Why do plants blush when they are hungry?

Foliar anthocyanins, as well as other secondary metabolites, accumulate transiently under nutritional stress. A misconception that only nitrogen or phosphorus deficiency induces leaf purpling/reddening has led to overuse of fertilizers that burden the environment. Here, we emphasize that several other nutritional imbalances induce anthocyanin accumulation, and nutrient-specific differences in this response have been reported for some deficiencies. A range of ecophysiological functions have been attributed to anthocyanins. We discuss the proposed functions and signalling pathways that elicit anthocyanin synthesis in nutrient-stressed leaves. Knowledge from the fields of genetics, molecular biology, ecophysiology and plant nutrition is combined to deduce how and why anthocyanins accumulate under nutritional stress. Future research to fully understand the mechanisms and nuances of foliar anthocyanin accumulation in nutrient-stressed crops could be utilized to allow these leaf pigments to act as bioindicators for demand-oriented application of fertilizers. This would benefit the environment, being timely due to the increasing impact of the climate crisis on crop performance.

Automated summary

Foliar anthocyanin accumulation is not only induced by nitrogen or phosphorous deficiency but also by other nutritional imbalances. This misconception has led to excessive use of fertilizers, causing environmental burden. Anthocyanins have various physiological functions, and this article discusses their synthesis pathways and functions in nutrient-stressed leaves. Integrating knowledge from genetics, molecular biology, ecophysiology, and plant nutrition, the mechanisms and reasons for foliar anthocyanin accumulation under nutritional stress are deduced. Further research can utilize this knowledge to use leaf pigments as bioindicators for demand-oriented fertilizer application, benefiting the environment in the face of increasing climate crisis impacts on crop performance.