CROP SCIENCE, VOL. 49, NOVEMBER– DECEMBER 2009 2187 RESEARCH M aize (Zea mays, L.) kernels with a high oil concentration (>70 g kg –1 ) are preferred in feed rations of livestock and poul- try because of their energy value and as a substitute for animal fats in feed rations (Thomison et al., 2003). Nevertheless, traditional maize hybrids commonly cultivated in Argentina yield kernels with an oil concentration of ~60 g kg –1 (Maddonni and Otegui, 2006). Oil concentration of maize kernels exhibits a significant degree of genetic variability that enables breeding for this trait. For example, after 89 generations of breeding, two contrasting kernel oil populations (10 and 200 g kg –1 ) were obtained (Dudley and Lambert, 2004). A study performed with these populations revealed that kernel oil concentration is a highly polygenic trait with a largely additive heritance (Laurie et al., 2004). Hence, maize hybrids with a high kernel oil concentration could be obtained by crossing parental lines selected for this trait. Unfortunately, the yield and other agronomic characteristics of these high oil popu- lations were poor so the population itself was not used in com- mercial production (Lambert, 1994 cited by Laurie et al., 2004). Likewise, yields of high-oil (~75 g kg –1 ) maize hybrids were about 15% lower than those of normal high-oil content (~50 kg kg –1 ) hybrids (Lambert et al., 1998). Consequently, profitable maize Maize Kernel Oil and Episodes of Shading during the Grain-Filling Period Walter Tanaka and Gustavo Ángel Maddonni* ABSTRACT Previous studies documented the stability of maize ( Zea mays L.) kernel oil concentration for a wide range of kernel weights promoted by contrasting post-flowering assimilate availabili- ties per kernel (i.e., source–sink ratios). These studies mainly modified the sink size with a low impact on the source size. In this study, we focused on kernel oil concentration response to source–sink ratio alterations promoted by differ- ent timings and intensities of shading during the effective grain-filling period. Two crosses with contrasting kernel oil concentration (‘DK752’ × DK752 and DK752 × ‘5MG’) were tested. Kernel oil concentration was positively related to the embryo–kernel ratio (r = 0.96, P < 0.001) and embryo oil concentration (r = 0.94, P < 0.001) and crosses differed in both traits. Severe shad- ing (85% reduction of incident solar radiation) at early stages of kernel growth reduced the final embryo–kernel ratio and the embryo oil con- centration of both crosses. Contrarily, moderate shading (45% reduction of incident solar radia- tion) did not modify the kernel oil determinants. Kernel oil concentration and kernel weight declined when severe shading shortened the kernel growth period. Our results collectively sustain the hypothesis that maize kernel oil concentration is commonly sink-limited. We established that kernel oil concentration of both crosses was reduced when post-flowering plant growth rate per kernel was less than 50% of ker- nel growth rate. W. Tanaka and G.A. Maddonni, Dep. de Producción Vegetal, Fac. de Agronomía, Univ. de Buenos Aires, Av. San Martín 4453, Ciudad de Bue- nos Aires (C1417DSE), Argentina. Financial support, from the National Council for Research (CONICET. PIP 5540). G.A. Maddonni is a mem- ber of and W. Tanaka has a scholarship from CONICET. Received 5 May 2009. *Corresponding author (maddonni@agro.uba.ar). Abbreviations: KGR, kernel growth rate; PGR, plant growth rate; PGRK, plant growth rate per kernel. Published in Crop Sci. 49:2187–2197 (2009). doi: 10.2135/cropsci2009.05.0238 © Crop Science Society of America 677 S. Segoe 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. Published November, 2009