Species name

Introduction

Portulaca oleracea, commonly known as purslane or little hogweed, **is an annual flowering plant with succulent leaves and stems which is adapted to warm climates. Typically P. oleracea has a prostate growth habit with a rosette-like branching pattern. The word portulaca is derived from the combination of two latin words, “portare” and “lac”, meaning “carry” and “milk”, which refers to the milky sap extruded from damaged plant tissue.

P. oleracea growth habit, stem color, and flower color.

P. oleracea is thought to originate from Northern Africa and the Middle East, where it has been cultivated for more than 4,000 years (Chan et al. 2000). Human aided dispersal brought it into Europe, North America, and Asia, where it has naturalized in most temperate and sub-tropical land masses across the globe (Chan et al. 2000).
P. oleracea is highly nutritious, with elevated levels of vitamin A, C and calcium, iron, magnesium and potassium and antioxidants. P. oleracea’s ability to relieve burns, remove infections, and heal wounds have made it a prized plant for traditional and modern healing (Rashed et al., 2003).
Portulaca is readily cultivated through seed or vegetative propagation and it grows quickly and easily. It’s succulent leaves and deep taproot provide drought tolerance. P. oleracea is used to develop ornamental cultivars, which are widely used in gardens across the world.

Ornamental purslane bred for garden use.

Photosynthetic Systems

Portulaca o. is unique because it is capable of altering its photosynthetic pathway from a C4 to a crassulacean acid metabolism (CAM) system. Portulaca normally uses C4, but switches to CAM during periods of intense heat and drought. The plant displays plasticity, where it can switch between the two processes depending on the moisture conditions (Ferrari et al, 2020).
C4 evolved to improve the efficiency of photosynthesis in warm climates. Rubisco, which binds CO2, has a tendency to instead bind to O2 in high temperatures. An added step in C4 creates a higher concentration of CO2 around rubisco to reduce accidental O2 binding and to increase the efficiency of carbon uptake.
The CAM photosynthetic pathway evolved in dry climates conserve water. It is derived from C4 and uses additional mechanisms to capture carbon dioxide at night, then convert and store it as malic acid. During the day the malic acid is used to supply carbon and fuel photosynthesis. This adaptation allows for stomata to stay closed during the day to conserve water (LÜTTGE, 2003). It also gives some CAM plants it a tart flavor, especially in the early morning, this is a true of Portulaca.
In warm and dry environments C4 and CAM photosynthesis have independently coevolved many times, both highlight how plants evolve to better adapt to extreme environments.

Portulaca’s ability to tolerate extreme moisture environments, it’s ease of cultivation, prostrate growth habit, and it’s nutritional content, are all favorable traits for space cultivation.
Portulaca oleracea was first flown to space July 2018 on a Space-X Dragon, during the CRS-15 mission to the ISS. In the ExoLab-4 experiment, Portulaca o. was grown from seed along with Amaranthus, Brassica rapa, and Berberis vulgaris within an Exolab growth chamber.
The ExoLab module is just 10 x 10 x 20 cm (L x W x H) and has sensors in the plant growth volume to measure the air temperature, humidity, carbon dioxide concentration, and light intensity. Exolab was developed and managed by Magnitude.io.

Exolab growth chamber diagram, courtesy of Magnitude.io.

Portulaca flew to the ISS again in December 2018, aboard a Cargo Dragon for the CRS-16. Portulaca o. shared an Exolab module with Amaranthus, Brassica rapa, and Eutrema japonicum (Wasabi).