Agriculture and Science 2023/5

Magnesium and potassium deficiency symptoms and remobilization in melon and cucumber

Former Graduate School of Natural Science and Technology, Okayama University
Masaharu Masuda

Introduction.

The author has summarized the physiological disorders of vegetables in a single table7) , showing the site of occurrence, the stage of growth, and the factors responsible for their occurrence. Even now, some 20 years later, no new findings have been added to the factors responsible for these symptoms.

　This suggests that physiological disorders in vegetables have not become a major problem in recent years. In the case of seed-breeding vegetables, physiological research itself seems to have been reduced, as much research has focused on the growth characteristics of new varieties and diseases caused by continuous cropping. In nutrient-breeding vegetables such as sweet potatoes, taros, potatoes, garlic, and strawberries, virus-free seedlings are stored, propagated, and marketed as seedlings, but usually growers become newly infected with the virus after one or two years of propagation, and the virus is not found in the seedlings.

　It is difficult to distinguish pathological or physiological factors from the outward manifestations. Of course, there are many cases of Mg deficiency, which are caused by magnesium and calcium deficiency, antagonistic inhibition, and trace element deficiency; Mg deficiency sometimes resembles disorders caused by trace amounts of nitrite or ammonia gas and is often very similar to photochemical oxidants or spider mite damage, making it difficult to immediately determine the cause. It is often difficult to determine the cause immediately.

　The former gas disorder tends to occur in greenhouse cultivation, especially with the heavy use of organic fertilizers such as oil cake and chicken manure. In addition, the gas disorder is also more likely to occur in open field cultivation of direct-seeded vegetables such as spinach, radish, and komatsuna, because their leaves are in contact with the ground surface. The mechanism of gas generation has already been clarified, and it is necessary to control the acidity of soil pH and the amount of nitrogen fertilizers, especially ammonia-form nitrogen fertilizers, to suppress gasification.

　For the latter, as shown in Fig. 1 (1) to (3), photochemical oxidant damage, high light and high temperature damage, etc.
The following are some of the most common chlorosis. These symptoms may be similar to the commonly known chlorosis between the veins of melon leaves shown in photo (4). This paper attempts to discuss the relationship between the symptoms occurring on the nodal leaves of melons and Mg deficiency. Photochemical oxidants, such as ozone (O3) and peroxyacetyl nitrate (PAN), which are formed when nitrogen oxides (NOx) and volatile organic compounds emitted into the air from factories and automobiles are exposed to ultraviolet light, damage plant and animal cells. (O3) and peroxyacetyl nitrate (PAN)), which are formed when volatile organic compounds are exposed to UV light.

　The morning glory shown in the photo (1) in Fig. 1 is a typical example of such a disturbance, and was an indicator plant for photochemical smog in the 1970s, which the author also investigated at a farm affiliated with Kyoto University about 50 years ago. At that time, I learned that taros were sensitive to photochemical smog among vegetables. Even recently, when cucumber leaves temporarily encounter intense light in early summer, the veins of the leaves become mottled and whitened on the following two days. This phenomenon is probably caused by the inability of the reactive oxygen species (ROS) scavenging system to instantaneously process the excess light energy, but no evidence has been obtained.

　In the Earls melon shown in Photo (2), high temperatures and temporary intense light also cause overnight discoloration of some of the spaces between the veins of the leaves. Although the earl melon is an extremely high temperature tolerant crop (no damage occurs on the leaves on the opposite side of the strong light even at 43°C), the high temperature combined with strong light causes damage to the leaves.

　Unlike nitrite, ammonia gas, photochemical oxidants, or high light damage, the mite damage shown in photo (3) is characterized by blurred leaf symptoms, and as the disease progresses, many mites can be seen on the underside of the leaves with the naked eye. However, the initial symptoms are similar to those of Mg deficiency and are difficult to distinguish. In particular, it is better to suspect mite damage first rather than physiological disorders in vegetables at the beginning of the hot season.

Mg deficiency symptoms and component migration in melon defoliation and cucumber

(1) Overview of melon defoliation

　In general, cucurbit crops often develop chlorosis between the veins of neighboring leaves after the middle stage of growth, especially when the fruit is enlarging. In such a situation, Tsutaka12) of the Hyogo Prefectural Agricultural Research Center reported that chlorosis was caused by K deficiency. To the best of the author's knowledge, this is the only paper ever published that attributed the disease to K deficiency.

