Core Ideas Rice grain‐Zn concentration increased with more aerobic soil conditions. Fertilizer‐Zn applied to soil had limited effect on rice grain‐Zn concentration. Rice paddy water management affected soil redox potential and DTPA‐extractable Zn. Combined water and Zn management are useful for agronomic Zn biofortification. Agronomic biofortification was not sufficient to meet rice Zn nutrition targets. Increasing rice (Oryza sativa) grain‐Zn concentration is important for achieving improved human nutrition. Our objective was to understand how agronomic management practices, including water management and fertilizer‐Zn application rate and timing, affect plant growth, grain‐Zn concentration, and yield of rice genotypes. In a series of four‐field experiments over three seasons, we tested multiple combinations of water management techniques and fertilizer‐Zn application techniques. The use of alternate wetting and drying (AWD) water management increased (p < 0.001) soil redox potential and diethylene triamine pentaacetic acid (DTPA)‐extractable soil Zn compared with continuous flooding (CF) in the 0‐ to 2‐cm soil depth, but not always in the 2‐ to 10‐cm depth. On average, AWD grain‐Zn concentration increased 9% over CF without any yield penalty. Fertilizer‐Zn application increased DTPA‐extractable Zn only in the top soil layer and only temporarily, with a corresponding increase in grain‐Zn concentration only at rates > 10 kg Zn ha−1. Different timings of fertilizer‐Zn application (from basal to flowering) had no effect on grain‐Zn concentration or yield. Overall, our results indicated that AWD had a consistent and larger positive effect than fertilizer‐Zn application on grain‐Zn concentration. However, the increase in grain‐Zn concentration due to fertilizer‐Zn or water management was small, up to 2 mg Zn kg−1 brown rice, implying that improved agronomic management alone is not sufficient to meet the target increase of at least 10 mg Zn kg−1, but is a useful complementary strategy for enhancing the performance of Zn‐enriched rice by improving soil‐Zn availability.

Four heterotic groups were identified for tropical indica rice germplasm to develop hybrid rice in the tropics based on two studies. Heterotic groups are of fundamental importance in hybrid crop breeding. This study investigated hybrid yield, yield heterosis and combining ability within and among groups based on genetic distance derived from single-nucleotide polymorphism markers. The main objectives of the study were to (1) evaluate the magnitude of yield heterosis among marker-based groups, (2) identify possible heterotic groups for tropical indica hybrid rice, and (3) validate heterotic patterns concluded from a previous study. Seventeen rice parents selected from improved indica germplasm from the tropics with high genetic divergence and 136 derived hybrids were evaluated in five environments. The hybrids had more yield than their parents with an average of 24.1 % mid-parent heterosis. Genotype × environment interaction was the major factor affecting variations in yield and yield heterosis, which raised a necessity and a challenge to develop heterotic rice hybrid adapted to different regions and seasons in the tropics. Yield, yield heterosis and combining ability were significantly increased in inter-group than in intra-group hybrids. Four heterotic groups and three promising hybridization patterns, which could be used in tropical hybrid rice breeding, were identified based on marker-based grouping, yield and yield heterosis analyses in the two studies. The study reveals that molecular markers analysis can serve as a basis for assigning germplasm into heterotic groups and to provide guidelines for parental selection in hybrid rice breeding.

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Cereal Chem. 94(5):887-891Apparent amylose content (AAC), as one of the most important quality traits of rice grain, is primarily controlled by the Wx gene, which encodes a granule-bound starch synthase. Effects of putative alleles of Wx of tropical rice hybrids generated from parents with three different nonglutinous Wx alleles (Wx a , Wx b , and Wx in ) were examined in a multienvironment trial. Genotypes of Wx b /Wx in and Wx in /Wx in were associated with intermediate AAC (20.2-23.7%), and genotype of Wx a /Wx a was associated with high AAC (24.9-28.0%). Wx alleles of Wx in /Wx in could be the most favorable combinations in selecting hybrid parents with desirable intermediate amylose content (20-25%) for rice consumers of South and Southeast Asian countries. Genotype and genotype × environment interaction were the major contributing factors to the variation in parental accessions' and hybrids' AAC, and in terms of AAC, hybrids were more sensitive than parental varieties to environment. Genotypes of Wx a /Wx a , Wx a /Wx in , and Wx in /Wx in showed higher AAC performance in the wet season than in the dry season. † Corresponding authors.