1594 WWW.CROPS.ORG CROP SCIENCE, VOL. 53, JULY– AUGUST 2013 RESEARCH S oybean [ Glycine max (L.) Merr.] is an important legume crop often exposed to high temperatures during reproductive stages of crop development in semiarid regions. It is predicted that global mean surface air temperatures will increase in the range of 1.5 to 4.5°C by the end of this century (IPCC, 2007). The magnitude of increase in temperatures will be greater during nighttime than in daytime (Peng et al., 2004). Several studies have investigated the combined effects of high daytime and nighttime temperatures (Prasad et al., 1999, 2006a, 2006b, 2008a) or nighttime temperatures (Manunta and Kirkham, 1996; Prasad et al., 2008b; Mohammed and Tarpley, 2009a, 2009b) on various crop species; however, studies comparing the effects of high daytime or nighttime temperatures on physiological processes and reproductive function are limited. Combination of high daytime and nighttime temperature sig- nificantly decreased the chlorophyll content, Photosystem II (PSII) quantum efficiency ( FPSII), photosynthetic rate, and yield in soy- bean (Djanaguiraman et al., 2011a) and cotton ( Gossypium hirsutum L.; Snider et al., 2009). Similarly, combination of high daytime and nighttime temperatures decreased pollen production and pol- len viability, leading to decreased seed set and seed numbers in rice ( Oryza sativa L.; Prasad et al., 2006b), groundnut (Arachis hypogaea High Day- or Nighttime Temperature Alters Leaf Assimilation, Reproductive Success, and Phosphatidic Acid of Pollen Grain in Soybean [ Glycine max (L.) Merr.] M. Djanaguiraman, P. V. V. Prasad,* and W. T. Schapaugh ABSTRACT Soybean [Glycine max (L.) Merr.] is often exposed to high daytime and nighttime temper- atures during critical growth stages. Threshold mean daily temperature for photosynthesis, res- piration, and reproductive process in soybean is ³26°C. In future, the magnitude of increase in nighttime temperatures will be greater than in daytime temperatures. The objectives were to determine effects of high daytime or nighttime temperatures on (i) leaf photosynthetic and res- piration rates; (ii) pollen germination, pod-set, and seed weight; and (iii) pollen phospholipids profile. Soybean plants were exposed to high daytime temperature (39/20°C), high nighttime temperatures (30/23°C, 30/26°C, and 30/29°C), or optimum temperature (30/20°C) for 10 d at flowering stage. High daytime temperature (39/20°C) or nighttime temperatures (30/29°C) increased leaf respiration rates and decreased leaf chlorophyll content, photosynthetic rate, photochemical quenching, and electron trans- port rate compared to optimum temperature. Likewise, high temperature decreased pollen viability and germination. Lower pollen ger- mination at high temperature may be due to decreased levels of saturated phospholipids and phosphatidic acid in pollen grains com- pared with optimum temperature. Pod-set and seed weight were decreased by high daytime or nighttime temperature. In conclusion, high day- time (39/20°C) or nighttime (30/29°C) tempera- ture decreased leaf photosynthetic rate and pol- len germination, leading to lower pod-set and seed weight. Dep. of Agronomy, 2004 Throckmorton Plant Science Center, Kansas State Univ., Manhattan, KS 66506. Received 22 July 2012. *Corresponding author (vara@ksu.edu). Abbreviations: DGDG, digalactosyldiacylglycerol; ETR, electron transport rate; MGDG, monogalactosyldiacylglycerol; NPQ, non–pho- tochemical quenching; PA, phosphatidic acid; PC, phosphatidylcho- line; PI, phosphatidylinositol; PSII, Photosystem II; qP, photochemical quenching; FPSII, quantum yield of Photosystem II. Published in Crop Sci. 53:1594–1604 (2013). doi: 10.2135/cropsci2012.07.0441 © Crop Science Society of America | 5585 Guilford Rd., Madison, WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.