Onion

Abstract

Onion (Allium cepa L.) is among the most important horticultural crops grown worldwide for its culinary preparations. The onion is the second most widely cultivated horticultural crop in the world. Limited water availability, increasing temperature, salinity and flooding are the major environmental factors in sustaining and increasing the productivity of onion worldwide. In onion, water is the main limiting factor for low productivity. Onions are considered a shallow-rooted crop and are more sensitive to water stress during bulb formation and enlargement than during the vegetative stage. In rainy season, the production of vegetables is often limited due to excessive moisture brought about by heavy rains. Temperature limits the range and production. Day length and temperature influence bulb formation in onions. Onions are sensitive to salt, are relative excluders of both Na and Cl and are sensitive to sulphate. Physiologically, excessive soil salinity imposes initial water deficit that results from the relatively high solute concentrations in the soil, causes ion-specific pores resulting from altered K⁺/Na⁺ ratio and leads to a build-up in Na⁺ and Cl⁻ concentrations that are detrimental to plants. Understanding the physiological, molecular and biochemical processes in relation to these changing climatic factors including increasing CO₂ constitutes the first step toward the development of strategies for designing stress-resistant genotypes of onion. Effects of environmental factors- water, temperature salinity and elevated CO₂ on whole plant physiology of onion crops as expressed by growth, yield, quality and photosynthetic features are discussed.