A simple, rapid and reliable bioassay for evaluating seedling vigor under submergence in indica and japonica rice (Oryza sativa L.)

Abstract

Submergence is a major stress causing yield losses particularly in the direct-seeded rice cultivation system and necessitates the development of a simple, rapid and reliable bioassay for a large scale screening of rice germplasms with tolerance against submergence stress. We developed two new bioassay methods that were based primarily on the seedling vigor evaluated by the ability of fast shoot elongation under submerged conditions, and compared their eVectiveness with two other available methods. All four bioassay methods using cultivars of 7 indica and 6 japonica types revealed signiWcant and consistent cultivar diVerences in seedling vigor under submergence and/or submergence tolerance. Japonica cultivars were more vigorous than indica cultivars, with Nipponbare being the most vigorous. The simplest test tube method showed the highest correlations to all other methods. Our results suggest that seedling vigor serves as a submergence avoidance mechanism and confers tolerance on rice seedlings to Xooding during early crop establishment. A possible relationship is discussed between seedling vigor based on fast shoot elongation and submergence tolerance deWned by recovery from submergence stress.

Submergence-InducedADHandALDHGene Expression inJaponicaandIndicaRice with Contrasting Levels of Seedling Vigor under Submergence Stress

“…After a survey of numbers of rice cultivars, we selected 3 japonica and 9 indica including two floating rice cultivars. Post-germination seedling vigor under submergence was evaluated based on the shoot growth by adopting the test tube bioassay method (12). Shoot lengths of these cultivars at the 5 th day of submergence are shown on Fig.…”

“…Plant materials and bioassay conditions for evaluating seedling vigor under submergence in this study we used 3 japonica and 9 indica cultivars including two floating rice, which were all selected after a survey of rice germplasms based on their seedling vigor evaluated by the test tube bioassay (12). 'nipponbare' was included as a high seedling vigor japonica check, 'FR13A' and 'iR42' as standard submergence tolerant and intolerant indica checks, respectively, and 'Kasalath' as a low seedling vigor indica check (12).…”

“…'nipponbare' was included as a high seedling vigor japonica check, 'FR13A' and 'iR42' as standard submergence tolerant and intolerant indica checks, respectively, and 'Kasalath' as a low seedling vigor indica check (12). Four chromosome segment substitution lines (cSSls), which were selected based on their vigorous and non-vigorous seedling Briefly, rice seeds were sterilized in 1% sodium hypochlorite (naclo) solution for 10 min and rinsed with distilled water three times.…”

“…Submergence stress significantly promotes shoot elongation in young rice seedlings (9,14). A study using fast shoot elongation growth as a basic parameter for screening rice cultivars under low dissolved oxygen levels suggested that faster shoot elongation was an adaptive response of water-seeded rice for acquiring oxygen (12,20). the trait is particularly important during direct sowing of rice seeds in paddy field (11,23).…”

“…We therefore studied gene expression profiles of ADH and ALDH at the post-germination early seedling stage under submergence using rice cultivars and lines showing contrasting levels of seedling vigor under submergence. We adopted the test tube bioassay condition that was applied to evaluate the seedling vigor based on the ability of shoot elongation for escaping submergence stress (12).…”