Studies on shedding habit of rice plant. I. Morphology of separation zone.

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Abstract
Relation between the strength of shedding resistance and morphology of separation zone (SZ) were investigated. Varietal differences in the morphological features of separation surface were observed with a scanning electron microscope. Separation in the abscission layer (AL) formed a "separation pit" at the base of rachilla and a "separation pileus" at the terminal of pedicel. Although diameters of separation pileus, vascular bundle and pedicel did not always relate with the strength of resistance, the height of pileus showed a highly negative correlative with the strength; whereas separation angle showed a very high positive correlation. In easy shedding varieties, separation started at the marginal portion of AL and proceeded inward with the progress of maturation and dehydration. The separation was rather noticeable on the abaxial side of AL. The cell walls of AL were thinner and melted more severely at the marginal portion. In shedding resistant varieties, however, formation of AL was obscure and separation usually occurred at a curved portion of pedicel by its breaking. Inferior spikelets as compared with superior ones had a poorly developed morphological structure of SZ and showed better separation of AL and weaker shedding resistance. A sclerenchymatous fiber tissue developed around the vascular bundle sheath. The development of the fibrous cell wall was remarkable in shedding resistant varieties. Pits were seen in the walls between abscission cells and adjacent cells. Density of the pits was lower in lesser shedding resistant varieties. In the wild species, the formation of AL extended far into the bundle sheath region and the development of fibrous tissue was very poor. Shedding, in general, may be regulated by a relative extent of the AL formation and the development of sclerenchymatous fiber tissue, though it depends upon the mechanical strength of fibrous tissue in the case of resistant varieties.

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