Contact: Matt Thompson

Our work in Chile focuses on the physiological and morphological features of desert plants that make their existence possible in extreme desert conditions. Our major focus is on the perennial shrub, Nolana mollis, found commonly in Parque Nacional Pan de Azúcar in Northern Chile (see right, below). We collaborate with a number of institutions on this project, both in Chile and in the United States.

Nolana mollis

The survival of Nolana mollis (Solanaceae) in the Atacama presents an interesting problem in plant biology - how can a perennial plant not only survive, but persistently flower, in an environment that receives no rain for up to ten years at a time? The Atacama is considered to be the driest desert in the world and it is common in many inland parts of the desert to find absolutely no plants at all.

Two aspects of N. mollis intrigue us: (1) its available physiological and developmental toolbox that allows it to survive in this environment, and (2) how it uses salt excretion as a means of limiting transpiration.

Phylogenetic considerations

The plant and its relatives in the genera Nolana and Alona are found on the Galapagos Islands and in Peru, as well as Chile, and is of floristic interest not only for its endemism to these areas (i.e., it originated in these areas and appears to be found nowhere else) but for its interesting fruit anatomy and historically contentious position within the plant family Solanaceae (the Solanaceae include such familiar plants as tobacco, tomato and potato). (See the Andean Botanical Information System for more information on this subject.) Nolana and its relatives have been moved in and out of the Solanaceae for some time in the systematics literature, but work by Olmstead and Palmer (1992; Ann. Mo. Bot. Gard. 79: 346-360) established the genus's firm position within the Solanaceae.

Physiological considerations

Nolana mollis has been long under study. In 1980, the first paper on its special physiological characteristics were published in the journal Science by Mooney et al. in 1980. N. mollis possesses specialized salt glands on the lower surfaces of their leaves that excrete a saturated salty liquid on the leaf surfaces. Although the relative humidity of the atmosphere near where these plants are found only very rarely exceeds 80 or 90%, the salt on the leaf surfaces acts to trap what water can be found in the atmosphere onto the leaf surfaces (called a hygroscopic effect), so that by morning the leaf surfaces are found to be covered in a copious amount of salty dew. The hypothesis presented by Mooney et al. in 1980 was that this salty dew might be used directly by the plant, either as dripped water to the soil surface where it could be taken up by surface roots, or directly into the leaf surfaces where it could be used by the plant during the day.

Lest it seem just a physiological curiosity, it should be noted that the importance of such a mechanism to N. mollis is plain upon visiting Pan de Azúcar. The plant is by far the healthiest of the dominant plants. After as much as 8 years of complete drought, N. mollis is the sole perennial non-succulent plant with green branches (although granted that many of its branches have senesced by this time). All other dominants (Gypothamnium pinifolium, etc.) are nearly completely senesced with little to show but a few ashy twigs and burnt out leaves.

Since then, numerous visits to Pan de Azúcar have shown the explanation of N. mollis's success to be more complicated than initially suggested by Mooney et al. It now appears that despite prolonged periods of complete drought (sometimes up to 10 years without rain), N. mollis is able to take water from the soil. But the plant appears to possess no deep tap root. In fact, water may be drawn from soil that contains no water!! Soil water balance measurements suggest that by the end of a long drought period, N. mollis is drawing more water out of the soil than could possibly have been placed there by rain.

In addition, visits to the park since 1990 have shown that neither mechanism proposed by Mooney et al. are particularly efficacious. Current work emphasizes the possibility that the salty dew prevents excessive transpiration during the day by both increasing the relative humidity of the boundary layer and by decreasing leaf temperature.

This material is based upon work supported by the National Science Foundation under Grant No. 9901329.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.