this give the plant a greater resistance against saline stress.
The objectives of this trial have been: (1) the evaluation of the response of tomato seedlings, cultivated in hydroponics, to saline stress in the early stages of its development, (2) their response after the application of a compound fertilizer based on Co and Mo and (3) the evaluation of different doses of this product to reducing oxidative stress.
Tomato was chosen to make this experiment given its socio-cultural importance at a global level, being one of the main vegetable crops in the world.
To carry out the trial, tomato cv. Mil Seny seeds were sown in a plastic tray with silica sand as an inert substrate and were maintained in a growing chamber. They were grown for 20 days and then were transplanted to opaque plastic containers with 200 ml of nutritive solution. Conditions in the growing chamber during the development of the experiment were normal regarding the temperature, light and humidity, setting day and night periods.
A total of 5 theses: a control treatment (CTL), in which the plants were developed under normal conditions, a saline control (CSA), in which the plants were developed in conditions of high salinity with 50mm of NaCl and 3 treatments of salinity 50mm of NaCl that contain the fertilizer product based on Co and Mo, which was applied directly to the solution, at different doses 0.5; 1 and 2 ml/l. Each treatment had 9 repetitions.
The analytical determinations carried out in this experiment were: physical parameters, where fresh and dry weight of the seedlings was monitored during the development of all theses, and on another side, the physiological parameters through a macro and micronutrient foliar analysis and evaluation of different parameters related to oxidative stress.
To evaluate the data obtained, a statistical analysis was carried out through a study of variance ANOVA of a factor and the Duncan test was applied, with a significance of 95% to establish significant differences between the means of the different treatments.
As shown in Figure 1 the results of foliar N obtained indicate that this nutrient is not affected by the saline stress at this dose of NaCl. However, increasing the dose of the product, a slight descent occurs. This may be due to the increase in the presence of molybdenum and its influence on the nitrate reductase that favors the transformation to nitrite, and its subsequent transformation and translocation.
As seen in Figure 2, the amount of foliar P suffers a decrease in all saline treatments, compared to CTL treatment, due to that the absorption of P is diminished when the plant is in a situation of stress. The content of P decreases by increasing the Co and Mo dosage, which can be due to that the plants take the molybdenum in the form of the anion molybdate and, due to the structural similarity between this Ion and phosphate, it is considered very likely that your entrance to the plant takes place through the same anionic transporters used by the ion phosphate, creating a competition between both anions.
The content of foliar K as observed in Figure 3, suffers a significant decrease in the thesis with higher doses of fertilizer rich in Co and Mo, compared to the other treatments. The decrease in the concentration of K, as well as the aforementioned N and P could be indicatives of possible toxicity by Mo or Co at high dose.
Regarding foliar Mg (Figure 4), a decrease is observed in Mg in the treatment CSA vs. CTL treatment. Treatments where the product based on Co and Mo was applied show a statistically significant recovery of the foliar Mg content compared to the CSA sample. This decrease in Mg in seedlings subjected to saline stress could cause serious physiological problems in the plant, but treatment with this fertilizer recovers the loss of Mg.
The Ca content in the different treatments (Figure 5) indicates that seedlings subjected to stress saline suffer a significant decrease compared to the control. Despite this, a significant increase in Ca foliar content is observed in SD2 and SD3 treatments in comparison with the CSA treatment. Calcium plays an essential structural role in the plant, stabilizing the cell membranes and reducing their permeability, which contributes to the improved membrane integrity under saline stress.
The results of foliar Na (Figure 6), show an expected significant increase in all treatments subjected to saline stress, in front of the CTL treatment. There is also a significant descent of foliar Na content with larger doses Co-and Mo-based fertilizer, between CSA treatment and the remaining three.
Regarding the Fe content (Figure 7), the increase in saline concentration in CSA did not produce chlorosis, while for the treatments with the fertilizer, the content of foliar Fe decreases as the dose increases. This result is most likely due to the cationic antagonism that exists between this nutrient and Co. This lack of iron was also observed in plants by the presence of chlorosis.
Saline stress causes an increase in the generation of reactive oxygen species and this oxidative stress leads to the destruction of the integrity of the structures of the membrane and the increase in its permeability. The results obtained (Figure 8) indicate a significant increase in the loss of electrolytes in the CSA treatment, compared to the other treatments, which is indicative of loss of membrane integrity in saline conditions. In addition, all the applications of Co and Mo reduce the loss of electrolytes in roots without significant differences compared to the control CTL. So, a better recovery of membrane integrity under saline stress is obtained thanks to the product based on Co and Mo.
As seen in the Figure 9 there are no statistically significant differences between the values of the three pigmentss on CTL and CSA treatments. However, SD2 and SD3 do show a progressive descent of pigments by increasing the dosage of the fertilizer. This loss of chlorophylls related with the results of foliar Fe, as this nutrient participates actively in the synthesis of chlorophyll.
The increase of ethylene generated by the plants under saline stress also causes the accumulation of sugars. This is clearly seen in the results of the (Figure 10) through the significant increase of total sugars which occurs in the CSA treatment compared to the control. On the other hand, the application of the product shows a reduction of the sugar content in SD1 and SD2 without significant differences compared to the control sample, indicating a decrease in stress with the application of the fertilizer. SD3 presents a peak, signal of possible excess stress of Co and/o Mo.
Catalase and Ascorbate peroxidase
As seen in the Figures 11 and 12, the contents of catalase and ascorbate peroxidase show a significant increase in CSA treatment against the CTL, a symptom that the plant is stressed. The results of SD1 and SD2 treatments show a significant decrease in both enzymes, which indicates that the product based on Co and Mo limits ROS production with these doses
From the results obtained, it can be concluded that:
By Elena Roca, Quality Manager of Stoller Europe