A rosette above the root
Obregonia denegrii forms a stem of overlapping triangular tubercles and fleshy, fusiform roots. In this structural plan, much of the plant lies beneath the substrate surface. Seedling growth can therefore be described by total biomass increase and by the proportions of root and shoot. These two pieces of information answer different questions. The first indicates how much new tissue formed. The second shows where it was built. [Martínez-Berdeja and Valverde 2008, pp. 1768–1769]
In the Jaumave Valley, associated with older observations of the species, plants occupy stony, calcareous habitats. Ecological descriptions indicate that seedlings can develop in shaded microsites. A nurse plant changes several conditions beneath its canopy simultaneously, including radiation input, temperature and water availability. Seedling presence beneath a shrub alone therefore leaves open the question of which factor favours growth. [Lucas and Synge 1978, IUCN p. 107, Martínez-Berdeja and Valverde pp. 1767–1768] [Martínez-Berdeja and Valverde 2008, IUCN p. 107, Martínez-Berdeja and Valverde pp. 1767–1768]
Separating light and water
Alejandra Martínez-Berdeja and Teresa Valverde prepared an experiment in a greenhouse in Mexico City. Seeds had previously germinated on filter paper in a chamber with a 12-hour light period and alternating temperatures of 18 and 32°C. Young plants were transferred individually to small pots and acclimated to greenhouse conditions for five months. The actual experiment began with approximately six-month-old seedlings in September 2005 and ended after a further six months in February 2006. [Martínez-Berdeja and Valverde 2008, pp. 1768–1769]
Transparent coverings provided the higher light level, while coverings with mesh provided about 40% of this exposure. A measurement between 11 and 15 hours on a warm May day gave about 243 and 102 µmol photons m⁻² s⁻¹. These numbers describe a particular measurement of photosynthetically active photon flux. Sunlight in the greenhouse experiment was weaker than in an open semidesert habitat. [Martínez-Berdeja and Valverde 2008, p. 1768]
For O. denegrii, two substrate hydration levels, 25% and 60%, were used, each crossed with the two light levels. Each of the four treatments included at least seven seedlings. Comparative plants of two other species also received a 5% treatment, but the limited number of Obregonia seeds allowed only the two higher levels to be studied. Pot positions on the bench were randomised again every two weeks to reduce the influence of greenhouse location. [Martínez-Berdeja and Valverde 2008, pp. 1768–1769]
What the hydration percentage means
The reference point was the mass of water retained by the substrate after saturation and drainage of excess water. After drainage ended, a further two hours were allowed before weighing the pot and subtracting the mass of dry substrate and container. The resulting water mass corresponded to 100%. Masses for the 25% and 60% levels were then calculated, and losses were replenished to the assigned value every other day. This was therefore gravimetric control related to the properties of one mixture and one container. [Martínez-Berdeja and Valverde 2008, p. 1768]
The higher treatment was intended to mimic more favourable hydration during and shortly after rainfall. Beneath a shrub canopy, this period can last longer because of the altered microclimate. The experiment nevertheless maintained relatively constant values, while in the field water rises and falls in successive episodes. The interpretation concerns resource availability within the range studied. A pot-watering procedure also requires information about substrate, temperature, developmental stage and drying rate. [Martínez-Berdeja and Valverde 2008, pp. 1772–1773]
Measurement of dry tissue
Ten seedlings of each species were collected before treatments began as a baseline group. Roots and above-ground parts were dried for 48 hours at 80°C and then weighed separately. Plants were processed in the same way after the six-month experiment. Dry-mass measurement limits the influence of momentary tissue filling with water on growth assessment. Because sampling was destructive, beginning and end were compared in separate plant groups. [Martínez-Berdeja and Valverde 2008, pp. 1768–1769]
