Short communicationImage analysis of grain shape to evaluate the effects of high temperatures on grain filling of rice, Oryza sativa L.
Highlights
► High temperature during ripening deepens creases on the rice grain surface. ► The inflection point of the grain perimeter was measured by image analysis. ► The inflection at the deepest crease indicates the effect of high temperature. ► Heat-tolerant ‘Nikomaru’ had shallower creases than ‘Hinohikari’.
Introduction
High temperatures during ripening reduce rice grain quality, causing the conditions “chalky grain” (Yoshida and Hara, 1977, Tashiro and Wardlaw, 1991), “fissured grain” (Nagata et al., 2004), and “immature thin grain with deep creases” (Nagato and Ebata, 1965, Morita et al., 2005a, Morita, 2009). This last effect is more serious because it causes not only lower grain quality, relating to lower milling efficiency, but also lower grain yield by reducing grain weight (Morita et al., 2005b). The physiological mechanisms underlying this problem are unclear, so there is little information about how to prevent it.
Research has been hindered by the lack of a high-throughput method for evaluating crease depth. Sasaki and Magoshi (1933) proposed a method based on microscopic analysis, but this method is too time-consuming to have been taken up. Breaks and cracks responsible for fissured grain have been assessed by image analysis of parboiled rice (Courtois et al., 2010), but creases formed along the junction of the lemma and palea have not.
We have proposed a method based on image analysis and have used it to assess the effects of growing condition (Morita et al., 2005a, Morita, 2009) and cultivar (Morita, 2009). We found that high temperatures during ripening promote creasing, and that the heat-tolerant cultivar ‘Nikomaru’ (Sakai et al., 2007, Morita and Nakano, 2011) creases less than the susceptible cultivar ‘Hinohikari’ under high temperatures during ripening, especially with low solar radiation (Morita, 2009). However, our pot experiments provided little information about the factors relating to the depth of creases on the grain surface.
Here, we elucidated the effects of high temperature during ripening and of cultivar on crease depth using data from growth chamber and field experiments at high and normal temperatures (Morita, 2009), and confirmed an appropriate index for immature thin grain with deep creases.
Section snippets
Pot experiment
We grew two cultivars: the heat-tolerant ‘Nikomaru’ (Sakai et al., 2007) and the heat-susceptible ‘Hinohikari’ (Yagi et al., 1990), a leading cultivar in western Japan. Ten seedlings of each cultivar were transplanted on 17 June into plastic pots (16 cm across, 20 cm tall) containing Gray Lowland soil (a Typic Endoaquept), the field soil at the National Agricultural Research Organization (NARO) Kyushu Okinawa Agricultural Research Center (KARC), in 2004. Tillers were removed periodically to
Results and discussion
Higher temperatures decreased r by similar amounts at both the dorsal (180°) and ventral (0°) ends in the pot trial (Fig. 2A); it also decreased grain weight in the pot trial (Fig. 3) and 1000-grain weight in the field trials (Fig. 4). Nagato and Ebata (1965) reported that the effects of temperature on grain growth differed between the ventral and dorsal sides: the dorsal radius, which develops early during ripening, was longer at higher temperatures, and the ventral radius, which grows later
Acknowledgments
We thank Mr. Hisashi Kitagawa, Dr. Hiroshi Nakano, Dr. Hiroshi Wada, Mr. Akitoshi Honbu, Ms. Kumi Matsufuji, and Ms. Emi Hashimoto, NARO/KARC, for their help and cooperation.
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Both authors contributed equally.