Abstract:
Rice varieties with higher yield potential can be developed by increasing the number of grains per area, by increasing the grain weight, or by a combination of both. Our studies on the structure, histochemistry, grain filling, and response to plant growth regulators (PGRs) have clarified certain unique features of the spikelet of rice grains and have identified strategies required to enhance the weight of rice grains, The most important barrier to grain weight is the space limitation imposed by the fertile glumes, the palea and lemma within which the caryopsis should develop. Experiments with intact plants and excised spikelets reveal that most PGRs, singly or in combination, do not influence the growth of glumes. Brassinoloide (BR) promotes growth of the palea and lemma when applied during the panicle initiation stage. BR at 10 sup(-7) M and benzylaminopurine (BAP) at 10 sup(-5) M applied together as a soaking spray increase grain size and dry weight by 39 percent. This is achieved through promotion of cell size and cell number, of both the palea and lemma, and the caryopsis within. The aleurone cells of the treated grains increase to 160,000 from 75,400 in control grains. Strategies for yield improvement could aim at increasing the dry grain weight, either by (1) breeding rice varieties with a larger palea and lemma or by (2) producing varieties that respond to PGR application by rapidly increasing the size of the palea and lemma.
