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Research on response of direct seeded rice (Oryza sativaL.) to interactions of seeding rate and N rate is very limited, although considerable work has been done in the tropical countries with transplanted rice. Understanding responses by U.S. rice cultivars to these factors is important not only for optimizing management, but also because it provides information concerning the need for further improvement of plant type. Therefore, our objective in this study was to determine what factors might limit performance of ‘Starbonnet’ rice when subjected to a range of seeding and N rates. This study was conducted in field plots for 2 years on a Crowley silt loam soil (Typic Albaqualf) at the Univ. of Arkansas Rice Branch Experiment Station, Stuttgart, Ark. A split plot design was used. Time of N application (preplant or topdress) was the main plot and all possible combinations of seeding rates of 67, 135, and 303 kg/ha and N rates of 67, 135, and 202 kg/ha were the subplots. All treatments were replicated four times. The following measurements were taken: grain yield; stand counts prior to tillering and at maturity; blanks per panicle; florets per panicle; dry weight at heading and maturity; percent N in the flag leaf at heading; leaf area index (LAI) at heading Excessive vegetative growth before anthesis limited grain yield from dense plant populations. This was evidenced by high LAI at heading, reduced dry matter accumulation after heading, and adequate N levels in the flag leaf under conditions of dense population. Further indication of the effect of excessive vegetation prior to anthesis was shown by the fact that number of florets per panicle had more influence on grain yield than did blanks per panicle. Increasing the N rate from 135 to 202 kg/ha increased grain yield only for the 67 kg/ha seeding rate in 1972. Time of N application (preplant or topdress) did not influence grain yield. Under the direct seeding conditions of this study initial plant population, rather than tillering, accounted for the number of panicles per unit area. As seeding rate and plant population increased, number of florets and blanks per panicle decreased. Increasing the N rate increased both florets and blanks per panicle. Both increased seeding rate and N rate resulted in increased LAI. Under the conditions of this test the major factors that limited yield under dense population appeared to be light and CO2, since extra N fertilizer was ineffective.

In recent years an expansion of rice (Oryza sativaL.) acreage in Arkansas has resulted in rice being produced on soils with a history of cotton production. Most of these cotton soils have had repeated applications of monosodium methanearsonate (MSMA) as a herbicide. There is some evidence that arsenical residues in the soil can lead to sterility in rice. In an effort to answer this question, we conducted a field experiment on a Crowley silt loam soil (Typic Albaqualf) in 1975 to evaluate the influence of MSMA on plant growth and yield of rice. Treatments included two water management regimes (continuous flood vs. one drain and dry), three levels of MSMA [0, 1.1, and 11.2 kg/ha of arsenic (As)] and 15 rice cultivars and experimental strains differing in susceptibility to straighthead disease (abnormally developed or sterile flowers resulting in reduced grain yields). Soil samples taken 65 days after arsenic application showed that As remained in the rice root zone. Vegetative growth prior to panicle initiation was not affected by any of the treatments; however, visual observations made near maturity showed that where MSMA had induced sterility, the plants were dark green suggestive of straighthead. Reproductive growth as shown by panicle weights and sterility ratings was affected most by high MSMA rates and continuous flooding. Under continuous flood, panicle weights were reduced and sterility ratings increased as MSMA rate increased. This effect was more pronounced in cultivars most susceptible to straighthead and was minimized by draining the flood and drying the soil. This data suggests that the MSMA residues induced straighthead. A significant and similar relationship was found between panicle weights and sterility ratings at maturity for both water management regimes.

