A manual of rice seed health testing - IRRI books - International Rice ...

CHAPTER 2
The rice plant and its environment
J.K. Misra, G.C. Loresto, and T.W. Mew
Oryza sativa L., the most commonly
cultivated rice species, belongs to
subfamily Pooideae of the grass family
Poaceae. The genus Oryza is divided
into several sections. O. sativa
comes under section Sativae and its
series Sativa. O. sativa is a diploid
plant (2n=24). O. glaberrima Steud.
(2n=24), anothcr Oryza species, is
cultivated in West Africa. The two
species have minor morphological
differences, but terms discussed here
apply to both. Both are annual
grasses adapted to habitats ranging
from aquatic to dry land.
Seedling morphology
Grains of nondormant varieties can
germinate immediately after ripening.
During germination, the
coleorhiza emerges with the radicle
first (Fig. 2.1a,b) in an aerated ervironment;
the coleoptile protrudes
first (Fig. 2.1 c) in a submerged medium.
The seedling soon develops
two or more secondary seminal
roots. As growth progresses, a secondary
system of adventitious roots
(Fig. 2.1d) originates from the first
node and replaces the seminal roots.
The cylindrical coleoptile encloses
the primary leaf. The mesocotyl between
the root and coleoptile grows
to push out the coleoptile above the
ground. The primary leaf, emerging
out of thc coleoptile, is green and
cylindrical, with no blade. The second
leaf differentiates into sheath,
blade, ligule, and auricles.
2.1 Oryza sativa
germination.
a, b. Coleorhiza
emerges, radicle
first in an aerated
environment.
c, d. Coleoptile
protrudes first in a
submerged environment.