　Fujimoto 2) investigated Mg content by leaf position in house melons for normal and defoliated plants and obtained the results shown in Figure 2. Fruits were attached to lateral branches that developed at nodes 13 to 16, and leaf analysis was conducted at the main stem node position from the lower to the upper leaves. Based on these results, he concluded that leaf senescence occurs at the 0.2% level of leaf Mg content, and that leaf senescence is caused by the translocation of leaf magnesium to the fruit as the fruit matures, resulting in yellowing between leaf veins and eventually a yellowish-brown color and death. This type of leaf yellowing is sometimes regarded as an indication of maturity and has little effect on yield and quality.

　The author4) also recognized the physiological disorder of autocotyl cucumber, the occurrence of brownish small group leaves at 0.19% or less, and that the Mg content of leaves with deficiency symptoms is indeed lower than that of normal leaves. However, these results do not indicate that magnesium is transferred from leaves to fruits. Although magnesium is conventionally considered a mobile component3, 10, 13), is it really mobile?

(2) Difficulty of component transfer within the cucumber tree

　First, the author compared the changes in the components of hydroponic cucumbers, especially magnesium, with those of other components. The author replaced the culture medium of cucumbers (main branches at 22 nodes and lateral branches at 3 nodes) grown in the standard culture medium with tap water, and investigated the dry matter weight and content rate of leaves and roots at the 5th node over time. Nitrogen and phosphorus decreased significantly in the dry matter content of the roots (61% after 4 days of water treatment), and potassium and magnesium decreased further (74% and 82% after 8 days of treatment).

　On the other hand, nitrogen, phosphorus, and potassium were greatly reduced in leaves. In particular, phosphorus and potassium were significantly reduced to 60% and 63% of their initial levels, respectively, after 8 days of treatment. In contrast, calcium and magnesium decreased only slightly5). Of course, during the treatment period, dry matter weight increased due to photosynthesis. Figure 3 shows the relative ratio of calcium and magnesium content in the leaves to the dry matter percentage at the beginning of treatment.

　In the roots, calcium did not change at all, and the rate of decrease was total N=K>P=Mg>Ca. On the other hand, in leaves, calcium and magnesium did not change at all, and the rate of decrease was P=K>total N>Mg=Ca. The fact that there was no change in magnesium, which is considered to move frequently, indicates that magnesium in cucumber leaves is a less mobile element than nitrogen, phosphorus, and potassium, at least in comparison to nitrogen, phosphorus, and potassium, considering that the nutrient removal from the solution (tap water) was short, only 8 days.

　(3) Magnesium sprayed on individual cucumber leaves does not move even on those leaves.

Next, cucumber plants were picked and double-trimmed, and magnesium was removed from the culture solution when the plants reached approximately 70 cm in height, and then 1% MgSO4, Mg(NO3)2, or MgCl2 solution was applied to one of the double-trimmed plants four times every 2 days. completely suppressed in all treatments (control: it occurs in trees without spraying). The fact that the effect of MgCl2 application did not differ between the front and back sides of the leaves suggests that the Mg applied to the leaves enters the cells through the cuticle, where it is transformed into various forms.

　The sprayed leaves themselves remained green, but the leaves of the upper and lower nodes showed chlorosis (photo omitted). This indicates how difficult it is for the sprayed magnesium to move.

　In order to further understand the difficulty of Mg translocation in individual leaves, we identified one medium-sized leaf that was expected to develop chlorosis and sprayed a 1% MgSO4 solution into it four times every other day by pressing an open-bottomed paper cup against a portion of the leaf. The results are shown in Figure 4.

　Sprayed areas remained green, while unsprayed areas exhibited typical intervein chlorosis. The Mg content in these sites is shown in Table 1.

　It is clear that the water-soluble Mg in sprayed sites A-1 and B-2 is about 6 times higher than that in unsprayed sites A-2 and B-1, and the content rate of the unsprayed site is the same as that of the whole-leaf unsprayed site, which was established as a control, so at least the sprayed Mg is not mobile in the individual leaves.

