A description of the fleshy fruit of Heeria argentea (Anacardiaceae)

@benjamin_walton @tonyrebelo @jeremygilmore @botaneek @robertarcher397 @adriaan_grobler @justinhawthorne @rcswart @dianastuder

During Easter of 1999, while visiting the Cederberg range (https://en.wikipedia.org/wiki/Cederberg) near Citrusdal, Western Cape, South Africa, I made the following notes on the fleshy fruit and other biological aspects of Heeria argentea (https://www.inaturalist.org/taxa/569483-Heeria-argentea) - one of the few spp. of relatively large woody plants restricted to the mediterranean-type climate in southern Africa.

The location was close to https://www.inaturalist.org/observations/97636858.

Habitat:

Heeria argentea is essentially a plant of fynbos vegetation, strongly associated with relatively low altitudes, a mesic climate, and boulder outcrops.

It is common on lower slopes and in in the mid-altitude shelf-zone associated with a shale-band, in the Cederberg range. It does not extend above the 'cedar zone' of Widdringtonia wallichii (https://en.wikipedia.org/wiki/Widdringtonia_wallichii).

The plants are inevitably scorched by the usual wildfires of fynbos. However, hugging the boulders (https://www.inaturalist.org/observations/60244770 and https://www.inaturalist.org/observations/107779464 and https://www.inaturalist.org/observations/115348334 and https://www.inaturalist.org/observations/115155395 and https://www.inaturalist.org/observations/119119988 and https://www.inaturalist.org/observations/112319739) protects H. argentea partly from damage, allowing partly epicormic regeneration without corky bark. It is slow-growing and probably long-lived.

Description of plant:

This is a stunted-looking large shrub (https://www.inaturalist.org/observations/141694068), up to 5 m high (https://www.inaturalist.org/observations/69106925).

Rather than having a bole, it is multi-stemmed at ground level (https://www.inaturalist.org/observations/72355976).

It is evergreen and semi-sclerophyllous (https://www.inaturalist.org/observations/141379584 and https://www.inaturalist.org/observations/140991942), and dioecious. Regeneration after scorching is vegetative (https://www.inaturalist.org/observations/17931554 and https://www.inaturalist.org/observations/39719663 and https://www.inaturalist.org/observations/51084583 and https://www.inaturalist.org/observations/112739359 and https://www.inaturalist.org/observations/51084583) and partly epicormic (https://www.inaturalist.org/observations/96891411).

However, it is typically protected from intense wildfire by rockiness, apparently depending on this protection.

Coexisting spp. of fleshy fruits:

Heeria argentea coexists with Maytenus oleoides (https://www.inaturalist.org/observations/101819035), which is of similar form in its stems and foliage. It coexists less intimately with Diospyros glabra (https://www.inaturalist.org/observations/16257473), which is a much smaller plant, and less intimately again with Myrsine africana (https://www.inaturalist.org/observations/126687588).

Description of fruit:

The growing fruits, which I did not see during my visit, are green, dull reddish, or dull yellowish (https://www.inaturalist.org/observations/30798653 and https://www.inaturalist.org/observations/119845046 and https://www.inaturalist.org/observations/129461183 and https://www.inaturalist.org/observations/18965106 and https://www.inaturalist.org/observations/129991138 and https://www.inaturalist.org/observations/135219686 and https://www.inaturalist.org/observations/134708630 and https://www.inaturalist.org/observations/134849458). I do not interpret this as a pre-ripe display, because the hues are so inconsistent.

Field guide-books describe the fruit as leathery, which is an unsatisfactory description.

Based on my limited observations, I would classify this as definitely zoochorous, moist but non-succulent, and probably not endozoochorous.

I picked several nearly mature fruits which did not ripen up properly over the next few days, and several mature fruits which may however not have been as ripe as they become while still attached.

The fruit (https://www.inaturalist.org/observations/111321790 and https://www.inaturalist.org/observations/129880458 and https://www.inaturalist.org/observations/107684330 and https://www.inaturalist.org/observations/71292756 and https://www.inaturalist.org/observations/22705718) is ellipsoid. Its surface is matt and smooth.

This possibly represents an enlarged version of the kidney shape of the fruits of closely-related Ozoroa (https://www.inaturalist.org/observations?place_id=any&taxon_id=185182&view=species).

