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Reproductive Stage
The reproductive stage is characterized by culm elongation, a decline in tiller number, booting, emergence of the flag leaf, heading and flowering. The reproductive stage usually lasts approximately 30 days in most varieties. This stage is sometimes referred to as the internode elongation or jointing stage and varies slightly by variety and weather conditions.
1. Panicle Initiation (PI) - is the time when the panicle premordia initiate the production of a panicle in the uppermost node of the culm. At this point, the panicle is not visible to the naked eye. It is sometimes referred to as the green ring stage in rice (Figure 1-5). A thin green band is visible just above the top node, and represents the very beginning of internode elongation and is evident for only a couple of days
2.Internode Elongation - begins about the time panicle initiation is occurring, continues until full plant height is reached and is followed by heading. The top five internodes may elongate. This stage is also referred to as the “jointing stage” (Figure 1-6). Stem internodes can be distinguished from root internodes by the green color in the stem wall.
3.Panicle Differentiation (PD) - is closely associated with “jointing” or the internode elongation stage. PD is roughly equivalent to 1/2 to 3/4 inch internode elongation (Fig 1-6).
At this point, the panicle is 1 to 2 mm in length and the branching of the panicle is visible (Figure 1-7). This is a critical stage during rice plant development. At this stage, the environment can have a major effect on rice plant development. The second yield component, number of potential grains per panicle, is set by the time this development stage occurs.
4.Booting - this stage is loosely defined as that period characterized by a swelling of the flag leaf sheath which is caused by an increase in the size of the panicle as it grows up the leaf sheath. Full or late boot occurs when the flag leaf has completely extended. Booting is the stage in which meiosis occurs. Environmental stress during this stage may reduce rice grain yield. Late boot occurs about 6 days prior to heading. This date can be predicted by the DD50.
5.Heading - is the time when the panicle begins to exsert from the boot. (Figure 1-8 shows common types of panicle exsertion.) Heading may take over 10 to 14 days due to variations within tillers on the same plant and between plants in the field. Agronomically, “heading date” is defined as the time when 50 percent of the panicles have at least partially exserted from the boot.
Some panicles may never emerge completely from the boot. The Arkansas DD50 Management Program uses this same definition of “heading date.”
6. Anthesis - or flowering (Figure 1-9) refers to the events between the opening and closing of the spikelet (floret) and lasts for 1 to 2 1/2 hours. Flowering generally begins upon panicle exsertion or on the following day and is consequently considered synonymous with heading. Anthesis generally occurs between 9 a.m. and 3 p.m. in Arkansas. The steps involved are:
a.The tips of the lemma and palea (hulls)
open.
b.The filaments elongate.
c.The anthers exsert from the lemma and
palea.
d.As the lemma and palea open further, the
tips of the feathery stigma become visible.
e.The filaments elongate past the tips of
lemma and palea.
f.The spikelet closes, leaving the anther
outside. Anther dehiscence (pollen shed)
usually occurs just prior to or at the time
the lemma and palea open (step a).
Pollen grains are viable for about five minutes after emerging from the anther, whereas the stigma may be fertilized for three to seven days. Rice is a self-pollinating plant, because it is usually pollinated before the lemma and palea open to release pollen into the air. Fertilization of the ovary by the pollen grain is generally completed within five to six hours after pollination; at that point, the ovary becomes brown rice.