Why are fleshy fruits rich in potassium?

Fleshy fruits are those adapted for dispersal by animals larger than insects. They are extremely diverse in form and phylogeny.

One of the most consistent aspects of fleshy fruits is that their fruit-pulp tends to be rich in potassium, even where it is poor in all other nutrient elements.

This consistent richness of fleshy fruit-pulp is particularly noteworthy because the food-rewards - such as sugars, lipids, and vitamins - offered in this form by plants to seed-dispersers are remarkably variable. Some fleshy fruits are sweet, others sour; some are rich in ascorbic acid, others not; a few are oily, most not so; and this variation applies even within the category of succulent fruits (viz. those with fruit-pulp containing >80% water, so that any crushing results in the visible exudation of juice).

This pattern has yet to be explained in adaptive terms.

There are two broad possibilities, which are not mutually exclusive.

Firstly, it is possible that the main function of potassium in fruit-pulp is to promote turgor pressure, in aid of maintaining succulence. However, the main problem with this is that even naturally dry fruit-pulp, such as that of carobs (fleshy pods of leguminous trees and shrubs in the Fabaceae).

Secondly, it is possible that the main function of the potassium is to provide nourishment for the animal concerned. However, the main problem with this is that potassium seems to be one of the 'cheapest' elements, cycling rapidly through the body, lacking any significant reservoir in the tissues, being easily excreted in urine, and having mainly a simple electrolytic (as opposed to catalytic) function.

Posted on 19 de novembro de 2022, 11:31 PM by milewski milewski

Comentários

An exception to the pattern of relative concentration of potassium in fruit-pulp is as follows.

Taxus (https://www.inaturalist.org/taxa/47553-Taxus-baccata) and Dioscorea (https://www.inaturalist.org/taxa/82691-Dioscorea-communis) both occur in the Mediterranean Basin. Both genera possess succulent fruits.

However, the fruit-pulp of the herbaceous plant contains 4.26% potassium. By contrast, that of the conifer contains 0.6% potassium. This is a seven-fold difference.

Because the fruit-pulp is succulent in both cases, this variation undermines the idea that the main function of potassium is to facilitate succulence physiologically.

Publicado por milewski mais de 1 ano antes

In general, plants with ectotrophic mycorrhizae (https://en.wikipedia.org/wiki/Ectomycorrhiza) lack fleshy fruits. This may perhaps reflect, at least partly, the regime of mineral nutrition facilitated by the fungi concerned, which does not particularly favour potassium.

The rosaceous genera Malus and Pyrus have succulent fruits, with fruit-pulp relatively poor in potassium.

Ectomycorrhizae occur in Rosaceae (https://www.researchgate.net/publication/51169238_Ectomycorrhizas_of_Cercocarpus_ledifolius_Rosaceae#:~:text=Unlabelled%3A%20Premise%20of%20the%20study%3A%20Woody%20species%20in,or%20linked%20with%20other%20trees%20through%20mycorrhizal%20networks.).

Ectomycorrhizal involvement is also recorded in Malus and Pyrus (see Manami Shishikura et al., 2021, Mycorrhiza 31: 31-42).

Publicado por milewski mais de 1 ano antes

Concentrations of potassium in foliage are generally <3% on a dry matter basis. On base-rich soils, the values are usually >1%. On relatively nutrient-poor substrates under mediterranean-type climates, the values are <1%. On the nutrient-poorest substrates in kwongan and fynbos, the values are <0.5%.

In the arid zone in Australia, concentrations of potassium in foliage range from 1% in sclerophyllous plants to 2% in orthophyllous plants, and up to 10% in fleshy-leafed plants such as rhagodias (Keay and Bettenay).

Concentrations of potassium in the fruit-pulp of fleshy fruits are generally <1% on a dry matter basis.

