An unusual type of heteroblasty: juvenile form within phyllodinous foliage in Acacia

@arthur_chapman @abedggood @mattbarrett @iancastle @jeremygilmore @alan_dandie @tonyrebelo

In Acacia, many species have phyllodes (https://en.wikipedia.org/wiki/Phyllode) - derived from ancestral petioles - instead of leaves.

For most naturalists familiar with these plants, 'juvenile foliage' means the reversion to pinnate leaves that occurs in seedlings and, sometimes, on shoots regenerating from damage on saplings (https://www.flickr.com/photos/handsoff/44327173262 and https://wellcomecollection.org/works/mfrr2th3/items and https://keyserver.lucidcentral.org/weeds/data/media/Html/acacia_melanoxylon.htm and https://keys.lucidcentral.org/keys/v3/eafrinet/weeds/key/weeds/Media/Html/Acacia_melanoxylon_(Australian_Blackwood).htm).

However, it may be news to most naturalists that, in at least two species of Acacia, even the phyllodinous foliage itself changes from a juvenile form to a mature form as the plant grows from sapling to adult.

This can be described as a heteroblastic pattern (https://en.wikipedia.org/wiki/Heteroblasty_(botany)) within the phyllodinous foliage. The ‘juvenile’ foliage of phyllodes (https://en.wikipedia.org/wiki/Phyllode) differs greatly from ‘adult/mature’ foliage of phyllodes.

This difference may also involve foliar spinescence, in which the phyllodes have ‘pungent’ tips (https://www.wordnik.com/words/pungent#).

The two species in question are

• Acacia estrophiolata, and
• Acacia peuce (https://en.wikipedia.org/wiki/Acacia_peuce and https://apps.lucidcentral.org/wattle/text/entities/acacia_peuce.htm).

Acacia estrophiolata (http://www.aridzonetrees.com/assets/acest07.pdf and https://en.wikipedia.org/wiki/Acacia_estrophiolata and https://apps.lucidcentral.org/wattle/text/entities/acacia_estrophiolata.htm and https://spapps.environment.sa.gov.au/SeedsOfSA/speciesinformation.html?rid=140 and http://www.flora.sa.gov.au/cgi-bin/speciesfacts_display.cgi?form=speciesfacts&name=Acacia_estrophiolata) is a small tree occurring in central Australia.

In maturity it has pendulous foliage typical for a wattle, with long, narrow, non-spinescent phyllodes.

However, when the growing plant is still shrub-size, the foliage is surprisingly different. The phyllodes

• have different aggregation and orientation to that seen in maturity, and
• may be spinescent, although I have yet to find a clear statement that the tips of the phyllodes are ‘pungent’.

In maturity A. estrophiolata can reach as high as 15 m, making it one of the taller trees of semi-arid central Australia. The wood is ‘very hard’ (Latz 1995), which suggests to me a wood density >1.1.

This is a relatively slow-growing and long-lived species among wattles, as opposed to a ‘fireweed’ or a short-lived, successional species. There is an implication that, because growth is slow, herbivory in the sapling stage can be a particular problem for this species.
 
What is clear is that A. estrophiolata varies greatly in foliage ontogenetically, as it increases in height above ground. This seems to be partly explained by its palatability to large herbivores.

However, I have yet to find a clear statement of the height-threshold at which the form of the foliage switches. My impression is that this is at about 3 m. If so, this is evolutionarily puzzling because no indigenous herbivore in the habitat of A. estrophiolata can reach higher than 2 m.

The anti-herbivore defences of A. estrophiolata present the same puzzle as for so many other plants in Australia: which herbivores were originally being defended against?

The literature fails to distinguish between kangaroos and introduced ungulates. However, A. estrophiolata is regarded as among the more palatable of the wattles of semi-arid central Australia.

Contrast this with e.g. Acacia maidenii (see my other Post), which is certainly foliar-spinescent. That species, which is more strongly associated with Triodia and never grows taller than 3 m, is regarded as unpalatable to the degree that it is never eaten by large herbivores.
 
Peter Latz (1995, page 97) states of A. estrophiolata:
“The narrow leaves [sic] are dense and pendulous, giving the tree a willow-like appearance. Juvenile plants, with wider, shorter, spiny leaves [sic], are quite different in appearance to the adult trees. (This juvenile foliage is often never lost when the trees occur in the harsher spinifex country.)”

However, I do not understand the relationship of A. estrophiolata to fire. This is because Latz describes this species as fire-intolerant, stating “This extremely drought-tolerant tree is often killed or at least severely affected by fire.” My confusion arises because 'spinifex country' is associated with wildfire.
 
Acacia estrophiolata is an extreme example of heteroblasty in its genus. This may be partly because it combines unusual palatability with unusually slow growth, in a semi-arid environment.

In Acacia peuce, the juvenile-form phyllodes are known to be spinescent. However, this awaits confirmation in A. estrophiolata.
 
Acacia excelsa (https://www.inaturalist.org/taxa/139917-Acacia-excelsa) is closely related to A. estrophiolata. It would be worth checking whether it shows the same heteroblastic pattern as seen in A. estrophiolata.
 
The following shows the height and form of the mature (adult) plant in Acacia estrophiolata: http://www.flora.sa.gov.au/cgi-bin/speciesfacts_display.cgi?form=speciesfacts&name=Acacia_estrophiolata and http://www.aridzonetrees.com/acacia-estrophiolata.html.
 
The following shows that the mature/adult phyllodes are not spinescent: http://worldwidewattle.com/speciesgallery/estrophiolata.php and https://en.wikipedia.org/wiki/Acacia_estrophiolata#/media/File:Acacia-estrophiolata-foliage.jpg and https://www.wikidata.org/wiki/Q4670928#/media/File:Acacia-estrophiolata-habit.jpghttp://worldwidewattle.com/speciesgallery/images/estrophiolata.jpg 
  
The difference in form between the ‘juvenile’ and the mature/adult form of the foliage of A. estrophiolata is indicated by the use of the term ‘spiky’ in http://worldwidewattle.com/speciesgallery/estrophiolata.php?id=3327.

The following relates the defensive ‘juvenile’ form of the foliage to the palatability of A. estrophiolata, presumably invoking domestic ungulates: https://web.archive.org/web/20080709022239/http://www.fao.org./ag/agp/agpc/doc/Gbase/new_grasses/acaest.htm.
 
There are few photos of the ‘juvenile’ foliage of A. estrophiolata on the Web, and all are only tentatively identified: https://bie.ala.org.au/species/https://id.biodiversity.org.au/node/apni/2898101 and https://www.inaturalist.org/observations/98461582 and https://www.inaturalist.org/observations/103872799 and https://www.inaturalist.org/observations/96854190 and https://www.inaturalist.org/observations/96081669.

I am puzzled because these phyllodes, although stiff, do not look spinescent. This seems to contradict the adjectives ‘spiky’ and ‘spiny’ used by several authors to describe this foliage.
 
The following photo, of a specimen cultivated in Arizona, shows the contrast in foliage-form within A. estrophiolata: http://www.aridzonetrees.com/assets/acest07.pdf.

The overarching puzzle, in this whole field of research, remains:
which herbivores exerted major evolutionary pressure on the foliage-form?

Publicado el junio 24, 2022 10:55 TARDE por milewski