The Nitrogen Stress Syndrome

This paper will present and discuss some basic physiological effects of nitrogen (N) nutrition in the cotton plant. The general thesis will be that N deficiency generates several related, yet discrete and experimentally identifiable, effects. Overall, these effects are integrated at the whole-plant level to produce systematic alterations in growth, yield, earliness, and other agronomic characteristics. Because the benefits or detriments of a particular level of N nutrition are conditional-dependent upon other factors-the concept of N deficiency must be defined closely here. By deficiency we mean a level of N nutrition which allows less than maximum dry matter production without regard to the nature of that dry matter. Thus, to the extent that N nutrition alters partitioning between vegetative and reproductive dry matter, a deficiency could actually be desirable. Indeed, this argument is made for cotton grown under certain conditions and probably is valid for indeterminate crops in general. There are three primary responses by plants toN deficiency which can explain most or all of their observed differences in growth and performance. These three effects are: (1) altered photosynthetic rate; (2) altered leaf expansion resulting from changes in hydraulic conductivity; and (3) altered responses to water stress. The first effect has received by far the most attention, presumably because chlorophyll depletion is the most obvious visual symptom of N deficiency. All three effects, however, contribute to alterations of whole-plant behavior. In fact, except for severe deficiency, the second and third effects may be the most important. Each of the three effects is discussed in a separate section of this paper, and an integrative overview is presented in the final two sections.