Relative growth rate was calculated from the difference between logarithms of final and initial dry mass divided by time. The root-to-shoot mass ratio shows the proportions of the two organs at collection. A separate index, K, divides relative root growth rate by relative shoot growth rate. A K value greater than one means faster relative root growth, expressed against its own initial mass. This proportion alone does not give the absolute number of grams built in each organ. [Martínez-Berdeja and Valverde 2008, p. 1769]
Changing proportions with water availability
At 60% hydration, O. denegrii grew faster than at 25%. At the same time, the root-to-shoot mass ratio and K index decreased. Each difference was significant in the species-specific analysis. The root remained an important part of the young plant in both treatments, but with greater water availability, the above-ground part developed more strongly relative to the underground part. The comparison combines both light levels. [Martínez-Berdeja and Valverde 2008, p. 1770]
| Substrate hydration relative to the reference value | Relative growth rate, g g⁻¹ day⁻¹ | Root-to-shoot dry-mass ratio | Ratio of relative root and shoot growth rates, K |
|---|---|---|---|
| 25% | 0,007 ± 0,002 | 1,106 ± 0,353 | 2,775 ± 0,895 |
| 60% | 0,009 ± 0,003 | 0,774 ± 0,388 | 1,51 ± 0,256 |
The authors interpreted this response as growth plasticity. Compared with the other cacti studied, Obregonia was distinguished by changes in all three indices. The relationship of this response to fleshy roots and a rosette habit was proposed as an explanation requiring further study. The experiment demonstrated a change in biomass allocation. It did not separately measure root water-storage capacity or hydraulic conductivity. [Martínez-Berdeja and Valverde 2008, pp. 1770–1772]
The scope of shade effects
No significant change in the growth parameters analysed was found between the light levels used. For seedlings, the benefit of greater water availability was therefore clearer than the response to the illumination contrast considered. The authors suggested that both light levels might already supply enough energy for young plants. Photosynthetic metabolism, saturation point and photodamage were not measured in this series, so this remains an interpretation of the growth result. [Martínez-Berdeja and Valverde 2008, p. 1772]
For cultivation of young Obregonia, the experiment indicates a connection between growth and water in the roots under moderate light. SNHF guidance recommends partial shade and a permeable substrate and links subsequent watering to drying of its surface during the growing season. During cool overwintering below 10°C, it recommends a watering pause. The result for active seedlings and winter management of a potted plant describe two developmental phases with different balances of growth and resource availability. [Martínez-Berdeja and Valverde 2008] [21-WWW]
Experimental data checked in the full text of Martínez-Berdeja and Valverde (2008), including Table 4 in the page image. The scope of reading of the IUCN book covers pp. 107–108.
Growth response of three globose cacti to radiation and soil moisture: An experimental test of the mechanism behind the nurse effect
Martínez-Berdeja A., Valverde T., Growth response of three globose cacti to radiation and soil moisture: An experimental test of the mechanism behind the nurse effect, Journal of Arid Environments, 2008, 72(10):1766-1774, doi: 10.1016/j.jaridenv.2008.04.010
A six-month experiment on seedlings of three cacti separates the effects of light and water availability on dry biomass growth and root and shoot proportions. Obregonia denegrii was compared at two substrate hydration levels crossed with two light levels. Greater water availability increased relative growth rate and reduced the root-to-shoot biomass ratio. The authors discuss the significance of these responses for nurse plant effects.
The IUCN Plant Red Data Book: Comprising Red Data Sheets on 250 Selected Plants Threatened on a World Scale
[Lucas G., Synge H., The IUCN Plant Red Data Book: Comprising Red Data Sheets on 250 Selected Plants Threatened on a World Scale, IUCN, Morges, 1978, s. 107–108, ISBN 2-88032-202-2]
The collection of descriptions of 250 plants contains a separate profile of Obregonia denegrii, with tubercle, areole and flower structure, limestone habitat characteristics and a historical description of collecting pressure. The chapter was prepared in collaboration with D. R. Hunt. The reading covers pages 107–108. Threat assessments and regulations in the book represent the situation in 1978.
Société Nationale d’Horticulture de France
Botanical descriptions and cultivation guidance prepared by the French horticultural society. The Obregonia profile was used.
Accessed 2026-10-09.
