Amaranthus L., 1996

Hassan, Walaa A., Al-Shaye, Najla A., Alghamdi, Salma, Korany, Shereen M. & Iamonico, Duilio, 2022, Taxonomic revision of the genus Amaranthus (Amaranthaceae) in Saudi Arabia, Phytotaxa 576 (2), pp. 135-157 : 137

Amaranthus L. , Sp. Pl. 2: 989. 1753.

Type (lectotype designated by Green 1929: 188): Amaranthus caudatus L.

Description:— Monoecious or dioecious herbs, usually annual (therophytes), sometimes perennial (hemicryptophytes). Stems erect, ascending, or prostrate, glabrous to tomentose (trichomes uniseriate, whitish to yellowish), green, white, brownish or red, usually branched. Leaves alternate, petioled, with blade lanceolate to ovate, elliptic to deltoid to rhombic; base cuneate to obtuse; apex acute, obtuse, or emarginate, sometimens mucronate; margins entire, sometimens undulate; blade glabrous to pubescent (sometimens hairs only along the veins), with trichomes whitish to yellowish, uniseriate. Synflorescences thyrsoid paraclades arranged in terminal and/or axillary spike- or panicle-like structures or only in axillary glomerules (for details see Iamonico 2015). Bracts 1–5, ovate to lanceolate, with membranous borders thinning to apex or abruptly interrupted at the half of the total length, sometimes keeled; apex acute to obtuse. Flowers unisexual, sessile. Staminate flowers with 3–5 free and more or less equal tepals, ovate to lanceolate, usually glabrous; apex usually acute; margin entire; stamens 3–5, anthers tetrasporangiate with 2 lines of dehiscence, filaments free to the base; pseudostaminodia absent. Pistillate flowers with (0–)2–5 usually free tepals, linear to ovate-lanceolate sometimes spathulate, usually glabrous; apex acute to emarginate (sometimes mucronate); margins entire; one pistil, one ovule, 2–5 stigmas. Fruit dry (dehiscent capsule, or indehiscent utricle), globose to ellipsoid, smooth to strongly rugose on the surface, with often persistent styles; seed one, usually lenticular, smooth to reticulate; embryo annular.

General note and diagnostic key:— Twelve non-hybrid species (sixteen taxa, by considering the infraspecific taxa) were here recorded in Saudi Arabia, of which two ( A. caudatus and A. tricolor L.) are cultivated only in the country. Four taxa ( A. graecizans subsp. graecizans , A. graecizans subsp. sylvestris , A. graecizans subsp. thellungianus , and A. sparganicephalus ) are authoctonous, whereas the other ones are aliens, mostly neophytes native to the Americas (Table 2).

A diagnostic key of the Amaranthus species occurring in Saudi Arabia follows (the characters of the flowers refer to the pistillate ones, since the features of the staminate flowers have a very low taxonomical value in the genus Amaranthus ; see e.g., Mosyakin & Robertson 2003, Iamonico 2015). We include also the two cultivated species ( A. caudatus and A. tricolor ) which could be found in wild in future. Diagnostic keys for infraspecific taxa of A. blitum L. and A. graecizans are reported after the morphological descriptions of them.

1. Tepals 3...............................................................................................................................................................................................2

- Tepals> 3 ...........................................................................................................................................................................................7

2. Stem white to white-greenish; bracts spinescent longer than the tepals .............................................................................1. A. albus

- Stem never white to white-greenish; bracts not spinescent, shorter than the tepals...........................................................................3

3. Synflorescence spike- or panicle-like.................................................................................................................................................4

- Synflorescence in axillary glomerules................................................................................................................................................6

4 Fruit indehiscent (utricle), as long as or longer than the perianth......................................................................................................5

- Fruit dehiscent (capsule) shorter than the perianth.......................................................................................................... 3. A. tricolor

5. Fruit smooth or sligthly on the surface...................................................................................................................... 2. A. blitum s.lat

- Fruit strongly rugose on the surface .................................................................................................................................. 4. A. viridis

6. Fruit shorter than the perianth ......................................................................................................................................... 3. A. tricolor

6. Fruit as long as or longer than the perianth ........................................................................................................................................8

8. Fruit up to 2.7 mm long 4, never longitudinally sulcate .................................................................................... 5. A. graecizans s.lat

- Fruit> than 2.7 mm long, the proximal half longitudinally sulcate............................................................... 6. A. sparganocephalus

7. Tepals usually 4; stem prostrate-diffuse or ascending; synflorescence arranged in axillary glomerules...................... 7. A. blitoides

- Tepals 5; stem erect; synflorescence arranged in spike- or panicle-like structures............................................................................ 8

8. Bracts of the first flower in the first cyme metamorphosed into a spine-like structure ................................................ 8. A. spinosus

- Bracts spine-like absent......................................................................................................................................................................9

9. Terminal synflorescence usually pendulous up to 70 cm long; tepals (at least the inner ones) obovate-spathulate.....9. A. caudatus

- Terminal synflorescence always erect; tepals ovate to lanceolate.................................................................................................... 10

10. Bracts up to 2 mm long and always shorter than the tepals ............................................................................................10. A. dubius

- Bracts> 2 mm long and always longer than the tepals ....................................................................................................................11

11. Bracts clearly longer (1.6–2.0 times) than the perianth; tepals with median vein usually dark-green ....................... 11. A. hybridus

- Bracts as long as or slightly longer (up to 1.5 times) than the perianth; tepals with median vein usually yellow-brown................... ..................................................................................................................................................................................... 12. A. cruentus