Stenocactus coptonogonus seedlings. Light, canopy shelter and summer warming

Seedlings a few weeks old beneath a mesquite canopy

Pérez-Sánchez and coauthors transferred Stenocactus coptonogonus seedlings about 4–5 weeks old to a site near San Juanico Chico in San Luis Potosí, at 1870 m elevation. Plants grew individually in biodegradable 295 ml containers with local soil. Places beneath mesquite canopies, identified in the study as Prosopis laevigata, were compared with open ground. Observations lasted 105 days, from 26 September 2012 to 9 January 2013.[Pérez-Sánchez et al. 2015, pp. 3–5]

Each treatment comprised five replicates with 41 plants of the species in each. Some were sampled on successive dates to measure mass and water content, while others were followed using chlorophyll fluorescence. The experiment used the canopies of 12 trees about 2,5–3 m tall. This arrangement compared actual plant shelter with adjacent ground, so changes in light and temperature occurred together.[Pérez-Sánchez et al. 2015, pp. 4–5]

Between 12.00 and 14.00, mean surface temperature was 24,62 ± 0,56°C beneath the canopy and 37,42 ± 1,07°C in the open. Photon flux density was 306,34 ± 11,45 and 926,75 ± 21,21 µmol m⁻² s⁻¹ respectively. These are means with standard errors. Measurements of moisture in the shallow soil layer at about 1 cm depth showed no significant difference between places.[Pérez-Sánchez et al. 2015, p. 5]

Photosystem II and electron transport rate

Photosystem II participates in capturing light energy and initiates electron flow in the photosynthetic apparatus. In S. coptonogonus, its photochemical efficiency was greater beneath the mesquite canopy. At the same time, estimated electron transport rate was greater in the open, where more light arrived. Greater efficiency in using available photons and a greater total electron flux can therefore occur under different lighting conditions.[Pérez-Sánchez et al. 2015, pp. 5–7]

Electron transport was calculated from photosystem II efficiency and measured photon flux, assuming light absorption of 0,84 and an energy division of 0,5 between photosystems. This index was not a direct measurement of carbon dioxide uptake. It depends on the optical assumptions used, which the study applied equally to the plants examined.[Pérez-Sánchez et al. 2015, p. 5]

Mass gain and hydration in the same trial

For S. coptonogonus, canopy shelter did not significantly change relative mass growth rate, net assimilation rate or the root-to-stem mass ratio. Relative tissue water content was greater in the open. It was 62,68 ± 1,35% in sun and 55,55 ± 1,56% beneath the canopy, reported as means with standard errors. In this season, better photosystem II efficiency under shelter was not associated with greater tissue hydration or significantly faster mass gain.[Pérez-Sánchez et al. 2015, pp. 7]

Relative water content compares the water present in a tissue with the amount the same tissue can hold when fully hydrated. It thus links plant condition to water-holding capacity. Its response in this experiment applies to small seedlings in autumn and winter under the described soil, container and shelter arrangement.[Pérez-Sánchez et al. 2015, pp. 4–5, 7]

Two-year-old plants during summer warming

Aragón-Gastélum and coauthors used two-year-old seedlings of S. coptonogonus, Echinocactus platyacanthus and Ferocactus histrix. The plants had previously grown in a greenhouse. From 7 June to 27 September 2013, they remained at an exposed, cleared site in San Luis Potosí, at 1844 m elevation. Open chambers with transparent walls increased temperature and changed air humidity.[Aragón-Gastélum et al. 2017b, pp. 93–95]

20 plants of each species were placed in each of six chambers and six control plots, giving 120 per species in each treatment. Four plants grew together in a pot 5 cm in diameter and 10 cm deep. The substrate consisted of gravel, sand and clay fraction in a ratio of 10:40:50, with no mass or volume basis specified. Plants were not watered during the trial. Rainfall during this period was 126,4 mm.[Aragón-Gastélum et al. 2017b, pp. 94–95]

Survival under a particular combination of conditions

In the warming chambers, all studied S. coptonogonus had died by day 105. On control plots, more than 22% of plants of this species remained alive at the end of the experiment. The other species died out in the chambers earlier, E. platyacanthus by day 65 and F. histrix by day 55. In the summer trial, the microclimate change greatly reduced young-plant survival.[Aragón-Gastélum et al. 2017b, pp. 95–96, Figure 1]

Mean temperature rose by about 1,9°C and mean daily maximum by about 5,8°C. Air humidity declined at the same time. Plants grew four to a small pot at a site without shelter from other plants. The result covers this entire arrangement and the weather at the time. It does not define a single temperature threshold causing death in S. coptonogonus.[Aragón-Gastélum et al. 2017b, pp. 94–98]

The two studies describe different juvenile stages and different seasonal windows. In the first, mesquite shelter improved photosystem II efficiency without significantly changing mass gain. In the second, hotter and drier summer air was associated with complete loss of plants in the chambers. The relationship between exposure, physiological processes and survival therefore requires retaining plant age and timescale.[Pérez-Sánchez et al. 2015, Pérez-Sánchez et al., pp. 4–7; Aragón-Gastélum et al., pp. 94–98][Aragón-Gastélum et al. 2017b, Pérez-Sánchez et al., pp. 4–7; Aragón-Gastélum et al., pp. 94–98]