Association of Aflatoxin Contamination with Physiological Responses of Corn Plants to Moisture Deficit and Heat Stress.

In the southern United States, corn production frequently encounters a period of drought and heat stress during flowering and kernel development and these weather conditions have been associated with increased aflatoxin contamination produced by the fungus Aspergillus flavus. This study was undertaken to evaluate corn genotypes for aflatoxin contamination in relation to their physiological responses to moisture deficit and heat stress. The study was conducted at the Delta States Research Center, Stoneville, MS in 2009 and 2010. Four commercial hybrids, P31G70, P33F37, P32B34 and DKC63-42 and two inbred germplasm lines with tropical background, aflatoxin resistant (PI 639055) and drought stress tolerant (PI 489361), were evaluated. A split plot experimental design was used with main unit treatments consisting of two soil moisture treatments, irrigated and non-irrigated, and six corn genotypes as sub plot treatment. To determine the extent of moisture deficit and heat stress imposed on the plants, measurements were made on air, soil and leaf temperatures and soil moisture status using an automated microcontroller-based monitoring system. To examine the response of the corn plants to these stresses, leaf water potential, photosynthetic pigments, cell membrane thermostability, and maximum quantum efficiency of PSII were determined. The plants were inoculated with Aspergillus flavus (K-54) at mid-silk stage and aflatoxin contamination was determined on the kernels at harvest. The physiological measurements and data from the microcontroller-based monitoring system suggested that PI 639055, PI 489361 and hybrid DKC63-42 were under more severe stress in the non-irrigated plots. Seed composition analysis showed higher protein concentration in these three genotypes. P31G70 was the most tolerant among all the genotypes. The aflatoxin resistant germplasm, PI 639055, even though it was under severe stress, had the lowest aflatoxin contamination (20 ppb) followed by P31G70 (37 ppb), whereas DKC63-42 and PI 489361 had the highest contamination, 202 ppb and 186 ppb, respectively. These results indicated that the genotypes with the most severe stress had the highest aflatoxin accumulation except for PI 639055. Possible reasons for these differences are presented.