Genetic Improvement of Grain Quality in Japonica Rice - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 145

in plant breeding, molecular biology, and plant
physiology is enhancing our understanding of
the gene pathways that control grain-quality
traits. These new research strategies will not
only advance our understanding of the molec-
ular mechanisms, but will also lead to high-
quality rice through efficient and targeted grain
improvement.
In this chapter, we review (1) component traits
of grain and eating quality, such as grain shape,
grain chalkiness, and contents of seed storage
components; and (2) genetic analyses related to
grain quality (QTL detection and gene cloning)
in japonica rice. We also discuss new approaches
and prospects for the future of grain-quality
improvement, including advances in phenotyp-
ing methods and genomics research.

defining market value. Cooking characteristics
typically include hydration and temperature dur-
ing cooking time, textural properties of cooked
rice, human evaluation for sensory qualities of
cooked rice, the ability to remain soft for sev-
eral hours after cooking, and aroma retention
after cooking. Textural properties of cooked rice
grain are important for consumers of japonica
rice, who prefer strong stickiness and softness of
cooked rice (Okabe 1979; Kim 2009; Sun et al.
2011). The types and amounts of each grain com-
ponent, such as starch, protein, lipids, oligosac-
charides, amino acids, vitamins, minerals, and
secondary metabolites, affect the various cook-
ing characteristics, sensory qualities, and nutri-
tional value.

Physical Appearance of Rice Grain

ComponentTraits of Rice Quality

GrainShape

Grain-quality traits in rice encompass a wide
range of characteristics that can be classified as
pertaining to physical appearance and cooking
characteristics (Table 9.1). The optimum score
for each grain-quality trait varies according to
the preferences of local consumers. The physi-
cal appearance of the rice grain includes its shape
(length, width, and thickness), surface whiteness,
and grain chalkiness (degree of translucence).
These physical traits are immediately obvious
to consumers and are among the major factors

Grain shape is a critical aspect of rice grain qual-
ity. Grain shape affects not only grain yield, but
also cooking and processing qualities (Fan et al.
2006; Song et al. 2007; Shomura et al. 2008).
In the United States, rice cultivars are classi-
fied into several categories according to grain
shape. Long-grain rice is approximately three
times as long as it is wide. Short-grain rice is
less than two times as long as it is wide. Medium-
grain rice lies in between (Bergman et al. 2004).

Components of grain quality in rice. Each component is intimately interrelated with the others to form overall

Table 9.1.

grain quality

Major classification Small classification Specific example

Physical appearance Grain shape Seed size, length, width, thickness and weight

Grain chalkiness Degree of translucence endosperm

Husking and milling quality Amount of cracked grain after husking or milling

Cooking characteristics Cooking property Gelatinization temperature, gel consistency and texture of

cooked rice grain

Sensory quality Glossiness, stickiness, hardiness and taste of cooked rice

evaluated by persons

Component of endosperm Starch (amylose and amylopectin), oligosaccharides,

protein, amino acids, lipids, vitamins, minerals and
second metabolite products

Aroma Amount of 2-acetyl-1-pyrroline (2-AP)

Nutritional Value Amount of amino acid, vitamin and mineral