Two experiments were conducted to study the effects of a high dietary potassium intake on magnesium metabolism. In experiment 1, eight 3-day balance trials were conducted with 12 crossbred wether lambs. Rations containing equal amounts of magnesium, calcium and sodium and 0.6 or 4.9% potassium were fed. Apparent absorption of magnesium was greatly depressed (P < .05) while apparent absorption of sodium and potassium were generally increased by the addition of 100 gm. of potassium bicarbonate to the ration. There was a trend for higher apparent absorption of calcium by the high-potassium fed lambs. Although blood serum calcium, sodium or potassium showed no distinct trends, there was a trend toward lowered serum magnesium levels after 14 days of potassium supplementation, which disappeared after 27 days. In experiment 2, six paired yearling wethers were fed two rations identical in ingredient composition to those used in experiment 1. All wethers received an intravenous dose of radioactive magnesium²⁸ and magnitude of fecal and urinary magnesium²⁸ excretion was determined and magnesium turnover rates calculated. The high-potassium fed wethers tended to secrete smaller amounts of magnesium²⁸ in the urine and slightly greater amounts in the feces. The high-potassium fed wethers had lower magnesium turnover rates indicating that high potassium levels interfere with magnesium absorption rather than drastically increase the excretion into the intestine. / Master of Science

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Typescript. / Bibliography: leaves 132-142. / xii, 142 l illus

Three experiments were conducted to study the effects of dietary potassium intake on magnesium metabolism. In experiment 1, 24 wether lambs were used in two metabolism trials to determine the minimum level of potassium that depresses magnesium absorption. Two magnesium levels (.1 and .2%) were fed with four levels of potassium (.6, 1.2, 2.4 and 4.8%) in a 2 x 4 factorial arrangement of treatments. Each trial consisted of a 10-day preliminary period followed by a 10-day collection period. Greater amounts (P<.05) of magnesium were absorbed and retained when animals were fed the high level of magnesium (.2%). Increasing the level of potassium in the diet increased (P<.05) fecal magnesium excretion. Magnesium absorption decreased linearly with increasing levels of potassium. The greatest depression in magnesium absorption occurred when the potassium level was increased from 1.2 to 2.4%. A further large decrease in absorption occurred when the potassium level was increased from 2.4 to 4.8%. Serum magnesium was depressed when higher levels of potassium were fed. Potassium absorption was not affected by level of dietary magnesium, but increased (P<.05) with increasing levels of dietary potassium. In experiment 2, two metabolism trials were conducted with 18 wether lambs, equipped with abomasal and ileal cannulae, in a 2 x 3 factorial arrangement. Two levels of magnesium (.1 and .2%) and three levels of potassium (.6, 2.4, and 4.8%) were used. Chromic oxide was used as an indicator to determine digesta flow. Each trial consisted of a 10-day preliminary period, a 7-day collection of feed, feces and urine, and a 6-day sampling of feed, abomasal and ileal fluid and feces. Magnesium was absorbed mainly prior to reaching the small intestine, followed by a low net absorption in the large intestine. Increasinq potassium level resulted in a linear decrease in magnesium absorption. Preintestinal magnesium absorption decreased (P<.05) with each increase in dietary potassium. Large amounts of magnesium flowed into the small and large intestines and was excreted in the feces when higher levels of potassium were fed. Lambs fed the high level of magnesium (.2%) absorbed larger amounts of magnesium from the stomach. Magnesium absorbed, expressed as a percent of intake, was not affected by level of magnesium. Experiment 3 consisted of three metabolism trials with six Angus x Simmental steer calves, per trial, equipped with abomasal and ileal cannulae. Steers were fed .1% magnesium and three levels of potassium, .6, 2.4 and 4.8%. Procedures were similar to those used in Experiment 2. Magnesium absorption decreased linearly (P<.05) when high levels of potassium were fed. The primary site of magnesium absorption was the preintestinal region, followed by a net secretion into the small and large intestines. Preintestinal magnesium absorption was decreased approximately 50% when steers were fed 4.8% potassium. Small amounts of magnesium tended to be secreted into the small intestine when the high level of potassium was fed. As observed in experiment 1, serum magnesium levels were depressed when increasing amounts of potassium were fed. Potassium absorption increased with increasing levels of dietary potassium. The effect of high dietary potassium on magnesium absorption was similar to those reported in experiments 1 and 2 with sheep. / Ph. D.