Root growth dynamics of rice (Oryza salivaL.) under submerged soil conditions are poorly understood. Two field experiments were conducted in 1986 to compare root growth of rice at two locations on different soils. The rice cultivar Bond was seeded in leveed plots on a Captina silt loam (fine‐silty, siliceous, mesic Typic Fragiudults) at Fayetteville, AR, and on a Crowley silt loam (fine, montmorillonitic, thermic Typic Albaqualfs) at Stuttgart, AR. Plots contained rectangular Plexiglass root observation tubes oriented at a 45° angle from the soil surface and extending 0.57 m (Captina) and 0.44 m (Crowley) below the soil surface. Root recordings were taken during vegetative and reproductive growth. Root growth was most rapid during vegetative growth at both locations, with maximum root length occurring at panicle initiation. Root lengths either plateaued or declined during reproductive growth. Root lengths were greater on the Captina soil than on the Crowley soil. The pattern of root length densities (RLD) was similar between soils; however, RLDs were generally lower on the Crowley soil. During early stages of rice growth, a greater percentage of rice roots was measured in the upper 20 cm of the Crowley soil. By harvest, about 50% of the rice roots occurred in the top 20 cm in both soils. Harvesting appeared to reduce the percentage of roots found below 20 cm on the Crowley soil, and to have little effect on the Captina soil. Results suggest that rice root growth can vary dramatically between soils, and is influenced by soil profile characteristics, soil moisture, air temperatures, and growth stage.

Field studies were conducted with rice (Oryza sativaL.) on Crowley silt loam soil using the important commercial cultivars ‘Bluebelle,’ ‘Nova 66,’ and ‘Starbonnet.’ We studied the effects of rate and timing of mid‐season N applications on grain yield, plant height, lodging, head‐rice yield, and grain weight (g/M) of varieties (cultivars) differing in maturity and plant type. All plots received 45 kg/ha of N as ammonium sulfate 15 days after seedling emergence, just before initial flooding. The remaining N needed to give total rates of 78, 112, or 146 kg/ha was applied to drained soil at specified intervals near midseason. Fifteen main stems, selected at random from the outside rows of each plot at the time of mid‐season N application, were used for determining internode lengths. Maximum grain yields were associated with N applied at median internode lengths averaging 21.0, 58.5, and 5.0 mm for Bluebelle, Nova 66, and Starbonnet, respectively. Delaying mid‐season N applications until these respective stages of plant development, resulted in shorter plants and less lodging, accompanied by increased grain weight and head‐rice yields. Variations in the median internode length at N fertilization, timed for a combination of maximum grain yield and minimum plant height and lodging, were closely associated with plant type. Starbonnet and Bluebelle, which have short, stiff straw and erect leaves, responded better to N applied at a shorter internode length than Nova 66, a taller, broader‐leaved variety. When N was applied too early, Nova 66 produced considerably more excessive vegetation than did Bluebelle and Starbonnet. Both Starbonnet and Bluebelle have plant types which approach the one currently favored by many plant breeders.

‘Bluebelle’ cultivar of Oryza sativaL. was grown in a growth chamber at 4 and 16, 10 and 21, 16 and 27, and 21 and 33C night (10 hr) and day (14 hr) temperatures, respectively. Flood water was applied 12 (early) or 24 (late) days after seedling emergence at 6‐ and 12‐cm depths to seedlings growing in saline, alkaline, and normal Crowley silt loam soil. Plant tops were harvested 53 days after seedling emergence and analyzed for dry matter production and composition of P, K, Mg, Na, Fe, Mn, and Zn. Water samples were collected at the soil‐water interface 1, 2, 5, 7, 8, 9, 10, 11, 14, and 15 days after seedling emergence and analyzed for pH, electrical conductivity, CO3‐, HCO3‐, and C1‐. When temperature was increased, the order of dry matter production by plants growing in the various soils was normal > saline > alkaline. Applying flood water 12 days after seedling emergence caused plants growing in saline and normal soils to produce more dry matter, whereas those growing in alkaline soil produced more dry matter when flooded 24 days after seedling emergence. Varying flood depth did not affect growth. Plants growing on saline and alkaline soils at 21 and 27C were chlorotic and P/Zn was less than 1.0. When temperature was increased to 31C chlorosis disappeared and P/Zn was greater than 1.0; however, the data did not explain the effects of flood time on growth. Bicarbonate level of water above the alkaline soil was higher when flooding was delayed. This suggested young seedlings were more sensitive to HCO3‐than were older seedlings, since growth was greater with delayed flooding. Flooding saline soil early produced lower EC and Cl‐values in the water than did late flooding. Thus, plants grew more when flooded early because salinity was lower.