Date of Graduation


Document Type


Degree Name

Doctor of Philosophy in Crop, Soil & Environmental Sciences (PhD)


Crop, Soil & Environmental Sciences


D.M. Oosterhuis

Committee Member

F.M. Bourland

Second Committee Member

M. Mozaffari

Third Committee Member

C. Rom

Fourth Committee Member

B.L. McMichael


Chlorophyll a fluorescence, Cotton (Gossypium hirsutum), Photosystem II, Temperature tolerance


Cotton (Gossypium hirsutum L.) is sensitive to heat stress (HS) during reproductive development. The objective of this study was to evaluate different screening methods for identification of heat tolerance in cotton genotypes. Three growth chamber studies and four field trials were conducted from 2014 to 2017 using genotypes Arkot 9704, VH260, DP 210 B2RF and DP393. Measurements were made of membrane leakage (ML), chlorophyll fluorescence (ChlF), glutathione reductase (GR), and sucrose concentration. In the growth chambers, measurements were made at 30 and 40°C and at 2, 4 and 6 hours of HS, as well as 3 and 7 days after HS and 7 days after recovery. Both ML and ChlF were decreased at 40°C and genotypic difference were detected, with DP393 the least affected indicating heat tolerance. Arkot 9704 was affected the most indicated sensitivity to HS. The small genotypic responses to HS was related to modern genotypes having less tolerance to HS than older obsolete genotypes and wildtype cotton. Glutathione reductase was increased by HS and VH260 and DP393 increased the most in the growth chamber but not in the field studies. Sucrose concentrations were decreased by HS with no genotypic differences. Analysis of the fluorescence transient after HS was imposed showed that maximum fluorescence intensity, plant performance index (PIABS) and electron transport flux (ET/CS) provided more intrinsic quantitative measurements of the effect of HS on PSII function. For both ML and ChlF, for a one day heat stress period, measurements could be made at 2 hours, but for a longer heat stress, parameters should be measured 7 days after stress. The method of measuring genotype response to HS in the field by comparing cool versus hot days was not sufficiently accurate. A new method of comparing early morning cool 6.00 AM measurements versus hot midday measurements, showed genotypic increases in ML, but for ChlF only on clear, high radiation days. Differential genotypic responses to HS can be detected by ML and particularly by ChlF for ease of use and accuracy, with an analysis of the fluorescence transient responses to HS providing a clear means of differentiating between genotypes for thermotolerance.