Spinach (Spinacia oleracea L.) Seed Germination and Whole Plant Growth Response to Heat Stress and Association Mapping of Bolting, Tallness and Erectness for Use in Spinach Breeding

The effect of temperature on spinach seed germination was evaluated using a total of nine spinach genotypes and seven temperatures: 10, 15, 20, 25, 30, 32, and 35 C in growth chambers. Genetic variation was observed. ‘Donkey’, ‘Marabu’, and ‘Raccoon’ showed higher seed germination percentage with over 70% at 30 and 32 C, indicating the three spinach genotypes had heat-tolerance for germination. However, all spinach genotypes except ‘Ozarka II’ had lower germination percentages of less than 30% while ‘Ozarka II’ had 63% at 35 C, indicating ‘Ozarka II’ may be a source of heattolerance for seed germination. Seed germination may be useful as an early screening method for heat-tolerant spinach genotypes. The whole-plant experiment was conducted using two temperatures, 20 and 32 C, in growth chambers. Variation was observed among spinach cultivars for leaf area and shoot dry weight at 32 C. ‘Samish’ was the only cultivar to have no significant difference between 20 and 32 C for leaf area, while the leaf area for ‘Ozarka II’, ‘Donkey’, and ‘Marabu’ decreased from 20 to 32 C. This may indicate that ‘Samish’ is heat-tolerant. However, significant differences were not observed among cultivars for shoot dry weight. The results from whole-plant growth at high temperature did not parallel those of the seed germination study, and the evidence does not indicate that seed germination can be used as a screening tool for heat tolerant cultivars. Bolting, tallness, and erectness are important morphological traits in spinach breeding. A total of 288 USDA spinach accessions were used as the association panel in this research. Single nucleotide polymorphisms (SNPs) discovered through genotyping by sequencing (GBS) were used for genotyping. Three SNP markers, AYZV01001038_398, AYZV01031624_1060, and AYZV01088923_95 were found to be associated with bolting. Eight SNP markers, AYZV01011130_540, AYZV01180397_2162, AYZV01069590_19842, AYZV01105690_376, AYZV01058838_64, AYZV01152613_2532, AYZV01113619_2197, and AYZV01003134_248 were associated with tallness. Four SNP markers, AYZV01137843_229, AYZV01158294_79, AYZV01023368_256, and AYZV01097131_197 were associated with erectness. These SNP markers may be used in spinach molecular breeding to select spinach bolting, tallness and erectness through marker-assisted selection. Acknowledgments It takes a village to raise a Masters student, and I have been profoundly blessed by the wonderful village of people at the University of Arkansas and elsewhere who have helped me find my way during this time. I owe my success in completing this degree to all of them, and I cannot thank them all enough. First and foremost, I thank God for putting me in the all the right places in all of His perfect timing, and for the people He put in my path to act as guides, friends, and more. It took a little faith and a ton of work, and I was not always sure it was going to work out, but I am guessing He did because He never gave up on me, regardless of how I felt about myself. It was an honor to be the very first graduate student of Dr. Ainong Shi. We may not have always understood each other’s words, but the hard work, dedication, and support he showed me went beyond what words could describe. I thank Dr. Shi for trying to teach me everything he possibly could in the short two years we have known each other, from software to Chinese words. I will do my best to retain every bit. My committee members have been absolutely outstanding and I owe them a great big thank you as well. Dr. John Clark, Dr. Michael Evans, Dr. Pengyin Chen and Dr. David Hensley have all given me an incredible amount of support, information, and patience. I thank them for every bit of help they offered along my journey. I am so grateful for the folks at the Vegetable Research Station in Alma (Kibler really), who always had time for me and my questions, no matter the circumstances. Thank you especially to Dennis who probably knows more about spinach than any other person I have ever met, and to Stephen and Larry who never hesitated to give me direction or tell me a joke. There are very few faculty members in the Horticulture department who have not had me in their office for advice at one time or another. I thank them for showing me such a gracious spirit and allowing me to take their time. Thank you to Dr. David Hensley, who was the department head at the time of my acceptance and the first person I met when I came to the U of A, for the guidance you have always offered. I am also very appreciative of Dr. Wayne Mackay who, though he has only known me a short while, has had an incredible impact on my present and my future. I owe the greatest thanks of all to my family for being such a strong support network during these two years. To Nate, thank you for being by my side every step of the way unless you were doing the chores while I finished school work. Thank you to my mom for always believing in my abilities whether or not you understood what I was talking about, and to my dad for always helping me see the good and lighter side of any obstacle I was facing.