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Abstract

Currently, median seed survival period (P₅₀) is a seed storage index estimated from a probit model Y = a + bX, where Y = probit transformed viability proportions and X = in effect seed lifetimes; therefore, P₅₀ = (5 − a)/−b, where 5 is the probit of 50% germination. Since use of the probit model is based on the assumption of normally distributed seed “lifetimes,” our purpose was to eliminate this restriction by means of a new substitute model. Pearl millet [Pennisetum americanum (L.) Schum.] seeds were stored under constant conditions at 21°C and seed moisture contents of 18% or 14%. The seeds stored at 14% moisture content exhibited a statistically normal distribution of lifetimes, while those stored at 18% moisture content underwent rapid viability loss concomitant with a statistically non-normal, positively skewed distribution. Several empirical models other than the probit were used to describe deterioration of both populations; however, we propose a nonlinear four-parameter Weibull distribution for seed lifetimes. A useful property of the function is flexibility in modeling data, which may be normally distributed or positively skewed. Parameter initialization is facilitated by plotting the data on paper with transformed scales. Weibull parameters were used to generate seed mortality rate curves, P₅₀, and a new storage index, M/MV, where M is mode and MV is modal value. Inaccuracy in estimating initial seed mortality (Y intercept) is a negative feature we found associated with most of the empirical models investigated, including the Weibull. Evidence is presented to show the extreme dynamics of the system in the vicinity of the Y intercept, particularly in the case of seeds with 18% moisture content, which had deteriorated after 4 months of storage at 21°C and 90% RH.

A simulation model was built as a decision aid for management of five weed species in direct seeded irrigated onion (Allium cepa L.). The model uses the state variable approach and simulations are driven by temperature and sunlight as photosynthetically active radiation (PAR). It predicts yield reduction caused by competition for PAR according to the ratio of crop leaf area index (LAI) to weed LAI and respective light extinction coefficients (k). Input variables are plant density by species and average number of leaves by species. Number of leaves per plant is used by the model to provide an estimate of initial leaf area per plant. The model calculates initial species LAIs by multiplying species density times average leaf area per plant. The model accurately describes competitive interactions, taking into account respective plant densities, time of emergence, and time of weed removal. It permits economic evaluation of management factors such as handweeding, chemical weed control, herbicide phytotoxicity due to early application, and control of weed flushes during the season. The model is also used to evaluate mechanisms of plant competition for sunlight. In a sensitivity analysis, onion yield loss was more sensitive to weed PAR interception than to PAR use efficiency, the latter a species-dependent constant in the model.

Abstract

Effects of deletion and enhancement of 280–315 nm ultra-violet (UV-B) radiation on pea (Pisum sativus L.), potato (Solanum tuberosum L.), radish (Raphanus sativus L.), and wheat (Triticum aestivum L.) plants were examined at a 3000 m elevation field site, which provided a 49% increase in midday effective (biologically weighted) UV-B radiation relative to near sea level. Wheat plants grown under Aclar or cellulose acetate film, which transmit UV-B radiation, were shorter than plants grown under Mylar film, which excludes UV-B radiation. Effective UV-B radiation levels at solar noon in the open and under the Aclar and cellulose acetate were 61, 55, and 44 mW m⁻², respectively. There were no significant differences in potato, radish, and wheat dry weights attributable to UV-B radiation among the above treatments. The dry weights of all 4 species and pea and wheat plant height were not significantly decreased by a supplemental enhancement of 32 mW m⁻² effective UV-B radiation from cellulose acetate filtered FS-40 sunlamps for 6 hours each day, relative to control plants that received only solar UV-B radiation. The results suggest that wheat plant height is more sensitive to deletion of UV-B radiation than the other parameters measured, and that the 4 species studied can tolerate the level of enhanced UV-B radiation used with concomitant high levels of visible radiation with no significant reduction in dry weight accumulation.