Francis M. Mathooko

Regulation of genes encoding ethylene biosynthetic enzymes in peach (Prunus persica L.) fruit by carbon dioxide and 1-methylcyclopropene

Abstract We have cloned one member (PP-ACS1) of the 1-aminocyclopropane-1-carboxylate (ACC) synthase and two members (PP-ACO1 and PP-ACO2) of the ACC oxidase gene families in peach (Prunus persica L.) fruit and studied their expression characteristics during fruit ripening and treatment with CO2 and 1-methylcyclopropene (MCP), inhibitors of ethylene action. Northern analysis showed that the abundance of PP-ACS1, PP-ACO1 and PP-ACO2 mRNAs increased with fruit ripening in parallel with increases in ethylene production and activities of ACC synthase and ACC oxidase. The abundance of PP-ACO2 mRNA was much lower than that of PP-ACO1. CO2 and MCP treatment inhibited ethylene production, ACC synthase activity, and accumulation of PP-ACS1 mRNA. Although CO2 had little effect on ACC oxidase activity, it inhibited the accumulation of PP-ACO1 and PP-ACO2 mRNAs to the same levels as MCP. Wound-induced ethylene production, ACC synthase activity, and the abundance of PP-ACS1 mRNA were blocked and stimulated by CO2 and MCP, respectively. CO2 and MCP had no effect on wound-induced ACC oxidase activity but inhibited the accumulation of its mRNA. Wound-induced activities of ACC synthase and ACC oxidase, and abundance of their mRNAs were inhibited and stimulated, respectively, by exogenous ethylene. The translational inhibitor, cycloheximide inhibited wound-induced ethylene biosynthesis but super-induced the accumulation of PP-ACS1 and PP-ACO1 mRNAs, suggesting that their induction is a primary response to the inducer. These results suggest that the expression of PP-ACS1 and PP-ACO1 genes play a key role in the regulation of ethylene biosynthesis in peach fruit during ripening and in response to wounding. The results also indicate that wound-induced PP-ACS1 and PP-ACO1 genes are under negative and positive control, respectively. Further, using MCP we provide evidence indicating that CO2 does not regulate ACC synthase activity and expression of the PP-ACS1 gene in peach fruit during ripening and in response to wounding by antagonizing ethylene action.

Regulation of respiratory metabolism in fruits and vegetables by carbon dioxide

Abstract The respiratory rate of fruits and vegetables can be used as an indicator for designing storage conditions to maximize the longevity of these commodities. One postharvest technique that has been used to prolong the storage life of some of these commodities is the use of a controlled atmosphere. The modulation of respiratory metabolism of such commodities held in controlled atmospheres containing reduced oxygen and/or elevated carbon dioxide levels has been thought of as the primary reason for the beneficial effects on the commodities. However, the mechanism by which elevated carbon dioxide influences the regulation of respiratory metabolism is still obscure and several hypotheses have been proposed for its mode(s) of action. The regulation may be directed towards the glycolytic pathway, the fermentative metabolism, the tricarboxylic acid cycle or the electron transport system, presumably through its influence on the synthesis, degradation, inactivation and/or activation of the respective enzymes. It may also be through the antagonistic effects of carbon dioxide on ethylene action as well as its influence on secondary metabolism through an alteration in cell pH. This article discusses the recent developments on the biochemical and physiological fronts as well as the possible mode(s) of action of elevated carbon dioxide in the regulation of respiratory metabolism in fruits and vegetables.

Ascorbic acid changes in three indigenous Kenyan leafy vegetables during traditional cooking

The ascorbic acid (vitamin C) contents in three fresh green leafy vegetables commonly consumed in Kenya namely, Amaranthus hybridus, Gynandropsis gynandra and Solanum nigrum L. and the changes during traditional cooking were measured. In the fresh leaves, the ascorbic acid varied from 123.8 mg/100 g fresh weight in A. hybridus to 189.2 mg/100 g fresh weight in G. gynandra. Cooking for 20 min in boiling water led to losses of between 75 and 89%. Smaller losses in ascorbic acid were observed when the leafy vegetables were cooked in two and four volumes of water. The loss in ascorbic acid as a function of cooking time was highest in S. nigrum while its loss as a function of volume of cooking water was highest in A. hybridus. Using loss in ascorbic acid as an index, it is proposed that controlling the time and amount of water used in cooking without sacrificing palatability could help, at least in part, in preventing loss in other essential nutrients during traditional cooking of the vegetables.