Avocados are one of the major food sources in tropical and subtropical regions and are an important horticultural crop in South Africa. Avocados are exported over long distances and may have storage times of up to 30 or more days at temperatures of about 5.5oC. This procedure increases the risk of poor fruit quality, including physiological disorders, early softening and postharvest disease incidence. A major component of the postharvest diseases is Anthracnose caused by Colletotrichum gloeosporioides. Anthracnose infects unripe fruit and once infected, the fungus remains dormant in the fruit until ripening begins. This leads to a problem for producers and packers, as the presence of the disease cannot be detected on the pack line, and fruit is not removed. Anthracnose control is normally done through pre-harvest treatment with copper-based fungicides. While effective such treatment needs to be repeated frequently, resulting in copper residues on the avocados. The study was conducted to investigate the effects of phosphoric acid and potassium silicate on known antifungal compounds and critical enzymes of the pathways elemental for systemic resistance inducers, so as to evaluate the potential for using them as alternatives to or in conjunction with, copper fungicides in the control of Anthracnose in avocado fruit. The study included storage temperature and time variations, to take account of the logistics in shipping avocado fruit to distant markets. Pre- and postharvest applications of phosphoric acid and potassium silicate were used, and after harvest, fruit were either ripened at room temperature (22oC) without storage or stored for 28 days at temperatures of 5.5oC or 2oC before analysis. Concentrations of phenolics, activity of the enzyme phenylalanine ammonia lyase (PAL) and a known antifungal diene were determined in the fruit exocarp. Pre-harvest treatments of phosphoric acid showed that the highest phenolic concentration was found in fruit harvested 14 days after application for fruit stored at room temperature. For fruit stored at 5.5°C it was seen that as fruit softened, phenolic concentrations increased compared with hard fruit immediately after storage, with the highest increase noted for fruit harvested 7 days after application. When comparing the three storage temperatures, phenolic concentrations were enhanced most when fruit was stored at 2°C. Postharvest treatments showed a significant increase in phenolic concentrations for potassium silicate treated fruit stored at room temperature and 2°C when determined immediately after storage. Fruit stored at 5.5°C showed an increase in phenolic concentrations as it became softer. When considering PAL enzyme activity, it was found that postharvest treatments of both potassium silicate and phosphoric acid influenced enzyme activity, with potassium silicate having greater effects. Similarly, an increase in PAL activity was noted in the pre-harvest phosphoric acid treatment harvested 14 days after application for fruit ripened immediately as well as fruit stored at 5.5°C. Fruit stored at 2°C showed the highest PAL activity for fruit harvested 7 days after application. No results were obtained in the analysis of antifungal compounds for both pre- and postharvest treatments. However, it is suggested that the antifungal diene could follow similar trends to those found for phenolics. It is concluded that applications of both phosphoric acid and potassium silicate do create changes in phenolic concentrations and the activity of the enzyme PAL which is involved in the synthesis of phenolic compounds known to possess antifungal properties. It is therefore possible that phosphoric acid and potassium silicate may be used as part of an integrated programme for Anthracnose control, and should be tested as potential alternatives for high volume copper-based fungicides. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2012.

The public concern over synthetic pesticides in foods and the environment has created an interest to find effective and safe non-fungicide means of controlling postharvest pathogens. The overall objective of this thesis was to evaluate the effect of potassium silicate, yeast antagonists and hot water dip treatment to control postharvest grey mould and blue mould of pome fruits, caused by Botrytis cinerea and Penicillium expansum, respectively. Botrytis cinerea and Penicillium expansum were isolated from infected strawberry and pear fruits, respectively. These isolates were found to be non-resistant to YieldPlus® (Anchor yeast, Cape Town, South Africa), a biofungicide containing a yeast Cryptococcus albidus. A total of 100 epiphytic yeast isolates were obtained from the fruit surface of “Golden Delicious” apples and “Packham’s Triumph” pears, and screened against B. cinerea and P. expansum. Fifteen yeast isolates reduced grey mould incidence by > 50%, when applied four hours before inoculation with B. cinerea. Similarly, seven yeast isolates reduced blue mould incidence by > 50%, when applied four hours before inoculation with P. expansum. YieldPlus® and yeast Isolate YP25 provided the best control of B. cinerea, while Isolate YP60 and YieldPlus® provided the best control of P. expansum on “Golden Delicious” apples. A mixture of YP25 and YP60 provided complete control of both B. cinerea and P. expansum, when applied to “Golden Delicious” apples before inoculation with either B. cinerea or P. expansum. Electron microscopy studies showed that yeast Isolates YP25 and YP60 inhibited the mycelial growth of B. cinerea and P. expansum, respectively. Preventative and curative application of potassium silicate resulted in reduced incidence of B. cinerea or P. expansum of “Golden Delicious” apples. Electron microscopy studies indicated that potassium silicate inhibited the growth of B. cinerea and P. expansum. Furthermore, treatment of “Golden Delicious” apples with either potassium chloride or potassium hydroxide resulted in reduced incidence of both B. cinerea and P. expansum. In vivo tests showed that the disease incidence of P. expansum and B. cinerea on “Golden Delicious” apples was reduced by hot water dip treatments at 58-60°C for 60 to 120 seconds, compared with the control fruit treated with sterile distilled water, without causing skin damage. The use of potassium silicate, yeasts (Isolates YP25 and YP60), YieldPlus® and the antagonists mixture (YP25+YP60) in combination, resulted in the control of B. cinerea and P. expansum of “Golden Delicious” apples compared with Imazalil® treated fruit. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2012.