　Despite the difference in leaf color between the sprayed and unsprayed sites, there was no difference in the magnesium in the chlorophyll backbone, i.e., 95% methanol-soluble Mg. This suggests that the amount of magnesium constituting chlorophyll is too small to be measured by ordinary instruments.

(4) White and green ring leaves of cucumber are induced by ammonia-form nitrogen.

　White discoloration of grafted cucumber (pumpkin rootstock) leaves began to occur frequently in Kochi and Miyazaki in the late 1970s, and many researchers were involved in investigating the cause of this phenomenon for more than ten years. Initially, many reports attributed this symptom to Mg deficiency, but this phenomenon could not be explained by Mg deficiency alone, because it was not so-called yellowing (chlorosis) but necrosis (white necrosis). In many cases, the leaves are called "green ring leaves" because the leaf margins remain green, and when severe, browning occurs overnight, and white necrosis forms when exposed to light. Matsumoto et al. 8) of the Kochi Prefectural Horticultural Experiment Station have successfully elucidated the factors that cause such leaf development. A brief summary is as follows.

　At that time, methyl bromide was used as a soil disinfectant in greenhouse cultivation. Oil cake, an organic fertilizer, was applied to the soil. The oil cake is converted to ammonia-form nitrogen by the action of microorganisms (ammonia-forming bacteria). Ammonia nitrogen is converted to nitrate nitrogen by nitrate-forming bacteria, and under low soil temperatures, the activity of the nitrate-forming bacteria is low (it is generally known that nitrate-forming bacteria are less sensitive to low temperatures than ammonia-forming bacteria), so the concentration of ammonia nitrogen increases. Furthermore, in soils treated with methyl bromide, the activity of nitrate-forming bacteria is weakened and the accumulation of ammonia nitrogen becomes more pronounced. Even if magnesium is present in the soil, its absorption is inhibited by antagonism when ammonia nitrogen is high. The greening of cucumber leaves is caused by an excess of ammonia nitrogen in the soil and inhibition of magnesium absorption.

　Nevertheless, the author still felt that it was necessary to reproduce the symptoms observed in farmers' greenhouses in order to prove the factors that induce white discoloration of cucumber leaves, so he repeated trial and error in hydroponic cultivation of self-rooted cucumber and grafted cucumber. As a result, we found that white discoloration and green ring leaves appeared on cucumbers on pumpkin rootstocks when the concentration of ammonia nitrogen and magnesium in the nutrient solution were increased and decreased, respectively (Figure 5).

　The technical term "interveinal necroticleaves" was first introduced in the revised Glossary of the Horticultural Society of Japan (2004), and green ring leaves were defined as a part of interveinal necroticleaves.

(5) Leaf damage and variation in Mg and K content with fruit set in melons - magnesium is sure to be re-migrated.
　is it?

Generally, when melons approach harvest time, chlorosis appears on the leaves near the fruit. This has long been a good indicator of harvest time. Textbooks state that this symptom is caused by the transfer of magnesium from the leaves to the fruit. I think I also learned this in a college course. In the case of Aalsfevorit, a slight yellowing between the veins of the leaves begins around 40 days after fruit set and becomes more intense around 50 days, when the leaves begin to brown. Is this a symptom of magnesium transfer from the leaves to the fruit?

　To investigate the presence of fruit and the degree of leaf damage, fruit node leaves were collected from each plant at 40, 50, and 60 days after fruit set and analyzed 70 days later6) . As shown in the right side of Figure 6, female flowers are attached to the first node of the lateral branch (cotyledon) extending from the 13th node of the main stem. The leaves in the vicinity are A, B, and C. Their shapes and colors are shown on the left. The upper panel shows the leaf color of a plant that removed fruit 40 days after fruit set (no fruit for 30 days), and the lower panel shows the leaf color of a plant that kept fruit until the end (70 days).

　As can be seen from the photo, when the leaves are picked at 40 days, all the leaves remain green, but if not, the leaves are deformed, with chlorosis between the veins and necrosis at the leaf margins. The symptoms are more pronounced in the fruiting node leaves. The fruiting period was set as long as 70 days in order to clarify the relationship between the fruiting period and the presence or absence of fruit. The magnesium, calcium and potassium contents of these leaves are shown in Table 2.

