Why are lauraceous fleshy fruits potassium-rich despite being non-succulent?

Fleshy fruits are those fruits that present a food-reward to seed-dispersing animals.

Such fruits contain:

• the seeds, which remain intact and undigested by the agents of dispersal and sowing, and
• the fleshy part, called the fruit-pulp, that the plant explicitly allocates for consumption and digestion.

The fruit-pulp of fleshy fruits contains easily-digestible, energy-rich substances, subsidising the locomotion of the relevant fruit-eating animals. Carbohydrates are the most common. However, lipids (https://en.wikipedia.org/wiki/Lipid) take their place in a few cases, such as the fruits of various Lauraceae (https://en.wikipedia.org/wiki/Lauraceae).

In many kinds of fleshy fruits the ripe fruit-pulp is succulent, with water content 80% or more. This exceeds the water content in most tissues of plants or animals, making the fruits juicy. The main adaptive function of this extreme hydration seems to be to enlarge the fruit-pulp and to enhance palatability.

Assuming that fleshy fruits offer energy as their main reward to the agents of dispersal, one hardly expects that elemental (or 'mineral') nutrients, such as phosphorus, would be concentrated in fruit-pulp relative to other tissues such as leaves. This is because:

• fruit-pulp, once ripe, consists of metabolically inactive cells that merely maintain themselves,
• the plant does not need to reward seed-dispersers with anything more than energy-rich substances, and
• vitamins generally do not contain metals or other elemental nutrients.

And indeed no human consumer of ripe fruit-pulp expects much nourishment beyond vitamins and relatively 'hollow calories'.

However, potassium seems to be exceptional among the elemental nutrients.

Commercial fleshy fruits are well-known to be rich in potassium (https://www.healthline.com/nutrition/foods-loaded-with-potassium#TOC_TITLE_HDR_8) despite being poor in most other elemental nutrients. And consistent with this is the fact that potassium is crucial in the horticultural fertilisation of orchards (e.g. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8706047/).

It seems unlikely that potassium is a significant part of the reward to seed-dispersing animals. This is because potassium is:

• one of the most abundant of nutrient elements in plant matter generally,
• not involved in biological catalysis to the same extent as several other metallic elements (https://en.wikipedia.org/wiki/Cofactor_(biochemistry)#Metal_ions and https://en.wikipedia.org/wiki/Potassium#Biological_role), and
• unlikely to be in particular demand by frugivorous or omnivorous animals.

And so it has seemed reasonable to assume that the main function of potassium in ripe fruits is to aid succulence by the ionic maintenance of turgidity in the cells (https://en.wikipedia.org/wiki/Turgor_pressure and https://www.jstor.org/stable/2844617 and https://www.jstor.org/stable/2389867).

But is it really true that ripe fleshy fruits tend to be rich in potassium merely on account of their succulence?

One way to test this is to compare the relative concentration of potassium in succulent fruit-pulp with that in:

• fleshy but non-succulent fruit-pulp, and
• other plant tissues such as leaves.

The fruit-pulp of Laurus (https://www.inaturalist.org/observations?place_id=any&taxon_id=54760&view=species) is non-succulent, possibly because it is rich in lipids instead of carbohydrates. This genus is therefore suitable for such a test.

My source of information is http://www.plant-animal.es/pdfs/Herrera.1986.Estrada.pdf.

In Laurus nobilis (https://www.monaconatureencyclopedia.com/laurus-nobilis/?lang=en and https://www.inaturalist.org/taxa/54759-Laurus-nobilis), 90% of the content of elemental nutrients (excluding carbon, hydrogen, oxygen, and nitrogen) in the ripe fruit-pulp consists of potassium. For Laurus azorica (https://www.inaturalist.org/taxa/336800-Laurus-azorica) the figure is 83%. In both cases, only small contributions are made by calcium, magnesium, sodium and phosphorus; and iron, manganese, zinc and copper are present only as 'trace elements'.

This indicates that the richness of potassium in the fruit-pulp of Laurus cannot be explained by an ionic role in maintaining turgidity. The fruit-pulp is oily and dryish compared to that typical of fleshy fruits, yet turns out to be enriched with potassium relative to all other elemental nutrients.

The fruits of Persea americana (https://en.wikipedia.org/wiki/Avocado) are greatly enlarged by selective breeding. However, in being lipid-rich and non-succulent (water is 72% of the ripe fruit-pulp), they are otherwise similar to those of Laurus - and belong to the same family.

My sources of information are https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3664913/#:~:text=One%2Dhalf%20an%20avocado%20is,%2D6%20(0.2%20mg)%2C and http://www.dietandfitnesstoday.com/calcium-in-avocado.pup.

Once again, the fruit-pulp is rich in potassium, which contributes nearly 90% of the content of elemental nutrients.

How does this compare to green tissues, particularly the leaves of various plants?

My source is https://www.researchgate.net/publication/324079510_Nutritional_and_mineral_composition_of_selected_green_leafy_vegetables. The leaves analysed here are not regarded as succulent, but contain a greater proportion of water than the leaves of most shrubs or trees, including Laurus.

The above shows that, in leaves soft enough to be edible to humans, potassium contributes only about 20% of the content of elemental nutrients. This is presumably because other metallic elements, particularly magnesium and calcium, are vital for photosynthesis.

All fleshy fruits are green when developing, losing their photosynthetic capacity only once ripe. This implies that the plant actually withdraws magnesium and calcium from the fruit-pulp, but hardly withdraws potassium.

Given that even non-succulent fruit-pulp of Lauraceae retains potassium without the need to maintain turgidity:

is it possible that certain plants do - after all - offer potassium to seed-dispersing animals as a particular form of nutritional compensation? And, if so, how is this extra potassium valuable to these animals?

The following show the ripe fleshy fruits of Laurus nobilis:

https://www.organicseeds4you.com/uk/home/1000-seeds-bay-laurel-bay-tree-true-laurel-laurus-nobilis.html

https://garden.org/plants/photo/604646/

https://www.alamy.com/stock-image-laurus-nobilis-fruits-169512260.html

https://www.inaturalist.org/observations/106119777

https://www.inaturalist.org/observations/103309949

Posted on March 19, 2022 01:00 AM by milewski