**4. Mechanism of kernel elongation**

Although the mechanism of kernel elongation of rice is not clear. Water uptake might be the only trait showed positive and significant influence on kernel elongation. The anatomical study of the endosperm of elongating and non elongating varieties showed variation in the shape and arrangement of cells. Elongation pattern might be influenced by either size and arrangement of cells in endosperm or structural arrangement of starch molecules within cells and in non elongating rice, long belt of radially arranged cells might restrict linear expansion of cells while cells in the intermittent region may tend to expand towards the periphery leads to breadthwise elongation [12]. Aging also influences the kernel elongation [13].

Both genetic and environment factors influence the kernel elongation. The ambient temperature of about 25°C during day time and 21°C during night at the time of ripening is a favourable condition for maximum elongation [14]. Presoaking before cooking also play major role in kernel elongation [15].

Grain width and length, endosperm cell size and starch granule structure and arrangement are influenced the cooked kernel elongation. No specific mechanism to determine kernel elongation, but it might be due to physical and chemical phenomenon occurred after cooking [16].

Soaking of grains upto 30 min increases kernel elongation during cooking and leads to grain cracking. Elongating and non-elongating rices showed no difference in the composition of endosperm cell walls.

In basmati rice, equidistant pentagonal or hexagonal cells arranged in honeycomb fashion in the elongating variety and in breadth wise swelling types, long, rectangular and arranged radially in columns extending from the centre to the periphery. Microscopic examination of endosperm sections of the experimental materials revealed differences between elongating and non elongating groups of rice in respect of shape and arrangement of cells as represented by Sood et al. in **Figure 1**

#### **Figure 1.**

*Anatomical study of elongating and non elongating rice kernels. (1, 2) Transverse sections of elongating and non elongating types (3, 4) Longitudinal sections of elongating and non elongating types. Source: Sood et al.*

*Breeding for Grain Quality Improvement in Rice DOI: http://dx.doi.org/10.5772/intechopen.95001*

Internal kernel cracks were higher in aged rice kernels than the non-aged rice kernels in three popular Malaysian rice cultivars (Mahsuri, Mahsuri Mutant and Puteri). The number of cracks in kernel during cooking increased at higher temperature (110°C) compared to lower temperature (90°C). Kernel aging and temperature positively associated with internal physical structure of the rice kernel. From the above study, it is concluded that ageing and internal kernel cracks might influence the cooked kernel elongation [17].
