Awika, Henry Ochieng (2016-09). Determining How the Physiological and Genomic Interaction between Wax and Staygreen Affects Yield and Water Use in Sorghum Bicolor under Drought and Heat Stress. Doctoral Dissertation.
Thesis
The balance between plant internal heat requirements and water use is achieved through a combination of physiological, biochemical and genetic processes. The use of physiological traits in crop modeling and breeding programs has been notoriously inaccurate. A contributing problem is the low understanding and low emphasis on the complexity of the subtle interactions between the physiological traits and their genetic regulation. The goal of this study was to highlight how the characteristics of the leaf surface can contribute to staygreen, water use and yield. This was investigated using the following objectives: 1) define the relationship between wax and transpiration in regulating TD among staygreen NILs and non staygreen inbreds and how this affects yield potential under heat, drought and a combination of both stresses, 2) determine if there is physiological and reproductive significance of the bloom (Bm, glaucous) phenotype compared to the bloomless (bm, non-glaucous) phenotype, and 3) determine allelic diversity and significant gene interaction controlling the bloomless trait.in sorghum. We used sorghum recombinant inbred lines (RILs) and staygreen near isogenic lines (Stg NILs) derived from Tx642 and Tx7000. We also used ethylmethanesulfonate (EMS) mutagenized Tx623 to investigate allelic diversity, inheritance and the photochemical effect of the bloomless mutation. Under objective 1, partitioning data into near high and low leaf wax load (WL) genotypes with uniform duration to flowering (DTF) was important. It revealed that the relationship between wax and canopy temperature depression/ cooling (TD) adopts a quadratic trend among the high wax genotypes under stress with a novel threshold beyond which further increase in wax load does not result in further cooler canopy. Genotypes that expressed the staygreen (Stg) trait and had high WL clearly outperformed all the other phenotypic classes in grain yield. We also noted that with the advancement in phenology, the fluctuations between whole plant evapotranspiration (ET) and WL were antagonistic and polynomial. Canopy temperature changes were better predicted by the interaction between ET and WL than by the individual traits separately. Under the second objective, we determined that the bm mutation is associated with altered heat receptors important in signaling the regulation of stomatal aperture. These alterations may be linked to a modification in the C4 pathway that increases their overdependence on CO2 influx and open stomata under heat and drought. In the third objective, the results pointed to a potentially large diversity in the alleles, including dominant mutant alleles that can influence the production of the bloomless sorghum. Our observations agreed with both the one-gene and the two-gene models. However, based on the observed complex interactions between some of the alleles, loci and gene products, we have proposed that more detailed studies and validation steps may be required to ascertain the different inheritance patterns.
