AOBPreview originally published online on September 30, 2005
Annals of Botany 2005 96(7):1191-1198; doi:10.1093/aob/mci272
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Aerenchyma Formation and Recovery from Hypoxia of the Flooded Root System of Nodulated Soybean
1 Departamento de Fisiologia Vegetal and 2 Departamento de Botânica, Instituto de Biologia, Universidade Estadual de Campinas, C.P. 6109, 13083-970 Campinas – SP, Brazil
* For correspondence. E-mail lsodek{at}unicamp.br
Received: 3 June 2005 Returned for revision: 15 September 2005 Accepted: 15 August 2005 Published electronically: 30 September 2005
• Background and Aims Flooding results in hypoxia of the root system to which N2 fixation of nodulated roots can be especially sensitive. Morphological adaptions, such as aerenchyma formation, can facilitate the diffusion of oxygen to the hypoxic tissues. Using soybean, the aim of the study was to characterize the morphological response of the nodulated root system to flooding and obtain evidence for the recovery of N metabolism.
• Methods Sections from submerged tissues were observed by light microscopy, while sap bleeding from the xylem was analysed for nitrogenous components.
• Key Results Flooding resulted in the rapid formation of adventitious roots and aerenchyma between the stem (immediately above the water line), roots and nodules. In the submerged stem, taproot, lateral roots and adventitious roots, lysigenous aerenchyma arose initially in the cortex and was gradually substituted by secondary aerenchyma arising from cells derived from the pericycle. Nodules developed aerenchyma from cells originating in the phellogen but nodules situated at depths greater than 7–8 cm showed little or no aerenchyma formation. As a result of aerenchyma formation, porosity of the taproot increased substantially between the 4th and 7th days of flooding, coinciding with the recovery of certain nitrogenous products of N metabolism of roots and nodules transported in the xylem. Thus, on the first day of flooding there was a sharp decline in xylem ureides and glutamine (products of N2 fixation), together with a sharp rise in alanine (product of anaerobic metabolism). Between days 7 and 10, recovery of ureides and glutamine to near initial levels was recorded while recovery of alanine was partial.
• Conclusions N metabolism of the nodulated soybean root system can recover at least partially during a prolonged period of flooding, a process associated with aerenchyma formation.
Key words: Glycine max, soybean, secondary aerenchyma, oxygen deficiency, waterlogging, nitrogen metabolism, nitrogen fixation
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