Okra is a nutritious vegetable that belongs to the family Malvaceae. It is widely grown in the tropical, sub-tropical, temperate and Mediterranean regions of the world. However, little evidence of the impacts of high temperature stress on various physiological, morphological, biochemical and metabolic processes is available for okra nor is the extent of genetic variation known. This study characterized and evaluated 176 diverse okra germplasm under constant high temperature to assess morphological and physiological changes to growth and development and to identify molecular markers linked to heat tolerance. In the process, the ploidy level and genome size of diverse materials were established.
Significant marker-trait associations (MTAs) were found for various morphological and phenological traits. These included days to fruiting, plant height and stem diameter, that once validated, could be used for marker-assisted breeding. Fruit nutrient analysis identified significant genotypic differences for Ca, Fe, and Na and some organic metabolites including sucrose. The accumulation of Ca, Na and Fe and sugars in the fruit of some genotypes acted not only as osmolytes or protectants during fruit development, but also influenced signal transduction and the maintenance of cell membrane integrity. High temperature also impacted pollen micromorphology. Tolerant genotypes had dehisced anthers and fully turgid and spined pollen grains with improved germination compared to sensitive genotypes. The optimal temperature for pollen germination was observed to be 25°C while temperatures above 45°C caused significant damage. Flow cytometry indicated that the relative number of chromosomes varied from 84-189 and ploidy level from 7x to16x. The genome size data were inadequate to accurately indicate the number of chromosomes. A negative correlation between relative ploidy and genome size showed a downsizing of the genome with increased ploidy level.