The size of the mature fruit is variable. The diameters of several specimens that I measured averaged: long axis 2.6 cm, and short axis 1.95 cm.

The fruit-pulp is only 1 mm thick.

The odour of the damaged, unripe fruit is mango-like, typical of the family Anacardiaceae.

Picked fresh from the plant, the just-mature fruits are pale, dull, pastel creamy-fawn, with negligible green (https://www.inaturalist.org/observations/14948101).

Left lying around in the heat for a day, the fruit turns darker (dull brown, https://www.inaturalist.org/observations/11177340), which is possibly owing to the reaction of its oil-content.

The thin fruit-pulp is certainly more oily than succulent. However, at least at the not fully ripe stage I inspected - with the kernel still pale green and the membrane between the testa (https://www.merriam-webster.com/dictionary/testa) and the kernel also with a trace of green - it is moist enough to be at the threshold of succulence.

Description of seed:

Please see https://academic.oup.com/botlinnean/article/122/4/335/2607947, which does not refer to the term 'testa' that I used in my field-notes.

The testa is not woody, but (at the stage sampled) it is moist, and carvable with the thumbnail. When fully mature, it is perhaps chartaceous (https://www.merriam-webster.com/dictionary/chartaceous). However, it is hardly protective.

The seed, with two clear cotyledons and wrapped in a hazelnut-like membrane within the testa, occupies >90% of the volume of the diaspore. I found it to be pale green, starchy rather than oily, crisp, and nearly succulent at the stage I sampled. Older seeds, picked up on a path (possibly discarded by Papio ursinus), had the fruit-pulp stripped off the testa, with the kernel brown, not green, and somewhat dehydrated (certainly not succulent).

My interpretation:
This is an example of a large seed adapted to a nutrient-poor environment, but unprotected physically. It is invested in a definite (thin, both relatively and absolutely) fruit-pulp, that attracts dispersers mainly by virtue of its oil content.

I tasted the fruit-pulp, finding it innocuous enough. The interior of the seed tasted like a resinous acorn, astringent but not bitter or spicy.

Phenology:

More individual plants were in flower than in fruit, based in cursory inspection during my visit. Field guide-books give the flowering season as late summer, and the fruiting season as early summer, which is the opposite of the usual pattern for succulent fruits generally. I infer that I found the last of the ripening fruits. Do the fruits take more than 9 months to mature?

Dispersal and sowing:

The acorn-size seed must be attractive to seed-eaters because of its large size. So, how does it survive the attentions of Procavia capensis (https://www.inaturalist.org/observations/65876534 and https://www.inaturalist.org/observations/9274878) and Papio ursinus (https://www.inaturalist.org/observations/38351714)?

Remains found on an open sandy path, near shallow diggings (probably for iridaceous corms) suggested that some at least are discarded by P. ursinus, then subject to post-dispersal predation by Hystrix africaeaustralis (https://www.inaturalist.org/taxa/44173-Hystrix-africaeaustralis) - for which this succulent, somewhat toxic 'white matter' would be typical food.

I do not know if the fully ripe fruit withdraws moisture from the fruit-pulp, to leave just a mealy, oily layer.

In any event, there is no indication of bright hue, this and size ruling out birds as dispersers. And I do not see how Procavia capensis could disperse the seeds.

The fruits (at least the fruit-pulp) is sure to be edible to Procavia capensis. However, I suspect that the real disperser is Papio ursinus, filling its cheek-pouches and then perching on a rock to consume the fruit-pulp, or this plus the seed.

A search-image for naturalists:
Does Papio ursinus chew up the whole diaspore, or does it first chew off the fruit-pulp, then peel off and discard the testa and membrane (which are fibrous enough to be worth rejecting)?

I doubt that Papio ursinus swallows the whole diaspore, because it is not protected by a hard/tough shell, and seems too large to swallow/defaecate whole. Papio ursinus may intend to eat the kernel, but would inevitably fumble/drop items irretrievably into fissures among the boulders, where germination could occur (https://www.inaturalist.org/observations/72290046 and https://www.inaturalist.org/observations/29769981). The animals might be sated by the astringent seed before the oily fruit-pulp, leading to deliberate wastage.

Posted on 19 de novembro de 2022, 07:41 AM by milewski