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- Author or Editor: Peter A. Jolliffe x
Effective gas flow between plant materials and their surrounding atmosphere is necessary to enable controlled atmospheres to act on stored produce and maintain produce quality. Gas flow can be motivated in two ways: by component gas concentration (i.e. partial pressure) gradients and by total as pressure gradients. Varying the total gas pressure about stored plant material should cause gas flow which supplements that induced by concentration gradients alone. Novel measurement techniques were developed to explore the effects of varying atmospheric pressure and gas composition on gas transfer rates. In apple fruit, gas transfer rate undergoes a several-fold increase with small pressure fluctuations. There is a direct increase in gas transfer rate as the amplitude of pressure variation increases, and optimum periods of oscillation fall in the range of 20 to 60 seconds. Apples, onions, and other commodities with large internal gas volume and intermediate peripheral resistance to bulk gas flow, seem to be most responsive to total pressure variation.
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.
Sunscald was induced by exposing fruit of tomato (Lycopersicon esculentum mill.) to intense solar radiation; similar injury was caused by radiation from incandescent lamps. Injurious radiation treatments caused fruit temperature to exceed 40°C and altered fruit respiration rates. High air temperatures enhanced injury, but exposure to 0 to 100% O2 concentrations during radiation treatments had little influence on fruit response. Infrared wavelengths (>0.7 µm) were effective in inducing injury. Tissue water may serve as an important absorber of radiant energy. Overheating of the fruit appeared to be the main cause of injury, and storage at different temperatures, photoperiods, or O2 levels did not reverse injury induced by previous irradiation.
Previous research showed a relationship between viability and electrolyte leakage; here, we develop an index of viability based on electrolyte leakage from sample populations of seeds soaked in deionized distilled water. Conductivity of leachates from individual seeds was determined for 10 lots of lettuce (Lactuca sativa L.), each germinating at 99%. Conductivity data for two lots of soybean (Glycine max L.) seeds germinating at 100% and 74%, respectively, were obtained from literature. Cumulative frequency distributions (CFD) with a class interval of one µA, were fitted with a natural logarithmic form of the Richards function, which requires no arbitrary starting values. The procedure provided an effective estimation of slopes [(dCF/dµA)MAX] of hypothetical lines tangent to inflection points of the respective sigmoidal CFD curves. We suggest that this maximum slope, or internal slope can be used as a seed viability index. The index is unaffected by outlier µA readings and reflects the shape of the CFD. It is also a measure of seed-to-seed variability in leachate conductivity. The se of the 10 internal slopes derived from the 10 lettuce seed lots was 3.8. The viability index is sensitive, since nearly a four-fold difference in internal slope was found for the two soybean seed lots. The greater the internal slope, the less the variation among individual seed conductivities and the higher the seed quality.