Cost-benefit analysis of carbon gain and water use in green-stemmed desert species from southern California (Eleinis Ávila-Lovera)
In the field of plant physiological ecology, people are usually interested in how leaf carbon and water dynamics determine whole plant performance, including growth and reproduction, with stems being the organ responsible for water transport and mechanical support. In many plants from arid and semi-arid ecosystems in both temperate and tropical regions, green photosynthetic stems have evolved in many species, possibly as a response to water shortage at least during one period of the year, yet the possible advantages that green stems confer to such species remain unclear. Possible advantages include extra carbon gain, which is widely supported by scientific evidence, and high water-use efficiency, which has been supported by scientific evidence for some species but not for others. This study focused on evaluating the high water-use efficiency hypothesis. We measured water status, gas exchange traits and stable isotopes in leaves and stems of species with and without visible green stems, in a desert wash scrub in Joshua Tree National Park, California, USA. The work was performed in nine sampling campaigns from February 2016-March 2017. We found that all the woody species with green stems had small leaves in February-April 2016 and did not have any leaves from April 2016-January 2017. Green stems had higher water potential, photosynthetic rate and stomatal conductance than leaves of non-green stemmed species, leading to similar water-use efficiency in both organs. Leaf carbon isotope composition (δ13C) was higher than bark δ13C in 4/8 green stemmed species. Cuticular conductance (gmin) was higher in green stems than in leaves of non-green stemmed species, while leaves and stems of these plants had the same gmin. In conclusion, plants with green stems relied on their stem as the sole organ for carbon assimilation for most of the study period. I found partial support for the high-water use efficiency hypothesis, and that green stems might be losing more water through their cuticle than leaves or non-green stems. This raises questions on the possible trade-offs between carbon gain and water lost through the cuticle in woody green stems, and how it may affect species responses to drought.
Eleinis Ávila-Lovera won an ISHS student award for the best poster at the X International Workshop on Sap Flow in USA in May 2017.
Eleinis Ávila-Lovera, University of California Riverside, 950 W Linden St, Riverside CA 92507, USA, e-mail: firstname.lastname@example.org