From seed to giant. The changing physiology of Echinocactus platyacanthus

A beginning determined by water and temperature

An adult Echinocactus platyacanthus can reach several metres in height. Its stem contains extensive tissues, and the apex produces new organs each year. The beginning of such a plant's life depends on the much smaller space occupied by the seed and its immediate surroundings. In experiments, this stage is studied by observing the radicle. Its emergence marks the transition from seed to growing seedling, which must continue taking up water and developing its organs. [Herce et al. 2014][González-Salvatierra et al. 2026]

Water availability can be described by its potential, expressed in megapascals. An experiment published in 2026 compared potentials of 0, −0,2 and −0,4 MPa, obtained using polyethylene glycol solutions. Increasingly negative values meant that water uptake by the seeds became more difficult. Each level was combined with a temperature of either 25 or 32°C. This design allows the effects of temperature and water availability to be assessed separately and tests how the response changes when they are combined. [González-Salvatierra et al. 2026]

The window in which a seed can germinate

At 25°C, an average of about 95% of seeds germinated, compared with about 37% at 32°C. These values were calculated across the three levels of water potential. Mean germination time increased from about 6,5 to 13,7 days. In the most stressful combination, 32°C and −0,4 MPa, germination was completely inhibited over the 30-day observation period. Each combination was tested in five replicates of ten seeds. [González-Salvatierra et al. 2026]

The result establishes the sensitivity of the first developmental stage under the conditions studied. In the habitat, the sequence of rainfall, drying and daily temperature changes creates brief opportunities for growth to begin. The relationship revealed in the laboratory indicates that simultaneous heat and low water availability can greatly restrict such an opportunity. Temperature and water potential thus jointly shape the germination niche, the range of conditions allowing a seed to begin developing. Within this range, both the number of germinating seeds and the timing of radicle emergence vary. [González-Salvatierra et al. 2026]

The radicle emerges inside the fruit

In some plants, development begins before dispersal. In a ripe fruit still attached to the stem, some seeds produce a radicle. This is vivipary. In material collected at two localities in San Luis Potosí, it occurred in 4 of 44 plants and 9 of 262 fruits. The researchers described these plants as Echinocactus platyacanthus f. visnaga, now treated as a synonym of E. platyacanthus. [Aragón-Gastélum et al. 2017a]

Comparison of seed properties covered material that had not yet germinated at collection. Material from fruits containing germinating seeds was compared with that from other fruits on the same plants and from individuals without observed vivipary. The first two categories had greater viability and germinated better under controlled water availability. Traits associated with vivipary therefore extended to a broader set of seeds from the maternal plant. The seedling's subsequent fate depends on where it arrives and how long conditions favourable to growth persist. Assessing this part of the cycle requires observations after dispersal. [Aragón-Gastélum et al. 2017a]

Shade changes how a young plant functions

Pérez-Sánchez and colleagues compared young succulents, including E. platyacanthus, beneath tree canopies and in the open. Shelter reduced incoming light and soil surface temperature. Sheltered plants had a higher effective photosystem II yield but a lower estimated electron transport rate. In E. platyacanthus, relative mass growth rate remained similar in the two microenvironments. [Pérez-Sánchez et al. 2015]

The apparent discrepancy between these parameters arises from what they describe. Quantum yield is the proportion of absorbed energy directed into photochemistry. Electron transport rate also depends on the amount of incoming light. A shaded plant can use a larger proportion of a smaller photon flux while achieving a lower total electron flow. Shelter therefore changes the operating conditions of the photosynthetic apparatus, while the growth result is a property of the particular species and microenvironment. [Pérez-Sánchez et al. 2015]

Several years of growth, one year of observation

A further experiment involved 60 five-year-old plants approximately 4 cm tall and 6 cm in diameter. From June 2013 to May 2014, some were kept in warming chambers and others under control conditions. Mean daily temperatures were 19,3 and 17,7°C, respectively. All plants survived the year. During this period they received natural rainfall totalling about 383 mm. [Aragón-Gastélum et al. 2021]

Survival was accompanied by seasonal physiological changes. In summer, plants in the chambers had a lower effective photosystem II yield and lower estimated electron transport rate than the controls. These parameter values increased in autumn and winter. Fluorescence thus reveals changes taking place inside an apparently stable stem. It describes photochemical reactions at the time of measurement. The annual carbon balance also includes CO₂ uptake, respiration and organ growth. [Aragón-Gastélum et al. 2021]

The importance of developmental stage is highlighted by an earlier experiment with two-year-old plants of three species, also including E. platyacanthus (f. visnaga in the paper). In chambers with higher temperature and lower humidity, all plants studied died after 105 days, while some survived in the control. Comparing the studies indicates the importance of age, size and microclimate. The contribution of each factor requires comparison under identical conditions. [Aragón-Gastélum et al. 2017b]

An adult stem regulates its light balance through geometry

As the plant grows large, its surface becomes a mosaic of zones receiving radiation at different angles. In E. platyacanthus, the region of the flattened apex, where reproduction is concentrated, is particularly important. In the Metztitlán population, 337 stems examined showed a clear tendency to tilt southwards. The mean tilt angle was 17,4°. [Herce et al. 2014]

Metztitlán lies in the tropics. During part of the summer, the midday sun passes north of the zenith. A southward tilt of the apex then changes the angle of incidence and limits energy reaching its surface. The geometry of sunlight differs during the rest of the year. A model incorporating the sun's annual course showed a favourable combination of a greater total amount of radiation received during the year and a lower heat load during the hottest period. [Herce et al. 2014]

The authors also related this geometry to the number of retained reproductive structures. The association supports the hypothesis that stem orientation favours the functioning of its flowering zone. Lower temperatures during hot weather and access to photosynthetic products are two possible components of this effect. Stem observations, temperature measurements and the radiation model describe successive links in this mechanism. [Herce et al. 2014]

Population persistence arises from the whole life cycle

The seed, seedling and large stem use different ways of surviving successive life stages. Laboratory germination conditions define the beginning, the sheltered microenvironment affects the young plant, and seasonal physiology and stem architecture shape the functioning of older specimens. For the population, what matters is continuity along the entire path from seed formation to the next individual capable of fruiting. [González-Salvatierra et al. 2026][Aragón-Gastélum et al. 2017a][Pérez-Sánchez et al. 2015][Aragón-Gastélum et al. 2021][Herce et al. 2014]

In models of six E. platyacanthus populations, survival and growth of medium and large plants made a particularly strong contribution to population persistence. The result connects protection of adult individuals with conditions for recruitment. Each surviving plant can participate in many successive reproductive seasons, while each new generation again passes through the sensitive stages of germination and early growth. [Vázquez-Díaz et al. 2026]