Influence of Photosynthetically Active Radiation on Grain Density of Rice 1

Increasing the number of high density (HD) grains can increase the grain yield of rice (Oryza sativa L.). Studies were conducted on the effect of different levels of photosynthetically active radiation (PAR) on the density of grains. ’IR36’ was grown throughout its life cycle at 195, 390, and 780 ~mol m-2 s-j in the growth cabinets of the phototron. There was an increased production f spikelets and HD grains (specific gravity >1.20) as PAR increased. The low density (LD) grains also increased as PAR increased. When plants grown previously under natural light were xposed to different PAR after anthesis, the HD grains increased at higher PAR, while the LD grains decreased as PAR increased. The percentage contribution of HD grains to total grain yield emerged as a major determinant of grain yield; thus, cultivars possessing a higher proportion f HD grains would be advantageous even under low PAR. Individual grain weights were higher when plants were xposed to higher PAR after anthesis. The weight of individual grains howed a wider ange at lower specific gravities (1.16 to 1.20) than at higher specific gravities (> 1.20). Spikelet and grain number per hill showed high positive correlation with shoot weight per hill. High density grains had a positive but lower correlation with shoot weight, indicating a possible limitation in production r transport of assimilates a HD grains were not realized in all the filled spikelets regardless of PAR level. High PAR from flowering to harvest can greatly increase the number of HD grains, which contributes to higher grain yield. Additionalindex words: Anthesis, Grain yield, High density grain, Oryza satlva L. , Specific gravity. of potash. Two sets of experiments were conducted, each treatment having 15 pots with three uniformly grown plants (45 hills per treatment). One set was subjected to 195, 390, and 780 ~mol m-2 s-~ PAR throughout the crop period (from 10-day-old seedlings to maturity). Photosynthetically active radiation was varied in growth cabinets of the IRRI phytotron by adjusting plant distance from a combination f high discharge mercury lamps, fluorescent lamps, and incandescent bulbs. The Li-Cor (Lincoln, NE) quantum radio/photometer model LI-185B was used to measure PAR (photon flux density). The second set was maintained in the glasshouse under natural conditions until flowering. The average daily irradiance was 14.8 MW m-2. The pots were transferred to growth cabinets of the phytotron after anthesis and grown under the PARs: 195, 390, and 780 zmol m-2 s-~. The temperature of both sets was 29 °C during the day and 21 °C at night on a 12-h basis. Relative humidity was mainrained at 70%. Shoot weight, spikelet number, and grain number were recorded. Grain density was determined bythe specific gravity method (7, 13). High density grains had SG > 1.20, and low density (LD) grains had SG < 1.20. Statistical analysis identified significant mean differences. Correlations between shoot weight and number ofspikelets, grains, and HD grains were determined. H IGH DENSITY (HD) grains are those grains of cultivar that remain submerged in a solution of specific gravity (SG) > 1.20. Increasing the number of HD grains is one approach to overcoming the present yield barrier in rice (Oryza sativa L.). High density grains not only result in higher volume and weight (13), but also give higher milling and head rice recovery (11). Therefore, increasing the proportion of grains would enhance yield potential and milling quality. Grain yields are positively correlated with photosynthetically active radiation (PAR), especially during the reproductive and ripening phases (1, 3). The specific effect of PAR on grain density, however, has not been studied. This study evaluates the effect of PAR on grain density in order to characterize and quantify the nature of its influence. The relationship of shoot weight to number of spikelets, grains, and HD grains was examined under different PAR levels to assess the physiological basis of HD grain production. MATERIALS AND METHODS Seeds of rice ’IR36’ were planted in 4-L pots, and the seedlings were thinned to three plants per pot when they were l0 days old. Three kilograms of Maahas soil (Andaqueptic Haplaquolls) were placed in each pot with 4 g ammonium sulfate, 2 g single superphosphate, and 2 g muriate ~ Contribution from the Int. Rice Res. Inst. (IRRI), P.O. Box 933, Manila, Philippines. Received 6 Oct. 1986. 2 Visiting scientist, olant physiologist and head, and assistant scientist, resl~ectively, lalant Physiology Dep. , IRRI, P.O. Box 933, Manila, Philippines. Published in Crop Sci. 27:1210-1214 (1987). RESULTS AND DISCUSSION Decreased PAR during the reproductive phase decreases yield, primarily because of fewer filled spikelets (4, 5, 6, 9, 10). Photosynthetically active radiation one major climatic factor that influences crop productivity, and information on its effect on grain density would help explain the nature of HD grain production. Number of Grains of Different Densities Total spikelet number increased from 365 to 825 per hill as PAR increased from 195 to 780 umol m -2 s-~ throughout he crop period (Fig. 1). Although the number of HD grains increased with PAR, their percentage did not increase appreciably because of the larger total number of spikelets produced with higher PAR. Low density grains also increased, and there were more empty spikelets as well at higher PAR. Production of grains of different densities differed when plants were exposed to varied PAR only from anthesis until maturity (Fig. 1). Their proportion increased significantly with increased PAR. The LD grains decreased significantly when PAR was increased, as did empty spikelets. Weight of Grains of Different Densities Plants exposed to artificial light throughout their life cycle showed an increasing contribution to grain weight from HD grains with increasing PAR (Fig. 2). The grains comprised the majority of the grain yield at every PAR level. When the plants were exposed to different PARs 1210 Published November, 1987