Water from a cactus reaches its nurse shrub

An association beginning with a seedling

In the Tehuacán Valley, upright stems of Cephalocereus tetetzo rise among shrubs forming separated vegetation patches. Young cacti often begin life beneath these canopies. Mimosa luisana is one such nurse shrub. Its shelter reduces direct solar radiation where a seedling develops. Germination and survival studies associated this effect with better persistence of young cacti in shaded microhabitats. [Valiente-Banuet and Ezcurra 1991] [Montesinos-Navarro et al. 2019]

As it grows, the cactus rises above the sheltering shrub and stores water in its extensive stem parenchyma. The two plants may remain neighbours through successive developmental stages. This raises a question about the direction in which resources move within the association. Alicia Montesinos-Navarro and colleagues investigated whether water present in an adult cactus’s tissues could reach the shrub beneath which the young plant had established. [Montesinos-Navarro et al. 2019]

Deuterium identifies the water’s origin

The experiment took place in March 2014 in Zapotitlán, Puebla, near the end of the dry season. Nine pairs comprising an adult cactus and a M. luisana shrub were selected. The members of each pair grew close together within the same vegetation patch. The cactus is now treated as C. tetetzo, whereas the paper’s authors used the name Neobuxbaumia tetetzo. Both names refer here to the same species. [Montesinos-Navarro et al. 2019] [2-WWW]

Water strongly enriched in deuterium was injected into the water-storage parenchyma. This stable hydrogen isotope occurs naturally in small amounts. Increasing its proportion creates a tracer that can be detected after dilution and movement. Introducing it directly into the stem allowed the researchers to test whether a signal originating in the cactus’s water reserve would appear in a second organism. The labelling itself was an experimental intervention. [Montesinos-Navarro et al. 2019]

Shrub xylem samples were taken before labelling and three weeks later. Xylem contains the water-conducting elements of wood. Water was extracted from the sampled material and its isotopic composition was measured. Comparing the same shrub before and after treatment accounted for its natural deuterium background. The analysis also included control shrubs in neighbouring vegetation patches away from the labelled plants. [Montesinos-Navarro et al. 2019]

Sampling and storage procedures limited changes in water composition after the plant material had been cut. Xylem was collected in the morning, when atmospheric evaporative demand was lower. Samples were sealed in airtight containers, cooled during transport and stored frozen. Water was recovered from them in the laboratory by vacuum distillation. This extracted fraction was then analysed for hydrogen and oxygen isotopes. [Montesinos-Navarro et al. 2019]

Controls distinguish the tracer from evaporation effects

During the dry season, evaporation itself changes isotope proportions in the water left behind. Oxygen isotopes were therefore measured as well. Shrubs beside labelled cacti and those in neighbouring patches could experience similar environmental effects over the same period. The similarity of their oxygen signals provided an additional control for this process. Higher deuterium enrichment in one group could then be related to the introduced tracer. [Montesinos-Navarro et al. 2019]

After three weeks, xylem water in shrubs beside labelled cacti was enriched in deuterium relative to both sets of controls. Differences in the oxygen signal between shrubs in the same and neighbouring patches did not explain this result. Agreement between the comparisons supported the inference that some water had moved from cactus storage tissues to the nurse shrub. The degree of enrichment varied among individual pairs. [Montesinos-Navarro et al. 2019]

Several pathways could account for transfer

The result identifies the starting and receiving points of tracer movement. Its route through the root zone needs separate investigation. The authors considered water release from roots into the surrounding substrate followed by uptake by a neighbour. They also identified a possible contribution from mycorrhizal fungi. In this paper, both mechanisms remained explanations requiring experiments that separate the individual flow pathways. [Montesinos-Navarro et al. 2019]

The observation period also defines the scope of the inference. A sample taken after three weeks records tracer presence at the time of collection. Repeated measurements could establish when the signal first appears in the shrub, how long it persists and how rainfall changes it. Distinguishing these stages would relate transfer to seasonal water availability. The authors identified the need for further experiments on the course and magnitude of flow. [Montesinos-Navarro et al. 2019]

Detecting the tracer demonstrates that flow can occur, while assessing its ecological significance requires a water budget. The volume transferred naturally, the frequency of such events and the conditions in which they occur must be established. Measurements of growth, survival and reproduction in both plants are also needed. The Zapotitlán study provided a starting point for this work. An adult cactus’s water reserve can participate in an association with the shrub that previously sheltered its development. [Montesinos-Navarro et al. 2019]