　Magnesium values varied little in the 1.1% to 1.2% range, regardless of whether the fruit was present or not.
Calcium tended to decrease slightly. In contrast, potassium content in the leaves decreased the longer the fruit remained on the plants, and the potassium content in the leaves of plants that left fruit on for 70 days was less than half that of plants that did not leave fruit on 40 days (Nishimura et al. 9). Nishimura et al. 9) also analyzed leaves over time from the time of fruit set and found that Mg content did not decrease, but rather increased gradually (Fig. 7).

　Usually, leaves begin to turn yellow around 40 days after fruit set, and this change can also be detected by chlorophyll values (Figure 8). The leaf color is darkest on the 15th day of crossing, and then fades rapidly.

　In the Vegetable Horticulture Laboratory (Prof. Tadao Masuda) before the author was appointed to Okayama University, most of the research themes seemed to be set on melons. The data on magnesium, calcium, and potassium were extracted from the thesis of five years at that time and plotted by component. Figure 9 shows these data.

　As we will see, magnesium and calcium levels tend to increase gradually with fruit size, while potassium levels clearly decrease. Although intervein chlorosis and leaf margin necrosis would have appeared at 50 to 60 days after crossing, the relationship between the phenomena appearing on leaves and the analytical values has not been discussed. Observation of leaves in the vicinity of fruit set shows that intervein chlorosis caused by Mg deficiency in melons gradually intensifies as the harvest period approaches, so the view that leaf magnesium is transferred to the fruit is reasonable in nature and not at all surprising.

　However, there is no evidence to date that magnesium is transferred to fruits. The Encyclopedia of Agricultural Technology in the Rural Electronic Library (National Institute of Agro-Environmental Sciences) also states that magnesium is a mobile component that is transferred from leaves to fruits during fruit enlargement and ripening, and that deficiency symptoms may appear in the leaves around fruits.

Conclusion

　Vegetable nutrient physiology, especially nutrient deficiency, is often studied under two conditions: whether or not the same symptoms occur when one component is removed from the complete culture medium, and whether or not the same symptoms occur when other components are increased without removing the component. As shown in Figure 10, it is difficult to attribute leaf deficiency symptoms to a single component because of the interactive effects of ions on nutrient absorption in the roots.

　Plant roots selectively absorb nutrients from the anion and cation groups, and the total ion content (meq) in the xylem fluid is balanced between positive and negative ions. In the soil, there are many kinds of trace element ions including Na+, Cl-, and HCO3-. It is said that monovalent cations are more easily absorbed than divalent cations, and their antagonistic effects are also known (NH4+ is inserted based on the author's cultivation experience). White leaves of cucumber are found on pumpkin rootstock
and caused by Mg deficiency under NH4-N excess, and a slightly higher Mg concentration is more likely to result in green ring leaves. In other words, the author speculates that ammonia is likely to be directly involved because white-ringed leaves are a physiological disorder caused by two factors, and they show necrosis symptoms overnight.

　A similar phenomenon is also observed in tomato lintel disease. It is well known that tomato lintel disease is caused by Ca deficiency, but severe gangrene appears at the fruit apex when Ca deficiency under NH4-N excess is present. The chlorophyll decay and potassium retrotranslocation are also likely to cause the damaged leaves at the fruiting node of melons.

　Bukovac et al.1) noted that leaf-applied 28Mg was as unlikely to be translocated as 45Ca (no data for radioactive Mg, however). The spray experiments and analysis of individual cucumber leaves in this paper confirm that magnesium is not easily translocated, even to neighboring cells.

　Recently, Tanoi11) reported in his review article "Frontiers of Plant Mineral Transport Research" that nine Mg2+ transporter gene (MGT) families have been reported in Arabidopsis and rice, and 12 in maize, respectively, and described their magnesium transport mechanisms. If the expression level of MGT in leaves of melons is not increased by the applied magnesium, or if MGT function in leaves is weak, it is natural that magnesium does not stop at the site and move to that site. The intervein chlorosis in leaves near the melon fruit is caused by the decay of chlorophyll, and the fact that magnesium free from chloroplasts does not migrate to the fruit may be related to the expression level of the MGT gene.

