The World Health Organization uses the term “hidden hunger” to describe a condition that affects a third of the world’s population. Victims of hidden hunger aren’t necessarily starving (though they may be) but they’re undernourished from a dietary deficiency. And that poses health risks. A recent international study addressed this by spraying a cocktail of four minerals onto the surfaces of wheat leaves. The elements then found their way into the grains, from which they can be ingested. The micronutrients were zinc, iodine, selenium and iron. The crop study sites were in China, India, Mexico, Pakistan, South Africa, and Turkey.
There are a variety of tricky aspects here. High concentrations of these elements can be toxic to the plant. Also, normally they are absorbed at the roots, yet putting them into fertilizers invites major losses through drainage in the soil, especially at micro-concentrations. Moreover, the plant tends to partition minerals into specialized uses – in particular both iron and zinc have a role in nitrogen metabolism. And there’s something engineers call a mass balance issue: these metals tend to be present at 1000X the concentration in wheat kernels relative to the selenium and iodine – that’s a pretty big difference – which can create real difficulties in obtaining or maintaining a uniform mixture so as to deliver the intended ratios.
The trial had interesting results. After treatment the grain levels jumped by 9X (for iodine), 3X (for selenium), 65% (for zine), and 12% (for iron). Plants have respective transport and partitioning systems for each of these elements, at least from the root. What they would do for foliar (i.e., on-leaf) application was not entirely predictable in advance.
In reviewing the findings, interestingly, a physical chemist or electrochemist could rationalize them by simple diffusion. In general, anions (negative ions) diffuse through water much more easily than cations (positive ions, the metals) do, because cations attract a big cluster of water molecules around them. The cluster migrates about as quickly as dumplings ever do. And the lower the charge, the speedier the migration, regardless of whether the charge is negative or positive. Thus here we have I-1 > Se-2 > Zn+2 > Fe+2 or +3.
That’s just an observation. It could be just coincidence. But if it’s meaningful it suggests that it might be much more efficient to get zinc and iron micronutrients from treated leaves of broadleaf plants, such as spinach, collard greens, cabbage, lettuce, etc. Well, okay, the first two are dark green, meaning they’re already high in iron, but you get my drift. It might be a good way to deliver zinc. Then we wouldn’t be relying on the long diffusion path to the seed, but could ingest the mineral near the source.
Food for Thought
Primary article: C. Zou et al., “Simultaneous biofortification of wheat with zinc, iodine, selenium, and iron through foliar treatment of a micronutrient cocktail in six countries,” J. Agric. Food Chem., 67(29):8096-8106 (July 24, 2019). Abstract posted at: https://pubs.acs.org/doi/10.1021/acs.jafc.9b01829
Circling-V ™ 