The mystery of soils and wines, part 4

So, to continue our look at soils and vines, and the making of vines. In part 4, we are going to be focusing on vine roots.

Vine roots respond to the conditions of the soils they are growing in. First of all, a large permanent framework of roots is established, followed by a network of finer lateral roots, and finally even finer tertiary roots which are vital for uptake of water and nutrients. Nutrient uptake by the roots can be both passive and active. As the vine takes up water, it will usually take up whatever is dissolved in that water. But if it lacks specific nutrients, it can take them up actively, if they are present in the soil. There are some situations where the vine is fooled, though, by mineral ions that look quite similar, such that a deficiency of one can occur when there’s an abundance of another. And, for example, in soils with a lot of limestone, chlorosis can be a problem.

A special layer of material called the Casparian strip surrounds the root endodermis, the layer of cells that circle the vascular tissue. This strip contains suberin, and is impermeable to water. Thus water and solutes entering the roots have to pass through plasmodesmata (pores in the cell walls) and therefore through the cytoplasm of root cells, before they can be transmitted to the rest of the plant. This gives the vine a level of control to what is taken up. The plasmodesmata are significant because they allow direct communication between the cytoplasm of adjacent plant cells, through the otherwise rigid cellulose cell walls.

How do vine roots take nutrients from the soil? One of the key concepts here is cation exchange. Roots are able to exchange hydrogen ions, which they pump out, for the cations attached to the negatively charged soil particles such as clay and humus. Clay is always negatively charged, whereas humus – decayed organic material – carries both negative and positive charges, and so can hold both cations and anions. Cation exchange capacity (CEC) refers to the number of positive ions (such as calcium, magnesium, iron and the nitrogen-containing ammonium ion) that the soils can hold. Both clay and humus have a negative electrical charge, and this allows them to hold onto positively charged ions. Generally speaking, CEC correlates positively with soil fertility, because it determines how many plant nutrients the soils can hang on to. Soil pH also affects CEC: more acid soils (lower pH) have a lower CEC than more alkaline soils (high pH). One way to increase CEC is to increase the organic content of soils. This has the benefit of both increasing CEC, and thus fertility, and also increasing soil texture. Without organic material or clay, soils find it hard to retain nutrients. For example an excessively sandy or gravelly soil will allow mineral ions to be rapidly leached from the soil by rainfall.

The mineral uptake by the roots will affect the growth of the vine in significant ways.

But there’s also another important way in which vines affect the growth of vines. Significantly, root growth causes hormonal signals to be sent to the above-ground portion of the plant, and these act like instructions to tell the vine to modify its growth. Perhaps the best summary of what is happening here comes from the Western Australian plant biologist John Gladstones, in his book Wine, terroir and climate change (Wakefield Press, 2011), in which he brings together the existing literature and adds some theories of his own.

Gladstones points out that gibberellins (one of the major plant hormones) promote shoot internode growth through cell extension (the nodes are the bits of stem where the buds appear), and these gibberellins are formed in the region of cell division behind root tips. They also promote the formation of tendrils rather than fruit clusters in the newly forming lateral buds. Presumably, if conditions are good for root growth, then the vine is likely to favour vegetative growth, and gibberellins are sending up these instructions from the roots.

Cytokinins (another major plant hormone), produced in the root tips, promote new node and leaf formation, the branching of shoots, the development of existing fruit clusters and the fruitfulness of newly forming lateral buds. Gladstones observes that warm spring soils promote cytokinin dominance; cool soils gibberellins. Warm spring soils are therefore a good thing for grape production.

Abscisic acid (ABA) is probably the most interesting of the plant hormones for wine quality, though. Roots signal to the vine that water stress is coming using ABA, so that the leaves can respond appropriately, closing their gas exchange pores. ABA also acts in tension with another group of plant hormones, auxins, in controlling ripening. There’s a sort of tug of war here. The auxins, produced by developing seed, slow down berry development, prolonging the pre-veraison phase. ABA is pulling for berries to develop faster. In conditions of water stress, ABA is signalling to the berries to develop faster. A massive transfer of ABA to fruit coincides with veraison, possibly because of declining berry auxins at this stage. Therefore root moisture stress is contributing to berry ripeness. Gladstones concludes that ABA is the primary hormone imported into the grape clusters that both triggers and continues to stimulate ripening.

A recent study by Dr Hendrik Poorter and colleagues in Germany used magnetic resonance imaging (MRI) to look at root growth in potted plants. They were interested in finding out how big the pots have to be for experimental work, looking at a wide range of different pot-grown plants. The results emphasized how important root signalling is for the growth of the above-ground portion of the plant. The pot size restricted growth for a wide range of species, and doubling the size of the pot increased growth by 43%. The MRI results indicated that the plants were using their roots to ‘sense’ the size of the pot, and then signalling this to the rest of the plant.

Gladstones’ assessment of the literature on vine physiology agrees with this idea. Roots are signalling by means of hormones to the above-ground portion of the plant. The root structure is determined by soil conditions: deep soils with ample water supply prolong the phase of root development, and signal the above-ground plant to keep growing; shallower soils with limited water or a texture that obstructs root growth reduces vegatative vigour. Low vigour is best for wine quality.

More to come in part 5!

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