Antidesma bunius 0.14%
https://en.wikipedia.org/wiki/Antidesma_bunius

Morinda citrifolia? 0.49%

Pleiogynium timorense 0.52%
https://en.wikipedia.org/wiki/Pleiogynium_timoriense

Capparis mitchellii 0.27%
https://en.wikipedia.org/wiki/Capparis_mitchellii

Publicado por milewski mais de 1 ano antes

Ratios of potassium to sodium in foliage are generally about 30 (range 3-70) in acacias, and 7-65 in grasses, 2.1/1.2 in Portulacaria afra, and 3.2/0.5 in 'noors' euphorbia.

Plants in the arid zone of Australia have foliage with typically 2-3% nitrogen, corresponding to crude protein about 16%.

Publicado por milewski mais de 1 ano antes

Potassium is absorbed by plants readily. The more available, the more absorbed, even over and above metabolic needs (luxury consumption).

Potassium is freely leached and very soluble.

It is contained in plant tissues (including fleshy fruits but not particularly in these) in large amounts relative to other nutrients.

Acidic solutions release potassium from minerals such as feldspars.

An exception is that potassium is fixed in the crystal lattice of illite.

Vermiculite and illite far exceed montmorillonite in their fixing capacities for potassium.

Kaolin (which is particularly prevalent in Australia) is well-known to have limited ability to hold potassium.

Alternate freezing and thawing results in the release of fixed potassium under certain conditions (consider the profusion of fleshy fruits in tundra).

Lime induces fixation of potassium; in moderate amounts, it helps to prevent leaching, thus boosting availability to plants, overall; in large amounts, it results in deficiency of potassium (helping to explain why fleshy fruits are coomoner on calcareous sand than on limestone in the Cape floristic region).

There are three ways in which roots acquire nutrients: interception, mass flow, and diffusion.

It is well-known that, for agricultural crops, frequent light applications of potassium are more efficient and effective than infrequent large applications.

"as nutrients are absorbed by roots, a concentration gradient is set up between the zone immediately surrounding the root and the soil zones farther away. In response to this gradient, diffusion of ions toward the root surface takes place. For cations such as potassium and calcium diffusion js by far the most important means of supplying nutrients to plant roots."

Publicado por milewski mais de 1 ano antes

Most fleshy fruits have more potassium than nitrogen in their fruit-pulp.

Data in Debussche et al. (1987, https://www.jstor.org/stable/3565758) show that the fruit-pulp of fleshy fruits has ratios of potassium to nitrogen of about 1-4.

Only a few spp. have ratios (slightly) less than 1.

Examples are Laurus azorica (K/N = 0.98) and Laurus nobilis (K/N = 1.0-1.25).

The spp. with the greatest ratios seem to be herbaceous plants, which have fruit-pulp rich in potassium, but not correspondingly rich in protein.

So, expressing the potassium in fruit-pulp as K/N ratios does narrow the interspecific variations, but only partly.

But what about Acacia?

Publicado por milewski mais de 1 ano antes

It is the ratios, rather than the absolute quantities, of nutrients that have bearing on a surfeit of potassium.

The ratios of potassium to nitrogen, phosphorus, magnesium, and calcium all vary 10-fold or more among plant tissues.

For all these elements, the greatest ratios are found in fleshy fruit-pulp. Potassium outweighs all other nutrients, and this is >10-fold in the case of phosphorus, magnesium, and calcium.

In seeds and leaves, potassium is always outweighed by nitrogen.

In seeds, potassium outweighs phosphorus, magnesium, and calcium, but by <10-fold.

In leaves, potassium is outweighed by calcium in sclerophylls; and potassium outweighs magnesium by <10-fold.

In herbaceous tubers, the ratio of potassium to magnesium is consistently >10, as in fleshy fruit-pulp. However, tubers do not generally achieve ratios as great, of potassium to magnesium and other nutrients, as in fleshy fruit-pulp, even where of equivalent succulence.

