Hundreds of litres of water in one cactus

From a tissue sample to a whole plant

Pavón and colleagues selected 20 plants in Mexico’s Barranca de Metztitlán reserve. They measured height, basal diameter, branch number, and stem lengths and diameters. Mean height was 3.3 ± 0.147 m, and branch number ranged from 3 to 34. These were therefore substantial, branched cacti growing in the wild [Pavón et al. 2016].

Small parenchyma samples were taken from the main stems in April and August 2011. The difference between fresh and dry mass provided the water quantity in each sample. Plant volume was estimated by summing approximate cylinders representing its stems. Tissue water content was then extrapolated to that volume. This step links tissue analysis with whole-plant geometry [Pavón et al. 2016].

Storage depends on season and size

Mean estimated storage was 537.64 ± 71.59 L during the dry season and 692.24 ± 92.18 L during the rainy season. Values after ± are standard errors of the means. The average seasonal difference was approximately 155 L per plant. The authors also reported a maximum estimate of 3509.7 L for a 7 m plant. Storage size is therefore closely related to the development of the branched crown [Pavón et al. 2016].

Two seasonal measurements describe stored water at particular dates. Their difference reflects the combined effects of uptake, water loss and physiological changes. Assigning a specific share of that difference to flowering, transpiration or growth would require a separate flux balance. Tissue measurements reveal the scale of the resource, while its use over time is a further question [Pavón et al. 2016].

Physiological access to reserves also depends on which tissues release water. Nobel examined this process in rooted, unbranched segments of Selenicereus undatus, described as Hylocereus undatus. During drought, water-storage parenchyma shrank more strongly than adjacent chlorenchyma. Cell dimensions were compared with water potential, osmotic pressure and nocturnal CO₂ uptake. This combination explained how changes in storage coexisted with continued function of photosynthetic tissue [Nobel 2006].

Storage cells also expanded more strongly when tissue sections were perfused with distilled water, supporting a role for more compliant walls. In I. dumortieri, the seasonal stock of the whole plant was estimated, while the dragon-fruit experiment separated responses of two tissues. The studies describe successive levels of water management, the capacity of a branched stem and the use of its stored water [Nobel 2006] [Pavón et al. 2016].

Carbon stock and the scope of measurement

The same plants were used to estimate carbon content. Carbon represented 46.1 ± 2.34% of the analysed dry biomass, with an estimated stock of 16.75 ± 7.07 kg per plant, again reported with standard errors. The authors sampled soft tissues and excluded wood to limit plant damage. The result applies to that material and estimation model [Pavón et al. 2016].

A carbon stock describes the quantity present at the time of measurement. Long-term carbon retention also depends on growth, respiration, death and subsequent tissue decomposition. Converting kilograms held in a plant into annual sequestration therefore requires measurements over time. The study primarily demonstrates the importance of large cacti as a component of shrubland biomass [Pavón et al. 2016].

Flowering during the dry season

Phenological studies in the region document I. dumortieri flowering and fruiting during the dry season. Previously accumulated resources connect the rainy season with the later supply of nectar, pollen and fruits to animals. Across three years, stem growth and reproductive structure counts varied in different ways. This seasonality connects the physiology of individual cacti with the rhythms of the wider plant and animal community [Cruz and Pavón 2013] [Pavón et al. 2016].