You’re forgiven if you think that plant life is just about sunlight, water and photosynthesis. That’s only half the story. The other half takes place unseen in the dark, teeming underworld that scientists call the rhizosphere. What happens in the rhizosphere is highly complex, but it’s also fascinating. Does it explain why wines taste so different? Let’s start to dig.
What and where is the rhizosphere?
The rhizosphere is the area that immediately surrounds a vine’s roots. It teems with microbial life, and it’s where countless interchanges take place, not all of them understood. Life in the rhizosphere is far more active than in the surrounding soil. What affect does this have on grape vines and the fruit that they grow?
Roots: the basics
Vine roots do far more than just provide an anchor in the soil. They are the nerve center, engine house and hard drive of the vine rolled into one. They act as a carbohydrate store and produce hormones that tell the plant to grow in spring and go dormant in winter. Roots also govern the uptake of water and nutrients. But vines also give back to the soil, via fine root hairs that excrete sugars, amino acids and proteins.
Soils: much more than just dirt
“Soil contains dazzling numbers of microbes which interact with plant roots,” says Nicole van Dam, professor and head of Molecular Interaction Ecology at iDiv/FSU Jena in Germany, and Radboud University in Nijmegen, Netherlands. “The soil microbial community is extremely diverse, and [it] contains beneficial microbes as well as pathogens. Beneficial microbes can help plants to acquire nutrients which are essential for plant growth and production.”
“Plants are not passive bystanders, even though they cannot move.” —Nicole van Dam
Micro-organisms thrive due to the vine’s excretions, known as exudates. These microbes colonize the area around the roots and engage in a series of complex and mutually beneficial interchanges. Scientists are only beginning to understand how sophisticated these exchanges are. Interestingly, the vines influence the soil as much as the soil influences the resulting wine.
Denizens of the underworld: mycorrhizae and bacteria
Mycorrhizae are central to this exchange. They are minute fungal organisms that form a finely branched network which acts like an extension of the vine’s root system, increasing water and nutrient uptake. Interestingly, it’s more efficient for the vine to support these fungal networks than to grow extra roots of its own.
Mycorrhizae also have the ability to break down phosphorous compounds and make them available to the vine. Mycorrhizal populations also make the vine more resilient. If a vine is well-colonized by mycorrhizae, it’s much harder for pathogens to latch onto the root.
Vine roots do far more than just provide an anchor in the soil. They are the nerve center, engine house and hard drive of the vine rolled into one.
The exudates also enable various bacteria. Scientists claim each gram of soil contains up to four billion bacteria. Various species help decompose organic matter in the soil that allows plants to take in nutrients like nitrogen, often via the filament networks created by the mycorrhizae. Some bacteria can also decompose potentially harmful pathogens and protect the vine.
Plants can and do communicate
Interchanges in the rhizosphere go well beyond nutrition.
“Plants are not passive bystanders, even though they cannot move,” says van Dam. “They produce a wide range of chemical compounds in their root exudates that may serve as defense or attractants.
“Microbes can also ‘prime’ the plant to become more resistant to pathogens and herbivores. The effect of their interaction is that the plant’s immune system is boosted to respond faster when a pathogen or a herbivore infests the plant.”
Toni Bodenstein serves as winemaker at Weingut Prager in Weissenkirchen, Austria, and he’s a graduate of Vienna’s University of Natural Resources and Life Sciences. He says, “Scientists are currently examining the signals that plants and mycorrhizae give in order to exchange particular elements. Particular signals trigger particular exchanges. This happens not only within one plant species, but within diverse plant species, which adds another layer of complexity.”
Protecting or recreating a healthy rhizosphere in the vineyard
A healthy rhizosphere produces well-nourished and resilient vines. It’s a finely tuned underground ecosystem. Herbicides, pesticides and mineral fertilizers can upset this delicate balance. To Bodenstein, biodiversity is key.
“Every herb, every grass, every legume is vital, and the viticulturist has to pay attention not to upset mycorrhizae,” says Bodenstein. “Farmers really have to rethink soil. The more plant species [that] are allowed to thrive, the better the mycorrhizal partnership, the better the product.
“It is particularly in years of stress, like drought or heat, that winemakers benefit from soils that are well-colonized by mycorrhizae. It quickly becomes clear which vineyards have significantly fewer stress symptoms, due to their enhanced ability, despite stress, to absorb water and nutrients.”
Effects on taste
Bodenstein says that different rhizospheres account for different flavors in wine.
“Take a particular rhizosphere in a particular soil,” he says. “Depending on soil, climate, water availability and the surrounding humidity, temperature and countless other factors like tilling methods, particular chemical, physical and biological conditions exist that are specific to this particular site. Two, five or fifty yards away, the conditions of this microcosm may be fundamentally different.
“Therefore, plant roots interact differently with their environment, and the fruit is different, too. Just thinking about physical, chemical and biological differences between sites makes you realize that the fruit must be different, because no two situations are the same.”
Cracking the root exudate code
Study of the rhizosphere has already resulted in agricultural advances. Freeze-dried mycorrhizae have been used for years when planting new vineyards. But research continues.
“Currently, we are using many toxic and harmful pesticides to combat pathogens and herbivores,” says van Dam. “[Then] we throw tons of nutrients on our fields to enhance production. What if we could use beneficial microbes to do the job for us and our crops? That would be much healthier for humans and our natural environment.”