　Considering that, in general, about half of the magnesium in mature leaves is present in chloroplasts, some of which is chlorophyll-constituting magnesium, a small amount must be at issue with respect to retrotranslocation. However, it is not reasonable to assume that the free magnesium produced by chlorophyll decay is transferred back to the fruit, based on previous experimental results.28Magnesium has a short half-life of approximately 20 hours, so the tracer method is an essential tool for kinetic analysis, even with experimental limitations.

　Finally, one more question: why does chlorosis occur on the leaves near the fruit as it nears harvest time? If there is no fruit, it would not occur. At this time, some signal must be sent from the fruit to the leaves. What does this signal mean for the melon?

References

Bukovac,M.J. and S.H.Wittwer. 1957. Absorption and mobility of foliar applied nutrients.
　Physio. 32: 428-435.
2. Fujimoto, J. 2008; Development of Diagnostic Methods for Nutritional Problems in Horticultural Crops and Research on Preventive Measures. Shimane, Japan
　Journal of Agricultural Technology and Research. 8:1-45.
Jacob, A. 1958. Magnesium: The fifth majorplant nutrient. 34-54. Staples Printers Limited.
　Kingdom.
4. Masuda, M. 1984. Mg deficiency in grafted and self-rooted cucumber on high K and high NH4-N media.
　Differences. Agriculture and Horticulture 59: 1051-1053.
5. Masuda, M. 1989; Deficiency of Magnesium in Cucumber Leaves and Difficulty of its Migration. Journal of Agricultural Science, Okayama University.
　74: 7-14.
6; Masuda, M. and Kanazawa, T. 1993; Cation Content and Nutrient Deficiency of Melon Settling Node Leaves. Horticultural Society of Japan, Vol.4, No.2
　Chapter Conference Abstracts 32:31p.
7. Masuda, S. and Terabayashi, T. 2003. production technology. Illustrated Vegetable New Book. Asakura Shoten. Asakura Shoten. Tokyo.
　(as described on p. 131).
8. Matsumoto, M., Uesugi, I. and Yanai, T.: 1981. Mg deficiency of grafted cucumber in institutional cultivation (Gu.
　Leaning Syndrome). i. Effect of Nitrification Inhibition by MB Agents on Nutrient Absorption and Greenling Syndrome. Kochi, Japan
　Agriculture and Forestry Bulletin. 13: 1-10.
9. Nishimura, Y., Fukumoto, Y., Shimazaki, K., 2004. Inorganic Formation in Leaves of Armeron (Cucumis melo L.).
Effects of fruit set on the minute. Biological and Environmental Regulation, 42: 137-146.
10. Shimada, N. 1976. physiological action of magnesium. Physiological effects of magnesium.
　Yokendo. Tokyo.
11. tanoi, Keitaro. 2021. transport mechanism of magnesium. Japanese Journal of Soil Science and Fertilizers. 92: 108-113.
12. Tsutaka, T. 1982; Causes and control of leaf blight disease of Prince melon. Agriculture and Horticulture 57: 1162-1166.
13. Yamazaki, D. 1975. trace elements and multielements . 175-183. Hakuyusha. Tokyo.

No Soil - No. 21
　　Why is it said, "Soil is alive?" -Is soil a living thing?

Hokkaido Branch Office, JCM Agri Co.
Teruo Matsunaka Former Technical Advisor

　In this issue, we turn our attention to the soil itself. It is the soil that stably supports agriculture. The soil is the place where food that protects our lives is produced, and at the same time, it nurtures the lives of various living creatures. For this reason, it is often said that "the soil is alive" in awe of it. Why is this so?

1. Is soil a living thing?

　When asked again whether soil is really "alive," that is, whether it is a living thing, not many people would say that soil is a living thing.

　Living organisms in the general concept are multicellular organisms. To be a multicellular organism, it must satisfy the following three requirements: (1) differentiation and growth, (2) reproduction and heredity, and (3) autonomy in response to environmental changes (Okajima, 1989). However, no one would think that soil has parents who grow up, raise their children, and die by inheriting their genetic elements. Soil is alive" is merely a metaphorical expression of soil. We should be careful not to confuse soil with living beings out of respect for it.

　But even if it is a parable, the expression "the soil is alive" does not fail to capture our hearts. Why is that?

2. soil has properties similar to the autonomy of living things

　Soil has a property that gives a sense of the autonomy that living things have over their environment, i.e., the ability to maintain their own state of health in the face of external stimuli. This is the buffering power of soil.

