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C.A. Beyrouty1*, R.J. Norman1, B.R. Wells1, N.A. Slaton1,
B.C. Grigg1, Y.H. Teo1, and E.E. Gbur2
We have conducted a series of controlled environment and field studies over the past 12 years to understand the growth dynamics of paddy rice roots. These studies have used a combination of destructive and non-destructive methods for obtaining information about root growth as affected by cultivar, soil, water and fertility. Previously, very limited information was available from scientists in the United States that provided descriptions of paddy rice root growth. Yet, scientists acknowledge the importance of rice roots to nutrient and water uptake and to yield. However, until recently, the distribution of rice roots within the soil profile and the relationship of root growth to plant development was poorly understood.
Using the minirhizotron micro-video camera technique, we have developed a description of the seasonal pattern of paddy rice root growth under the culture practiced in Arkansas. Root growth, measured as root length, is rapid and linear during vegetative development of the rice plant. Maximum root length is observed by panicle initiation or booting and is maintained at nearly a constant level until heading. Following heading, root length declines until milk stage where it may remain at a reduced level or may increase by physiological maturity.
In our studies, less than 2% of the total rice root length was measured below the 40 cm soil depth. In addition, nearly 90% of the total root length produced by the plant and the corresponding N, P, and K uptake occurred within the upper 20 cm of the soil profile. The half distance between root axes was 0.1 cm at the 0- to 5-cm soil depth and between 0.3 and 1.1 cm at the 35- to 40-cm soil depth.
Using a mechanistic model, we have been able to predict N, P, and K uptake during vegetative growth for three rice cultivars grown under lowland irrigated conditions in the greenhouse and field. Sensitivity analysis showed that on a Crowley silt loam, root competition, soil solution concentration, and the soil buffering capacity had the greatest impact on N and P uptake. In contrast, K uptake was most affected by Imax , root radius and the Michaelis-Menten constant. Thus, under the conditions of our study, N and P uptake appear to be affected by a combination of plant and soil factors while K uptake appears to be most affected by plant factors. As a result, we have focused much of our recent attention on identifying cultivar differences in Imax for K and developing an understanding of the influence of environmental conditions on modifying values of Imax for a given cultivar.
The model assumption that nutrient movement to roots is primarily by mass flow and diffusion was verified in a field study on a silt loam soil for paddy rice. In the 0- to 5-cm soil depth, mass flow accounted for 9, 14, and 32% of NH4+, P, and K movement to roots, respectively, while diffusion accounted for 91, 86, and 68% of the movement of NH4+, P, and K, respectively. Contact exchange supplied negligible quantities of these nutrients. Mass flow has a greater influence on K uptake on submerged soils than is typically found on upland conditions.
However, our calculations comparing the amount of a nutrient supplied by mass flow with the respective Imax for that nutrient show that mass flow is not sufficient to supply enough NH4+, P, or K to the root to satisfy plant uptake.
Based upon effective diffusion coefficients, we found that the distance of NH4+, P, and K diffusion per day in the top 5-cm of soil was 0.47, 0.23, and 0.44 cm, respectively. Competition for a nutrient between roots occurs if the nutrient depletion zone around an individual root overlaps with the depletion zone of an adjacent root. This occurs when the distance between adjacent roots narrows and the diffusion distance of a nutrient increases. The half distance between roots in the top 5-cm of soil was less than the diffusion distance in a 24 hour period and thus, the potential competition for nutrients between adjacent roots would be high. Root growth has been shown to be affected by water and nitrogen management. However, alteration of root growth and distribution within the soil profile has not always resulted in significant changes in rice grain yield. Compared to rice subjected to the normal flood regime, a 14-day delay in the application of floodwater to rice resulted in a reduction in root length density between panicle initiation and booting. However, following application of a flood, root length density recovered to the levels measured for the control by anthesis. Grain yields were slightly lower for delayed flooded rice. We have shown that this reduction in root length density as a result of delayed flooding can be prevented by an earlier than normal application of nitrogen to the soil. In addition, any yield differences resulting from delayed flooding are removed with the earlier nitrogen application.
Removal of the flood water following heading did not appear to affect grain yield or grain quality. Thus, it would appear that the rice cultivars in the U.S. require the presence of a flood during reproductive development. Reduction in water inputs during vegetative development and following heading may be a feasible practice to conserve water resources and allow for additional land application of fertilizers and pesticides.