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Reproductive Stage Water Stress and Sterility. I. Effect of Stress during Meiosis¹

Increased spikelets sterility is the primary component of yield reduction in rice (Oryzas ativa L.) and other small grains subjected to water stress during reproductive growth stages. The causes of water stress induced spikelet sterility are not clear although the meiotic stage of pollen mother cells is often reported to exhibit yield-decreasing sensitivity. We exposed rice plants, in which pollen mother cells were actively undergoing meiosis, to the following series of water stress levels as measured by flag leaf water potential: –1.6, –1.9, –2.2, –2.5, –2.8, and –3.5 MPa for 18 to 24 h. Panicles were fixed in Farmer's solution and later examined microscopically for abnormal chromosomal behavior. The occurrence of chromosomal abnormalities such as univalents, lagging of chromosomes, noncongression in metaphase and micronuclei was evaluated in 3822 pollen mother cells in response to increasing water stress levels. Spikelet fertility of comparable panicles grown to maturity decreased linearly from 80% at –1.1 MPa (control) to 50% at –3.5 MPa flag leaf water potential. Chromosomal abnormalities appeared at relatively moderate stress levels (–1.1 to –1.9 MPa) and in general peaked at the –2.2 MPa stress treatment.

In general, water stress increased chromosomal abnormalities, except at severe stress levels (–3.5 MPa) where the meiotic process was partially arrested. Each meiotic stage had particular irregularities associated with water stress. Prophase I and II, univalents; Metaphase I, noncongression of bivalents and lagging chromosomes; Anaphase I and II, lagging chromosomes; Telophase II, micronuclei formation. Blasted or immature, small, colorless spikelets also were observed and their occurrence increased with stress level. Blasted spikelets, not included in the calculation of percent sterility, accounted for 10 to 20% of the total spikelets per panicle at moderate stress levels. The cause of this malady is unknown.

Key Words: Rice (Oryza sativa L.) • Chromosomal abnormalities • Micronuclei • Blasted spikelets

² Former research assistant and crop physiologist, respectively, Int. Rice Res. Inst., Los Bañs, Laguna, Philippines. Dr. O'Toole is presently located at the Texas Agric. Exp. Stn., P.O. Box 6112, Temple, TX 76503-6112

Received for publication March 25, 1985.

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