Publicado por milewski mais de 1 ano antes

Fire causes a short-term flush of potassium (in ash), but long-term depletion of potassium (in leachate), relative to other nutrients, particularly phosphorus.

Hence, we would expect some increase in the incidence of fleshy fruits after wildfire. However, we would expect plants with fleshy fruits to dominate the vegetation only in areas permanently protected from wildfire.

Publicado por milewski mais de 1 ano antes

Research is needed for nitrogen-fixing plants with fleshy fruits, particularly Morella and Myrica, Hippophae and other Elaeagnaceae, and Macrozamia.

Publicado por milewski mais de 1 ano antes

The caesalpinioid legume Ceratonia siliqua (https://www.inaturalist.org/taxa/82742-Ceratonia-siliqua) has fleshy fruits in the form of sugary pods, which are naturally dry when ripe.

The fruit-pulp of C. siliqua, although far from succulent, is rich in potassium.

The following is an analysis if the fruit-pulp, on a 10% moisture basis:
potassium 0.95%, magnesium 0.08%, calcium 0.22%, sodium 0.10%.

This means that the ratio of these nutrient elements is potassium 95: calcium 22: sodium 10: magnesium 8. The concentration of potassium is 12-fold that of magnesium.

Source: page 99 in 'The carob in California' by Paul Thomson, pp. 61-102 in California Rare Fruit Growers Yearbook, vol. III, 1971.

Also see https://www.researchgate.net/publication/8888671_Leaf_nutrient_variation_in_mature_carob_Ceratonia_siliqua_trees_in_response_to_irrigation_and_fertilization and https://www.doc-developpement-durable.org/file/Culture/Arbres-Fruitiers/FICHES_ARBRES/Caroubier/Leaf%20nutrient%20variation%20in%20mature%20carob%20tree%20in%20response%20to%20irrigation%20&%20fertization.pdf

Publicado por milewski cerca de 1 ano antes

Notes from Monselise (1986, https://www.taylorfrancis.com/books/edit/10.1201/9781351073042/handbook-fruit-set-development-shaul-monselise and https://www.amazon.com.au/CRC-Handbook-Fruit-Set-Development/dp/0849332605 and https://www.routledge.com/Handbook-of-Fruit-Set-and-Development/Monselise/p/book/9781315893945):

Potassium boosts the yield and quality of succulent fruits.

The balance between potassium and nitrogen is important. Nitrogen-rich soils can be extremely deficient in potassium, in horticulture of succulent fruits.

Calcium in soils tends to prevent the uptake of potassium.

In Carica papaya (https://www.inaturalist.org/taxa/62811-Carica-papaya), the concentrations of nutrients in the fruits are potassium > nitrogen > calcium > magnesium > phosphorus. At one location, the potassium removed in the fruits represented 56% of the amount applied as fertiliser.

Potassium boosts the size, rather than the number, of the fruits of Citrus.

In the banana (Musa acuminata, https://www.inaturalist.org/taxa/140907-Musa-acuminata), potassium boosts the size of the fruit (and growth generally), even more than in C. papaya. There is an additional effect on the quality of the fruit, including in storage.

In the grape (Vitis, https://www.inaturalist.org/observations?place_id=any&taxon_id=60773&view=species), potassium forms 50-70% of the cation content of the fruit.

The plum (Prunus domestica, https://www.inaturalist.org/taxa/63735-Prunus-domestica) is sensitive to potassium-deficiency. Potassium is clearly the nutrient most concentrated in the fruit-pulp of the plum (page 411 in Monselise 1986).

The following are the concentrations (mg/100 g fresh mass) of nutrients in the fruit-pulp of the plum:
potassium 150-170
calcium 11-12
phosphorus 18
iron 0.3-0.5
sodium 1-4.

(Note the great ratio of potassium to sodium.)

Publicado por milewski 9 meses antes

Adicionar um Comentário

Iniciar Sessão ou Registar-se to add comments