On Abaxial and Adaxial Sides of Rice

Abstract

Considering from the spikelet arrangement on panicle of rice plant, paddy or rice kernel may have two different flanks, i.e. abaxial side and adaxial side. The differences in these two sides were investigated mainly upon shape, hardness and ripening of the kernels of three rice varieties (Norin-6, Norin-8 and Fujisaka-5). The results and discussion are summarized as follows: 1) In many cases, both dorsal and ventral ridges of the paddy or kernel were seen indented towards the adaxial sides. And on both flower glumes and kernel of very young dry paddy, some-what crescent shapes with their adaxial sides concaved were observed on the cross section. 2) When observed on the cross section, each of the flanks of kernel has two protuberant parts: one close to the dorsal region and another close to ventral region. And the dorso-adaxial and ventro-abaxial protuberances were considerably biased towards dorsal and ventral sections respectively, showing well development of starch tissues in these parts. Above all, the most remarkable growth of tissues was seen around the dorso-adaxial region, so that the distortion of kernel was also clear as observed from the base or top of kernel. 3) Of young kernel, the vascular bundle was seen exactly on the dorsal ridge. But the ridge of mature kernel was formed frequently biasing a little to the abaxial side of the bundle, presumably on account of an extensional growth of tissue prevailing around the dorso-abaxial region. 4) Distortion in the dorsal part was first seen in the kernels of early ripening stage, and it became more distinct in the older kernels. On the other hand, the ventral distortion seemed to occur about 10 to 15 days after flowering. 5) Completion of ripening which was investigated by observing degree of translucency especially in the peripheral starch tissues of kernel proceded was earlier in the adaxial side than in the opposite side. 6) Results of hardness tests conducted on cross sections of kernel with a Vickers micro-hardness tester revealed that the region around the center was much harder than the periphery, and moreover that the adaxial region was harder than the abaxial region to some extent. It is presumed that the biased shape of flower glumes and unbalanced kernel growth incur the distortion of the kernel, which become clearer with the advance of maturation and is featured especially by the well growing of dorso-adaxial region, and that the adaxial region of kernel as a whole is superior to the opposite region in the ability of starch filling in the middle to late stages of ripening.