What triggers germination in southern cacti

Two species from the same area

Masini, Rovere and Pirk collected seeds of Maihuenia patagonica and Maihueniopsis hickenii in Auca Mahuida, Argentina. They described the second species as M. darwinii var. hickenii. After 60 days of storage at 5°C, they compared a control, moist cold stratification and two scarification methods. Each treatment had ten replicates of thirty seeds [Masini et al. 2014] [2-WWW].

Stratification lasted 40 days at 5°C. Mechanical treatments included abrasion and cutting with a scalpel. A 20/10°C cycle with a twelve-hour day was then maintained. In M. hickenii, a scalpel was used to remove a fragment of the outer covering of funicular origin, preserving the seed coat proper. This distinction reflects the structure of opuntioid seeds [Masini et al. 2014].

The result depended on the species

Mean final germination across treatments was about 8–27% in Maihuenia patagonica and 17–26% in Maihueniopsis hickenii. Observation lasted about two months. In the first species, the scalpel treatment exceeded the control. In the second, no treatment significantly increased the final percentage relative to the control, although stratification accelerated the onset of germination [Masini et al. 2014].

The viability of the remaining material was assessed after the experiment. The authors considered non-deep physiological dormancy, the influence of coverings and the temperature used in the experiment. The absence of a radicle over a short period therefore does not establish whether a seed has died or remains viable in a dormant state [Masini et al. 2014].

Diagnosing dormancy class draws on seed structure, water uptake and the behaviour of a viable embryo under favourable germination conditions. Physical dormancy requires demonstration that the coverings are impermeable to water. A review of Cactaceae research identified physiological dormancy and nondormant seeds as documented states in the material analysed. An improvement following cutting establishes a treatment response, while identifying the mechanism requires connecting it with those traits [Barrios et al. 2020b].

In the Patagonian comparison, cutting involved different structures. In Maihueniopsis, part of the funicular envelope surrounding the seed proper was removed. The treatment’s effect therefore also depends on which layer was altered. Faster germination after cold stratification and the final proportion germinating describe further, distinct responses. Combining anatomy, viability and germination timing allows each species’ outcome to be interpreted in relation to its own material [Masini et al. 2014][Barrios et al. 2020b].

Pterocactus tuberosus needs hydration

A separate study examined Pterocactus tuberosus seeds from La Pampa, stored dry for 2–10 months. Each treatment had five replicates of ten seeds. Within 30 days at 25°C, mean germination was 84% in light and 58% in darkness. Light increased the proportion germinating, but some could also develop without it [Kin et al. 2015].

Reducing water potential with polyethylene glycol solution strongly limited germination. At −0.7 and −1.0 MPa, most seeds remained ungerminated. After transfer to distilled water, some began growing, indicating retained viability despite the earlier adverse conditions [Kin et al. 2015].

The response after transferring P. tuberosus to pure water demonstrates the importance of an external condition. Low water potential restricted the onset of growth, and renewed hydration allowed some seeds to germinate. Seed physiology describes such inhibition by unsuitable conditions as quiescence. Dormancy is diagnosed from the behaviour of living seeds under conditions suitable for germination. This gives survival through drought, dormancy release and subsequent seedling growth their own assessment criteria [Kin et al. 2015][Barrios et al. 2020b].

Sowing temperature and subsequent growth

P. tuberosus germinated best in the treatments covering 20–35°C and in 25/15 and 30/20°C cycles. At 15°C, the proportion was low, and at 40°C seeds did not germinate during the experiment. The result describes a particular seed sample on moist substrate and indicates a range for planning further experiments with one's own material [Kin et al. 2015].

After germination, a seedling becomes a plant with its own water balance. In a greenhouse study of P. tuberosus, reduced watering decreased growth and mass even when plants remained viable. Survival under water shortage and effective growth therefore require separate descriptions. Detailed conditions are given in the species profile [Kin et al. 2015].