Germination, Growth, Physiology and Yield of Onion (Allium cepa L.) under Salt Stress

Abstract

Onion is one of irrigated crop produced by smallholder farmers and commercial growers for both local and export markets in Ethiopia. However, low productivity is reported by producers due to salinity. In view of this, study was initiated with the objective of determining the relative tolerability of onion cultivars to salt stress levels in laboratory and field condition on soil media filled in box, irrigated with salinized water at Melkasa Agricultural Research Center. Five onion cultivars (Adama Red, Bombey Red, Nasic Red and Nafis) and distilled water plus four salt levels (1.2, 4, 8 and 12 dSm-1 ) were factorially (5x5) arranged in Complete Randomized Design with three replications for germination test. Except Agrifound, four of the above cultivars and Awash water (0.3 dSm-1 ) instead of distilled water, were combined factorially (4 x5) arranged in Randomized Complete Blocked Design with three replications in the field. Data from laboratory and field were subjected to analysis of variance using SAS version 9.3. Laboratory result showed variations in seed germination percentage, seedlings root and shoot length, shoot to root length ratio, seedling vigor index, fresh and dry weight among salt levels (p≤ 0.001). Except, for germination rate and shoot to root length ratio non-significant differences were observed among cultivars (P>0.05). Germination percentages and seedling vigor index were higher with distilled water and gradually decreased as salt stress levels increased to 12 dSm-1 . The fastest germination rate was recorded for Adama Red and Nafis. Adama Red showed maximum (5.52) seedling shoot to root ratio while Agrifound (4.20) was the least. Field data analysis revealed variation in leaf length, leaf width, plant height, fresh and dry above ground biomass weight, Total Soluble Solid (TSS), bulb length and width among cultivars (p≤ 0.05) and salt stress levels (p≤ 0.001). Chlorophyll in SPAD- meter and stomatal conductance in porometer across three stages showed highly significant (p≤ 0.001) variations for main factors and interactions, whereas quantum yield showed significant difference at 40th DAT and 68th day after transplanting (DAT) among cultivars (p≤ 0.05) and salt levels (p≤ 0.001) at 40 and 54 DAT stages. The highest quantum was recorded for Nasic Red, Nafis and Bombey Red, where Adama Red showed the least. Leaf length and width, plant height, fresh and dry above ground biomass weight, fresh and dry bulb weights, TSS, bulb length and width were affected significantly by cultivars (p≤ 0.05) and salt stress levels (p≤ 0.001). Bombey Red, Nafis and Nasic Red cultivars showed the highest performance in leaf length and width, plant height, bulb length and width, fresh and dry bulb weight, dry above ground biomass. The highest leaf length, plant height, leaf width, TSS (12.78 o brix), dry bulb weight (11.32 g), fresh and dry above ground weight (70.01 g and 11.67g), bulb length and width were recorded with 1.2 dSm-1 salt level. Generally most of germination variables and early growth stages were not affected up to 4 dSm-1 , whereas the highest growth and yield performances were recorded with 1.2 dSm-1 salt stress levels in the field. Bombey Red, Nafis, and Nasic Red can be used for salt levels less than 4 dS/m. Indeed, experiment should be repeated under controlled environment including other cultivars and reducing salt levels to 4 dSm-1 in the future.