Fruit maturation preserves its connection with the plant
In Ariocarpus kotschoubeyanus, fruits mature in woolly axils, recessed between tubercles. Some emerge and release seeds, while others remain partly or completely hidden. Jiménez-Sierra and co-authors called this delayed release serotiny. In this case, seeds accumulate on the maternal plant's stem, so it can be described as an above-ground seed bank. [Jiménez-Sierra et al. 2022, pp. 362–366]
The degree of retention differs between fruits and plants. Some seeds from a fruit may leave the stem while the remainder stays in its axil. As the plant grows, older tubercle zones occupy more outward positions. This allowed seeds from different stem parts, associated with different growth histories, to be compared. [Jiménez-Sierra et al. 2022, pp. 363–365, Fig. 1]
What remained after one flowering season
In the Tolimán population, flowers of 30 adult plants were marked in November 2016, followed by monthly observations for ten months. The plants produced 53 fruits. Of these, 21 were extruded, 15 retained some seeds, and 17 remained with their entire seed supply. The difference between full and partial retention concerns seed fate within each fruit. [Jiménez-Sierra et al. 2022, pp. 362–364]
The degree of serotiny for this year's cohort was 37,3%, defined as the proportion of retained seeds among the seeds produced in the season. An older bank on the stem was found in 26 of 30 plants, or 86,6% of the group studied. These two measures describe, respectively, the partitioning of one year's seed cohort and the frequency of a bank accumulated over the plant's history. [Jiménez-Sierra et al. 2022, pp. 363–364, Table 1]
The age of stem zones was estimated from tubercle production
The authors distinguished five successive tubercle zones, termed rings. They estimated their mean ages as 2,7, 7,3, 13,3, 19,8 and 23,9 years. The reference was annual photographs of 30 other adult plants from 2012–2016, from which a mean production of 1,42 new tubercles per year was calculated. Tubercle counts within zones were then used in calculations for 27 plants. [Jiménez-Sierra et al. 2022, pp. 363–364, Table 1]
Describing the oldest seeds as approximately twenty-four years old rests on seed position and the estimated age of the zone in which they were found. Age was therefore determined indirectly by comparing different plant parts at one time, without tracking one marked batch for the full 24 years. The estimate indicates prolonged maintenance of germination ability on the stem. [Jiménez-Sierra et al. 2022, pp. 363–365]
Seeds from different zones were transferred to common conditions
Fresh seeds and seeds from five zones were sown on 1% agar without washing, scarification or other pretreatments. Dishes were maintained at 25°C with twelve hours of light per day. Radicle emergence was recorded daily for 20 days. Seed availability determined sample sizes. In the two oldest zones, these were 19 and 39 seeds, much fewer than in younger zones. [Jiménez-Sierra et al. 2022, pp. 364–365, Table 2]
| Material | Estimated zone age | Seeds / dishes | Mean germination |
|---|---|---|---|
| Fresh fruits | current year's cohort | 100 / 10 | 13% |
| Zone 1 | 2,7 years | 100 / 10 | 56% |
| Zone 2 | 7,3 years | 100 / 10 | 51% |
| Zone 3 | 13,3 years | 80 / 8 | 52% |
| Zone 4 | 19,8 years | 19 / 3 | 73% |
| Zone 5 | 23,9 years | 39 / 3 | 40% |
Seeds capable of germination occurred in every zone studied. Means for older material were higher than the mean for fresh seeds, but the large difference in sample sizes and the absence of observation of the same batch during ageing limit the comparison. The oldest zone and fresh seeds belonged to the same statistical group for germination proportion in the table. The results document retention of germination ability well, but provide weaker evidence about the effect of age itself. [Jiménez-Sierra et al. 2022, pp. 364–365, Table 2]
Release and germination between tubercles
The researchers observed that stem tissue tension changed during the wetter part of the year, and spaces between tubercles enlarged, releasing some seeds. Seeds from the outermost zones could also escape when old tubercles dried and fell away. These are two described ways of gradually opening the bank. The role of rainfall as a cue regulating release remains an interpretation of these observations. [Jiménez-Sierra et al. 2022, pp. 365–366]
Seedlings developing between outer tubercles, still on the maternal plant, were found in four plants. The authors called this vivipary in the sense of germination of retained mature seeds. The observation identifies a microhabitat in which seedling development can begin. Subsequent survival of these seedlings after detachment from the maternal plant was not assessed. [Jiménez-Sierra et al. 2022, pp. 364–366, Fig. 2]
The authors also considered after-ripening and periodic seed wetting as possible causes of the different germination ability of older material. These explanations are hypotheses. The experiment itself links retention of seeds on the plant with subsequent germination under uniform conditions, leaving the cause of differences between yearly cohorts to further investigation. [Jiménez-Sierra et al. 2022, pp. 366–367]
Serotiny as a reproductive strategy in the specially protected species Ariocarpus kotschoubeyanus (Cactaceae)
Jiménez-Sierra C. L., Arroyo-Pérez E., Flores J., Zavala Hurtado J. A., Serotiny as a reproductive strategy in the specially protected species Ariocarpus kotschoubeyanus (Cactaceae), Plant Species Biology, 2022, 37(6):361-368, doi: 10.1111/1442-1984.12391
Observations of 30 adult Ariocarpus kotschoubeyanus document partial retention of mature seeds between tubercles. Tubercle formation rate estimates the age of five stem zones, and a germination test compares their material with fresh seeds at 25°C. The paper also describes germination on the mother plant and possible gradual release mechanisms for the bank.
