Hermetospheres

Why does a plant begin to produce oversized leaves as soon as it is placed in a hermetosphere? Let us start the story at the beginning: I recently started a hermetosphere to represent New Guinea. Begonia bipinnatifida with its wonderful, reddish, bipinnate leaves should be the main actor and Strobilanthes reptans as well as Davallia parvula the supporting actors. But everything turned out differently. It soon became apparent that the Begonia was getting too big for the limited space in the 5L container, and I had to remove the plant, save a cutting and keep it for a future large-scale setting. So I focused on the two remaining plants, one of which behaved in an unexpected way.

Strobilanthes reptans (formerly known as Hemigraphis reptans) belongs to the Acanthaceae family and lives as a terrestrial or lithophe. Its native range are the islands between Australia and Southeast Asia. It possibly escaped from cultivation and naturalized in India, Myanmar, Thailand, Laos, Vietnam, Malaysia, Sri Lanka, La Réunion, Hawaii, French Polynesia, and other parts of world (Sarma e.a. 2021). In some places it grows on roadsides and is considered a weed (Wood 2011, p. 390) or even an invasive species (Sarma e.a. 2021).

After Ficus punctata, Strobilanthes reptans is the second plant that I observe to develop significantly differently under hermetospheric conditions than outside. The most striking feature of the change is the size of the leaves. From one node to the next, the length of the leaf blade increased from about 2.5 cm to over 7.5 cm; In addition, a noticeable lack of chlorophyll can be observed between the leaf veins (see figure below).

According to the descriptions of Wood (2011), Nilanthi e.a. (2021) and Sarma e.a. (2021) as well as in comparison with the numerous illustrations of preserved specimens (see the species’ record in the Global Biodiversity Information Facility GIBF), leaf lengths of well over 6 cm are very rare. This and the dramatic increase in leaf size from one node to the next lead me to believe that the observed change is the result of the plant’s acclimation to hermetospheric conditions. The most obvious difference between the growing conditions inside and outside a hermetosphere is the relative humidity. This is usually between 40 and 60% in inhabited rooms, and probably around 80% in greenhouses; in the hermetosphere, on the other hand, the relative humidity is close to 100% because there is little to no exchange of the water-saturated air. So, can the observed changes in the appearance of Strobilanthes reptans be explained by high relative humidity?

According to Amitrano e.a. (2022) “[…] leaf size has a central role in plant acclimation to environmental conditions. Variation in leaf size has been found along climatic gradients, often with increments in lamina [leaf blade] expansion in humid habitats.” In fact, there is ample evidence from experiments in controlled environments with various dicotyledonous plants that acclimation to high relative humidity can result in larger leaves (see table at the bottom of this post). Nevertheless, to what degree acclimation is possible and whether or not a change in leave size is part of an acclimation reaction remains highly species-specific (Stotz e.a. 2021). The question of how and to what extent plants can respond to changes in their environment through acclimation (short-term, within the life span of an individual) or adaptation (long-term, within a population) has gained much interest in recent years. The answers will help predict to what extent ecosystems are able to cope with expected man-made environmental changes.

And what about the change in leaf coloration of my Strobilanthes reptans? The interveinal chlorosis (lack of chlorophyll between the leaf veins) is most likely due to iron or/and manganese deficiency (see McCauley e.a. 2009). Gislerod e.a.(1987) observed that increasing the relative humidity in a greenhouse from 60% to over 90% reduced the transpiration rate of greenhouse plants by about half and that the concentration of some nutrients in the leaves was lower. It seems likely to me, therefore, that with the high humidity in the container, the plant was no longer able to maintain a transpiration rate sufficient to supply the large new leaves with sufficient nutrients.

If you have made similar observations, let me know.

Addendum, 18 August 2023:

Three weeks after the above pictures were taken, one of the plants in the 5-liter container formed two flowers. The picture below gives another impression of the size of the leaves compared to the size of the flower. Here again, the chlorosis between the leaf nerves is easily visible.