Tropics || Leaves-Dropping
Stepping into the world of plants is to enter a lively, ongoing conversation. These conversations feel invisible to most of us, but only because we’ve forgotten how to listen. With a little help, we can remember.
Plants are communicating all the time around us. They have 15-20 distinct senses (unlike our measly 5). And they use four main signaling “languages” - water, carbon, volatile chemicals and electrical potentials - to communicate about what they perceive, both within the individual plant and across whole ecosystem communities.
Water
Water connects all parts of a plant. It carries nutrients and maintains the plant’s metabolism, but it also provides a signaling highway along the soil-plant-air continuum. For example, when a plant’s stem is bent or pressed in one place, hydraulic pressure waves pulse warnings to the rest of the plant and, within minutes, the plant responds with hormone and gene signaling cascades. Liquid information also drives a plant’s decision-making around “eating” (photosynthesis) and “breathing” (transpiration). Drought-induced ABA hormone production causes the plant’s stomata (like mouths for breathing) to close, while flood-induced ethylene hormone buildup causes the plant to either “escape” (by growing rapidly upward) or “hold its breath” (by switching to anaerobic metabolism) before “drowning” (hypoxia) occurs.
Carbon
Plants - like all living things - are made of carbon and use carbon-based compounds as energy. But carbon also serves as the “currency” of plant communities in an ecosystem-wide “marketplace” facilitated by fungal mycelium. Unlike our markets, however, plant communities seem to function on a system of gift exchange. Plants send carbon to their neighbors in times of need, with the expectation of reciprocity later on. This phenomenon is usually measured in bi-directional transfer of radioactive carbon isotopes during sequential shading of two mycelially-connected plants. Although plants will send carbon to “stranger” neighbors, they give preferential treatment to their kin – often understood as a “mother” tree “feeding” its offspring. The carbon exchange also determines exchanges of other nutrients and signaling hormones, as well as informing plants about the nature of their environment and communities.
VOCs
Have you ever stopped to smell the roses? That’s the rose beckoning you over. Floral scents are just one of the many volatile organic compounds (VOCs) plants use to communicate with one another – as well as with other species. These diverse chemical signaling compounds become gaseous when released into the air and can travel long distances, carrying a wide variety of messages. For example, when a plant is attacked or otherwise stressed, it releases specific VOCs that other plants take in as early warning signals, enabling them to activate their defense hormones in preparation. In some cases, plants under siege also call on “bodyguard” insects. “Several species, including corn and lima beans, emit a chemical distress call when attacked by caterpillars. Parasitic wasps some distance away lock in on that scent, follow it to the afflicted plant, and proceed to slowly destroy the caterpillars.” Finally, plants use VOCs to exert their powers of attraction on insects and humans alike, even adapting their scents to cater to the exact tastes of their preferred reproductive assistants.
Electricity
Electrical potentials (EPs) enable a plant’s most speedy and complex decision-making. Electrical voltage moves through the plant in both gradual and sudden modes, with the most sudden responses called action potentials (APs). APs involve the sudden flow of positive calcium ions into the plant cell - known as “depolarization.” Reaching a threshold of this positive charge triggers an AP in the next plant cell and the next in a chain reaction, stimulating hormonal changes along the way. For example, when a caterpillar starts eating a plant leaf, the plant fires an AP to activate its defense chemicals. Within minutes, the catepillar is being poisoned and the feast is over. Incredibly enough, the same pattern occurs in response to the pre-recorded sound of a munching caterpillar in absence of any actual wounding. APs are ubiquitously useful to plants - from Venus Flytraps that “count” the number of APs triggered in their fly-sensing hairs before closing to ensure the highest probability of a real catch, to many other plants that produce APs at the onset of essential substances like blue light, water, and other nutrients.