This image from my ppt slide has pictures of 3 species of Tribulus. They are distinct in general morphology, style, fruits, thorns, indumentum, leaves etc. as you can see in the pictures.
Reproducing the key here:
A simple macroscopic key to distinguish the 3 southern indian species :
1. Style absent or very short (< 0.5 mm long) --- Tribulus terrestris
1. Style present, 1.5-2 mm long ———————- 2
2. Style glabrous ——————————————- Tribulus lanuginosus (syn. of Tribulus terrestris var. bicornutus (Fisch. & C. A. Mey.) Hadidi)
2. Style pubescent —————————————- Tribulus subramanyamii
Hope this gives some clarity in id of these 3 spp.
Look forward to your comments/observations.
Really useful information sir. Thank you very much for sharing and throwing light on this species complex.
Taxonomic Status of Some of the Tribulus Species in the Indian Subcontinent by Varghese, M.; Yadav, S.S. & Thomas, J. Saudi Journal of Biological Sciences 13 (1) 7-12: 2006 (keys)–
A provisional key for the determination of Tribulus L. in India
1. Mericarps winged 2
1+ Mericarps spiny or unarmed or look-like unarmed 3
2. Stamens usually 5; wings of mericarps 2-4 mm wide or represented by small spinules at the base of the mericarps; margin of the wing much dentate … T. pentandrus
2+ Stamens 8-10; wings of mericarps 6-10 mm wide; margin shortly or shallowly few dentate. … T. macropterus
3. Flowers smaller, 5-15 mm wide 4
3+ Flowers larger, 25-40 mm wide. … T. cistoides
4. Mericarps with two lateral spines and 20-25 unequal spines spreading throughout … T. terrestris var. rajasthanensis
4+ Mericarps with 2 lateral spines and two basal spines or the basal spines absent. 5
5. Style puberulous, intra staminal glands connate and ciliated. … T. subramanyamii
5+ Style glabrous, intra staminal glands free, not cilated 6
6. Flowers 5-7 mm wide; Stamens 5-8; fruit 6-8 mm in diameter including spines; lateral spines 1.5-3 mm long; basal spines small or reduced to tubercles. … T. parvispinus
6+ Flowers 7-15 mm wide; Stamens usually 10; fruit 9-15 mm in diameter including spines; lateral spines5-8 mm long; basal spines 3-5 mm long. … T. terrestris
Taxonomic Status of Some of the Tribulus Species in the Indian Subcontinent by Varghese, M.; Yadav, S.S. & Thomas, J. Saudi Journal of Biological Sciences 13 (1) 7-12: 2006 (Details with keys–
Tribulus parvispinus C.Presl,