Growth and viability of coleoptiles under oxygen deficiency in Oryza sativa L. from the collection of the Federal Rice Research Center

The distinctive ability of rice seedlings lies in the ability to germinate under conditions of oxygen lack. At the germination stage, the first to develop is the coleoptile, a juvenile organ that protects the true leaf in cereals. The mechanisms of regulation of growth and development of this organ have not been studied to a large extent. Special attention may be paid to a coleoptile in seedlings capable of germinating under oxygen conditions. In the presented study, for the first time, the importance of the growth rate and viability of coleoptiles of rice seedlings during flooding in determining survival and development was demonstrated. A total of 36 varieties and forms from the collection of the Federal Research Center for Rice, Krasnodar, were tested. Sprint and Kuban 3 were among the fastest growing varieties. Their coleoptiles reached 19-25 mm length both under normoxia and hypoxia. The slowest growing group included three Philippine varieties harbouring the SUB1A allele (HHZ11 Y6-Y2-SUB1, HHZ8 SAL 14 SUB1, HHZ9 DT12 SUB1), Chinese variety Xiannui and domestic varieties Amethyst, Zhemchug, Natasha, Rapan and Yuzhnaya noch. This group of varieties was characterized by inhibition of the growth of coleoptiles by 2.5-3 times under submergence. In main a positive correlation was estimated between the coleoptile length in normoxia and hypoxia (r = 0.70, p = 10-6), i.e. forms that actively grow in an aerobic environment also grow rapidly when flooded. Further detailed analysis of the growth of coleoptiles under hypoxic conditions showed that growth changes correspond to several patterns. The most common reaction to hypoxia in rice coleoptiles of the first group was growth suppression, but with the preservation of a small part of the plants that continue to grow. This group included all Philippine cultivars harbouring the SUB1A allele. In the second group, a significant proportion was plants which length under submergence exceeded that at normoxia. Approximately half of the plants of the third group slowed growth arrest, while the other part continued to grow as in normoxia. The fourth group included the variety Sprint, which was the only one to have traits of avoidance strategy (LOES - low oxygen escape syndrome) associated with growth enhancement, although this enhancement was not intensive. The variety Yuzhnaya noch had a unique growth pattern, the coleoptiles of which grew slowly both in normoxia and being submerged. In addition to growth, the viability of coleoptiles was analyzed in the work, which was assessed using a tetrazolium test. Under hypoxic conditions, the viability of all tested forms significantly decreased. In the fastest growing varieties (Sprint, Kuban 3), the color intensity of tetrazolium salts was higher both in the control (6-fold) and in the experiment (2-fold), compared to slow growing forms (Amethyst, Yuzhnaya noch, Philippine SUB1A varieties). In general, growth rate correlates with metabolic rate and submergence tolerance. Obtained results illustrate the tolerance to oxygen deficiency of the genotypes from the collection of the Federal Research Center of Rice, and show that coleoptile elongation can be used as a criterion for assessing the tolerance of rice varieties to the lack of oxygen.