　Let's take a look at the buffering capacity of soil specifically. Hydrochloric acid and sodium hydroxide are dropped onto the soil as an acidic substance and an alkaline substance, respectively. This means that the soil is stimulated from the outside. As shown in Figure 1, in the case of pure water (H₂O), the pH changes greatly to the acidic side and to the alkaline side with only a small amount of drop.

　On the other hand, the range of change is smaller for both Soil A and Soil B than for pure water. In other words, soil has a greater buffering capacity than pure water. However, soil A, which has less organic matter, has a larger range of change than soil B, which has more organic matter, and therefore has a larger buffer force
is small. Soil organic matter has the ability to electrostatically retain hydrogen ions (H+), which are responsible for acidity, and hydroxyl ions (OH-), which are responsible for alkalinity (the electrostatic ion retention capacity of soil was discussed last year in 2
See Part 9 of the March issue). For this reason, organic matter retains those ions and suppresses their movement, and external stimuli
Soil B has a greater buffering capacity than Soil A, based on the difference in soil organic content. The greater buffering capacity of soil B than soil A is based on the difference in the organic matter content of the soil. This buffering capacity of soil is similar to the autonomy of living organisms.

　However, the buffering capacity of the soil is not the only reason why the soil is said to be "alive. The activities of living creatures living in the soil are often invisible to our eyes, and appear to us as if they were performed by the soil itself. This is another major factor that makes us feel that the soil is alive.

3. proof of life of living creatures - soil respiration

　The fact that compost fed to the soil becomes nutrients for crops, and that leaves that fall on the soil in the fall and kitchen scraps disappear eventually if they are buried in the soil, are all the result of decomposition of organic matter by soil organisms.

　When soil organisms decompose organic matter such as compost, fallen leaves, and garbage, carbon dioxide (CO₂) is released as one of the decomposition products. This is called "soil respiration" because it resembles animal respiration. It is as if the soil is breathing. However, this is only a proof of the activities of living creatures living in the soil, not that the soil itself is breathing.

4. organic matter decomposition is a cooperative play among soil organisms

　Using the fallen leaves mentioned above as an example, let us take a look at the beautiful interplay between soil organisms as they decompose organic matter (Fig. 2).

　Fallen leaves (plant remains) on the soil surface do not change significantly when they remain dry. However, once they are wetted by rain, bacteria (bacteria) and fungi (filamentous fungi) attach to them and soften the plant remains to some extent.

　Then, large soil animals such as earthworms, borers, and sowbugs appear and feed on the plant remains, pulverizing them and dragging them and their remains into the soil. Medium-sized soil animals, such as mites and stoneflies, then take charge, feeding on the dragged organic matter and taking nutrients into their bodies, and excreting the unwanted material as feces.

　Bacteria and fungi feed on the excreted feces and the remains of plants that have been dragged into the soil, eventually transforming them into carbon dioxide, water, and inorganic substances. The inorganic matter produced by this process is absorbed by the roots as nutrients for the plants growing there. Thus, a cycle of nutrients is established. Of course, the same mechanism applies to animal remains as it does to plants.

　The various material changes that occur in the soil include changes involving soil organisms. Therefore, when we talk about material changes in the soil, if we blur the distinction between the function of the soil and the function of living creatures in the soil, we confuse the two.

5. distinguish between the workings of the soil and the workings of living things.

　The expression "the action of the soil" does not mean that the soil has a "will (purposiveness)" like a living creature and that the soil itself actively works by its own will. The expression "the soil supplies nutrients to the crop" is also inappropriate, because the soil itself does not intentionally supply nutrients to the crop. When the crop selects and absorbs the nutrients it needs, such as cations, from among the various nutrient ions dissolved in the water (soil solution) in the soil, the balance of the electro-neutral principle (equal charge of cations and anions) is upset. Various reactions take place in the soil to restore the imbalance, and cations are released into the soil solution to maintain the electroneutrality principle.

　This is the phenomenon described as "the soil supplying nutrients to the crop. This reaction is also governed by a certain principle (in this case, the principle of electro-neutrality in solution), and there is no room for the free will of the creature.

　I would like to understand the true meaning of the parable "the soil is alive" after distinguishing the function of the soil and the creatures in the soil.