Stem and underground system
Segments are cylindrical, brown-purple in the morphological comparison and lack conspicuous tubercles. Areoles do not have dense woolly hairs. The central spine is cylindrical, about 2 mm long in the material described in the table. Flowers develop at the segment tip. [Las Peñas and Bernardello 2021, Table 2]
Below the surface is a thickened root system associated with replacement of above-ground stems. The fruit opens with a lid and exposes seeds with a peripheral wing. Within the genus, this arrangement combines seed reproduction with the capacity for vegetative regeneration. [Las Peñas and Bernardello 2021]
Range and cytotypes
The 2021 account described the species' range from Salta to Río Negro. Material included the typical form and plants identified as the form lelongii. The typical form had a count of 2n = 22, and the form lelongii had 2n = 44. This difference is retained with data from these specimens. [Las Peñas and Bernardello 2021]
The epidermis study showed alternating bands of papillose cell projections and smooth sections. These microscopic papillae differ from the large tubercles visible on stems of other Pterocactus. In P. tuberosus, papillae are curved, and the ordinary cuticle surface has faint, irregular striations. [Faigón et al. 2011]
Germination in light and at different temperatures
Kin and coauthors studied seeds from La Pampa, stored dry at room temperature. Experiments were conducted 2–10 months after collection, with five replicates of ten seeds. Within 30 days at 25°C, a mean of 84.0 ± 6.8% of seeds germinated in light and 58.0 ± 4.9% in darkness. The reported dispersion is the standard error. [Kin et al. 2015]
In the 20–35°C treatments and the 25/15 and 30/20°C cycles studied, final germination exceeded 70%. It was much lower at a constant 15°C and under the 20/10°C cycle, and did not occur at 40°C. Germination was fastest at 30°C. The results describe seed germination on a moist experimental substrate. [Kin et al. 2015]
Water during germination
Water potential was lowered with polyethylene glycol solutions. Germination was high at 0 and −0.1 MPa. It fell sharply at −0.4 MPa, and most seeds remained ungerminated at −0.7 and −1.0 MPa. Some resumed germination after transfer to distilled water. [Kin et al. 2015]
The result indicates retention of viability by some seeds during a temporary reduction in water availability. Initiation of germination required more favourable hydration. A detailed comparison with Maihuenia patagonica and Maihueniopsis hickenii appears in the article on seeds of southern cacti. [Kin et al. 2015] [Masini et al. 2014]
Growth of young plants
One-week-old and six-week-old seedlings were studied separately. They grew in a greenhouse in Santa Rosa, in a 1:2 mixture of coarse sand and sandy loam soil. Watering every 7 days, every 21 days and only at the experiment's start was compared. The younger group was observed for 85 days and the older group for 100 days. [Kin et al. 2015]
More frequent watering produced longer stems and greater mass. Under the strongest water limitation, older seedlings shrank their stems and growth was much lower. Drought survival here is associated with economical functioning of the geophyte. Active growth requires suitable temperature, light and available water together, under conditions allowing effective root function. [Kin et al. 2015]
Synonyms and names in the literature
Opuntia kuntzei · Opuntia tuberosa · Opuntia tuberosa var. spinosa · Pterocactus decipiens · Pterocactus kuntzei · Pterocactus kuntzei f. lelongii · Pterocactus kurtzei · Pterocactus tuberosus f. lelongii [2-WWW]