Elemental composition
The effect of nutrient conditions was first checked on leaf and root elemental composition analyzed by ICP-OES (Fig. 1). As expected, there was a clear and substantial effect of K conditions on tissue K content in all organs. K conditions impacted on tissue Ca, Na, Mg demonstrating the antagonism between major cations (Fig. 1a-c). But conversely, there was no effect at all of Ca (and putrescine) on K tissue content. Low Ca conditions were associated with lower Ca content in all tissues and this was compensated for by higher Na content. There was little effect of K on micronutrients, except for Fe which appeared to be more abundant at low K in roots (other micronutrients are not shown). Taken as a whole, when all cations contents were used to calculate the total positive charge (cation charge balance), there was no effect of K and Ca conditions in roots. By contrast, there was a very clear effect in leaves which exhibited a much higher total cation charge balance at low K (Fig. 1d).