HEAT STRESS Soybean Pollen Anatomy, Viability and Pod Set under High Temperature Stress M. Djanaguiraman 1 , P. V. V. Prasad 1 , D. L. Boyle 2 & W. T. Schapaugh 1 1 Department of Agronomy, Kansas State University Manhattan, KS, USA 2 Division of Biology, Kansas State University Manhattan, KS, USA Keywords heat stress; high temperature stress; pod set; pollen anatomy; pollen viability; seed set Correspondence P. V. V. Prasad Department of Agronomy, Kansas State University, 2004 Throckmorton Hall, Manhattan, KS 66506, USA Tel.: +1 785 532 3746 Fax: +1 785 532 6094 Email: vara@ksu.edu Accepted September 6, 2012 doi:10.1111/jac.12005 Abstract High temperature (HT) stress is one of the major environmental factors influenc- ing yield of soybean (Glycine max L. Merr.) in the semi-arid regions. Experiments were conducted in controlled environments to study the effects of HT stress on anatomical changes of pollen and their relationship to pollen function in soybean genotype K 03-2897. Objectives of this study were to (a) quantify the effect of HT stress during flowering on pollen function and pod set and (b) observe the ana- tomical changes in pollen grains of soybean plants grown under HT stress. Plants were exposed to HT (38/28 °C) or optimum temperature (OT, 28/18 °C) for 14 days at flowering stage. HT stress significantly decreased in vitro pollen germi- nation by 22.7 % compared to OT. Pollen from HT stress was deformed; it had a thicker exine wall and a disintegrated tapetum layer. HT stress decreased pod set percentage (35.2 %) compared to OT. This study showed that decreases in pollen in vitro germination by HT stress were caused by anatomical changes in pollen, leading to decreased pod set percentage under HT stress. Introduction High temperature (HT) stress during reproductive stages of crop development can significantly decrease components of plant yield (Prasad et al. 1999, 2000, 2006a,b, Kebede et al. 2012). Crops in future climates are predicted to be exposed not only to higher mean temperatures but also to more frequent short episodes of HT (Solomon et al. 2007). Soybean (Glycine max L. Merr.) is sensitive to HT stress, particularly during flowering. Exposure to HT stress during flowering significantly decreases seed set in groundnut (Arachis hypogaea L.; Prasad et al. 1999, 2000), kidney bean (Phaseolus vulgaris L.; Prasad et al. 2002), sorghum (Sor- ghum bicolor L. Moench; Prasad et al. 2006a, 2008a), rice (Oryza sativa L.; Prasad et al. 2006b) and wheat (Triticum aestivum L.; Prasad et al. 2008b, 2011), and fertilization efficiency in cotton (Gossypium hirsutum L.; Snider et al. 2011) resulting in lower seed yield. Seed set primarily depends upon function of pollen and ovule, successful pollination, fertilization and post-fertilization processes. Many studies showed that pollen development during various phases of microsporogenesis was sensitive to HT stress in soybean (Salem et al. 2007). Few reports docu- mented the effects of HT on pollen morphological changes in soybean (Koti et al. 2005, Salem et al. 2007). In HT stress, the morphology of soybean pollen was affected more severely, particularly in temperature-sensitive genotypes. The altered morphology (flattened and collapsed) of pollen grain resulted in lower pollen viability in HT stress condi- tions (Salem et al. 2007). HT stress during reproductive development has been reported to cause similar abnormal exine with deeply pitted and smooth regions in soybean (Salem et al. 2007). As exine originates from the tapetum cells, the altered pollen morphology may be due to changes in the tapetal layer. Suzuki et al. (2001) reported that HT stress caused early degeneration of the tapetum layer and disrupted endoplasmic reticulum in snap beans (Phaseolus vulgaris L.). Anatomical evidence explaining reasons for failure of pollen germination under HT stress is not clearly documented and needs attention, particularly in soybean. We hypothesize that HT stress causes significant ultra- structural changes in pollen grains resulting in decreased pollen germination. Objectives of this research were to quantify the effects of short episode of HT stress (14 days) during flowering on changes in the anatomy of pollen grains and subsequent effects on pollen function and pod set. © 2012 Blackwell Verlag GmbH, 199 (2013) 171–177 171 J Agro Crop Sci (2013) ISSN 0931-2250