**3.5 Cotton secondary metabolites as a potential stress assessment tool for field crop management**

Although the general concept of plant secondary metabolites and their ecological functions became evident around the mid-twentieth century, gossypol was first extracted from cotton seeds in 1899 [103], and cotton pigment glands were correctly interpreted in 1906 [67]. Hence, we have accumulated over 124 years of knowledge of cotton secondary metabolites. The knowledge of cotton secondary metabolites has benefitted cotton breeding and human and animal health regarding contraception, byssinosis, and utilization of cotton seeds. Now, we have known draught [84], salinity [81], chilling [104], insect [86], pathogen [59], nutrient deficiency (Yue, unpublished data), herbicides [98, 99], and even mechanical damage [82] of the cotton plants can induce production of cotton terpenoids in cotton plants, especially root gossypol concentration to increase. In addition, the root system is the main organ to synthesize gossypol [61]. The root system also increased its gossypol concentration while supplying gossypol to aboveground parts except cotyledons. Hence, cotton root gossypol concentration is sensitive to all the above factors and is a cumulative index of cotton plant stress and defense status. In other words, cotton root gossypol concentrations are the common currency in cotton plant stress and defense response.

Furthermore, our ongoing projects showed that adjacent plants can increase cotton root gossypol concentration compared to isolated cotton plants without adjacent plants [105]. Nutrient deficiency stress increased cotton root gossypol concentration comparing to cotton plants fertilized by Hoagland solution (Yue, unpublished data). By analyzing all the previous data on cotton root gossypol concentrations, it is found cotton roots have the highest gossypol concentration on a dry matter basis (6.3% in Ref. [66]) among different cotton plant parts. Cotton roots have a similar gossypol concentration as seeds at the harvesting stage on a fresh weight basis, around 1% (Yue, unpublished data).

Cotton root gossypol concentration analysis is potentially a stress assessment tool for field crop management, just like body temperature measurement in medical examinations. The basis is that we set up a series of standard values of cotton root gossypol concentrations at different ages or sizes without those stresses.

It is time to push our understanding of the cotton secondary metabolites into productivity. Direct application of cotton secondary metabolites in crop management will convert the traditional knowledge accumulation pattern of cotton secondary metabolites from funding agency publication to application data production, making us acquire knowledge of cotton secondary metabolites faster. A specific farmer might not be able to analyze these secondary metabolites, but our research and extension centers should be able to do it.
