New publication: How Joshua trees switch up their photosynthesis game

Joshua trees can beat the heat of the desert with the help of a special form of photosynthesis, according to data presented in the latest peer-reviewed paper from the Joshua Tree Genome Project collaboration. The study is the first “fruit” of a multi-year experiment in growing Joshua tree seedlings in experimental gardens distributed across the Mojave Desert, led by plant physiologist Karolina Heyduk at the University of Connecticut and USGS ecologist Lesley DeFalco.

Photosynthesis is an everyday miracle: Green plants use the energy in sunlight to convert carbon dioxide into sugars they can use to power their metabolism and build new tissue, assembling themselves from not much more than light and air. But it comes with a conundrum for plants that grow in hot, dry conditions. Plants must open stomata — tiny pores on their leaves — to take in carbon dioxide from the atmosphere. But the same open stomata that let carbon dioxide in can allow water vapor to escape.

Many plants solve this conundrum by using an alternative form of photosynthesis called CAM. CAM lets a plant perform the part of the photosynthesis that takes in carbon dioxide (the “dark reactions”) at night, then complete the process (doing the “light reactions”) in the daytime, with their stomata closed. CAM-style photosynthesis is detectable in a couple of ways. First, by measuring a plant’s carbon dioxide uptake at night — non-CAM plants won’t take up carbon dioxide when there’s not enough light for photosynthesis. Second, by tracking the acidity of leaf tissues, since CAM plants store up carbon dioxide as organic acids. And third, by testing for the expression of CAM-associated genes.

Our new paper, just out in the journal New Phytologist, presents all three kinds of evidence to show that Joshua trees use CAM. The JTGP collaborators worked over multiple years to plant out Joshua tree seedlings grown from seeds collected in diverse habitats in a series of experimental gardens, part of a larger network of sites that Lesley DeFalco and her team developed to test the adaptation of Mojave Desert plants. Karolina Heyduk, as the plant physiologist in the collaboration, took charge of measuring the seedlings’ physiological and gene expression responses to conditions in the different gardens. In the very first round of measurements she decided to do some pilot sampling late at night, just to see what she’d find — and she found the first hint of CAM activity.

Confirming that first hint took more systematic measurements in Joshua tree seedlings, over multiple late nights in another year’s field season, as well as RNA sequencing to assess gene expression activity, and tests for acid accumulation in the leaves of adult trees back in the populations where we’d sourced seeds to plant in the gardens.

One surprise in the data was that seedlings from source populations in hotter parts of the Mojave were not more prone to use CAM than seedlings from other locations. It looks like most Joshua trees are able to do at least a little CAM as part of their baseline physiology, and perhaps they can turn up the CAM as a “plastic” response to heat and drought stress. It’s possible that different Joshua tree populations have different capacities for that plastic response, or different thresholds at which they switch to CAM — but it’ll take much more granular data to demonstrate this, and that’s still in the pipeline.

For more detail, check out the full paper, which is Open Access on the New Phytologist website.