When the root system of a nutrient deficient plant is supplied locally with the nutrient, the roots grow and proliferate in the nutrient-rich zone, sometimes spectacularly so. In addition, uptake per unit of root is faster in the nutrient-rich zone. These are often thought of as compensatory responses. The extra surface area and faster uptake from the localized supply offset the inability of the rest of the root system to contribute to nutrient acquisition. This is the picture obtained from a few, well-known studies of young cereals. But is it supported by all the evidence that is available?
This review assesses how various attributes respond to localized supplies of nutrients. The attributes are: root growth and development; uptake rates; nutrient contents; root;shoot interactions; and whole-plant growth. The nutrients considered are K, NH4+, NO3− and P. The experiments reviewed used techniques ranging from solution cultures to field studies. Most used as controls plants that were amply and uniformly supplied with the nutrient; some used plants uniformly deprived of the nutrient.
One third of the measurements of root growth showed little or no response to a localized nutrient supply. Many others did, and the general tendency was for a stimulation of growth in a nutrient-rich zone to be mirrored by a suppression of growth elsewhere. Responses varied with time, species and nutrient. The response of a root system to locally available nutrients can be predicted in general terms. But the precise degree and direction of growth cannot. Simple rules of root development sometimes apply. But these cannot account for many other observations of development that do not follow any simple rule.
Uptake per unit of root was usually up to c. three-fold faster when roots of nutrient-deprived plants were supplied locally. Exceptionally, increases of five- to ten-fold were measured. Generally, this increase was related inversely to the fraction of the root system with access to the nutrient. But within particular studies that relation did not hold.
Root:shoot ratio usually increased or hardly changed in locally supplied plants. Their nutrient content usually declined more than growth. On average, therefore, the nutrient was less concentrated in the tissues of a locally supplied plant. This may indicate the diversion or mobilization of nutrients from potential storage pools to supplement the amounts taken up from the localized supply. An exception to this was for N in the one C4 species studied, Zea mays.
When comparing these attributes, the growth of whole plants was least affected by a non-uniform supply of nutrients, and least variable. This is circumstantial evidence that some of the responses did compensate partially for non-uniform supplies of nutrients. But within species and nutrient treatments, there was no consistent association between specific responses and any apparent compensation.