The teardrop explodes, and spreads Orixa japonica

It can take a lot of effort for a plant to attract a mate and grow a seed. After all this, it would be a pity if the seeds fell to earth in the shade of the parent. Ideally, the seeds would get transport to a new location. Some fly on the wind. Some travel on or in animals in fruit. Orixa japonica, a shrub found in eastern Asia, has a different technique. Lan-Jie Huang and Wen-Long Fu found that this plant forms a slingshot to fling the seed away. What makes O. japonica unusual is the shape of its fruit.

For a plant to explosively scatter seeds reliably, it needs to have the right fruit. To build up energy, plants can grow fruit that dry. As they dry, they build up tension, creating a store of energy that can be explosively released when the last cells in a chain give way. But O. japonica doesn’t have a long fruit that can build tension along its length. It has something more like a teardrop. Yet, it can still catapult seeds. Huang and Fu set about finding how the fruit could work.

The botanists didn’t skimp on the observation. First, they scanned the fruits before dehiscence (splitting open) using micro-computed tomography. This is a technique that allows scientists to build a 3D X-ray image of a subject. Next, they fixed the seed using polyethylene glycol and sliced it into sections to examine it under a microscope. These slices were just 10μm thick, thinner than a human hair.

To watch the actual seed dispersal event, Huang and Fu used a camera capable of shooting 10000 frames per second. After comparing the fruits before and after dehiscence, they could then build a model to see how the fruit could fling its cargo.

As expected, the explosion relied on the pericarp, the part of the fruit around the seed. However, the exocarp, the outermost part of the fruit and the endocarp, the innermost part, did not act the same way. It’s this difference that is the key to O. japonica‘s success, say the authors.

“There are strong differences in the deformation of the exocarp and endocarp during fruit dehiscence, and they play different roles in the explosive seed dispersal of O. japonica,” write Huang and Fu. “The endocarp provides power for the seed launch through dehydration, while the exocarp resists the opening of the endocarp and provides other additional functions. The pericarp opens very slowly during the explosive seed dispersal of O. japonica, and the seed launch only takes place at the last stage of the opening. This process is different from that of explosive seed dispersal in other plants, in which pericarp opening is generally fast, and the seed launching occurs simultaneously with the pericarp opening… “

It’s this difference that provides the energy for the seed’s flight, say the authors.

“Before the seed of O. japonica leaves the fruit, the split of the exocarp driven by the endocarp is crucial for the seed launch. The top opening formed by the opening of the exocarp provides a passage for the separation of the seed; whereas, the ventral edge of the exocarp provides an acceleration slide for the detachment of the endocarp from the exocarp. When the arm-shaped endocarp rapidly slides out of the exocarp, the seed is accelerated to launch speed in a very short time by the endocarp. The kinetic energy of seed and endocarp flight is derived from the elastic potential energy released by the endocarp, which has been accumulated by the exocarp resistance to the deformation movement of the endocarp.”

Effectively, the exocarp and endocarp work against each other in the beginning. When the outer layer of the fruit gives way, the inner part shoots out. As the endocarp isn’t aerodynamic, there’s a final step to get the seeds out of the fruit as it launches.

“This process is accomplished by squeezing the ovary area of the opening endocarp to the seed, although this squeezing does not directly provide energy for the seed launching. The mechanisms of movement of the pericarp and acceleration of seed launch make the two layers of pericarp form a structure similar to a special-shaped slingshot, which can launch the seed from the fruit in the form of a bullet,” write Huang and Fu.

The method isn’t 100% successful. The botanists found that sometimes the seeds do not immediately escape the endocarp. This failure might be an example of the plant hedging its bets. The endocarp could be protecting the seeds from predators. So while not perfect for dispersal, combining the two strategies could help O. japonica cope in less seed-friendly habitats.