Growth and nutrient absorption of onion (Allium cepa) in the tropics in response to potassium nutrition.

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Ramirez-Guerrero, Hugo. 2001. Growth and nutrient absorption of onion (Allium cepa) in the tropics in response to potassium nutrition. Ph.D. Thesis. Imperial College, University of London, England.

The effect of potassium supply on leaf and bulb growth and mineral nutrition of short day onions was analysed under controlled and field conditions in a calcareous soil in a semiarid tropical area of Venezuela. Additionally, the effect of sowing date, and plant density on the growth and development of two short-day onion cultivars (hybrid and open-pollinated) was determined. The experiments confirmed that onion growth was promoted by long days rather than high temperatures. A supply of 4 mM K in solution culture under controlled conditions and 150 kg.ha-1 KCl in field conditions promoted onion growth and development and provided adequate mineral nutrition, attributed mainly to the role of K in translocation of photosynthates and conversion of assimilates. These supplies were sufficient to ensure maximum onion growth and development, maintaining an adequate nutritional balance. Under field conditions there was less response to K supply which was attributed to an adequate availability of soil K and the uptake efficiency of the onion roots. Excessive K supplies induced an unfavorable nutritional balance which was reflected in antagonistic interactions between K and P, Ca, Mg and Na. The antagonistic effect between K and Na is crucial for the plants to counteract salinity stress, particularly in relation to this salt sensitive crop (onion) growing under semiarid tropical conditions and in soils which can be potentially saline and sodic. The effect of K on postharvest quality was also subject to an adequate K balance in the plant. The onion bulb yields and postharvest quality responded positively to K fertilisation. The use of KCl as a K fertiliser in this soil and with this crop did not cause negative effects at any time of the year. The hybrid cultivar Canaria Dulce at a plant density of 250000 plant.ha-1 consistently produced the greatest yields. The small photoperiod variations had some influence on leaf and bulb growth, whilst precipitation influenced root growth and nutrient uptake and both factors influenced yields and postharvest quality. However, the rain also promoted bacterial disease infection and this resulted in many negative and erratic responses.