Oxygen Transport and Root Respiration of Maize Seedlings

Oxygen uptake and ATP/ADP ratio were simultaneously monitored during incubation of excised maize (Zea mays L. INRA 508) root tips under varying O(2) partial pressure. Both variables were independent of O(2) tension until a critical O(2) pressure was reached. Below this pressure, ATP/ADP ratio and respiratory rate declined. However, in tissues having a high glycolytic capacity, the correlation between the ATP/ADP ratio and the respiratory rate breaks down as O(2) tension decreases, due to the increasing contribution of fermentative processes.In presence of 2 millimolar NaF, the ATP/ADP ratio varied solely as a function of the O(2) tension, without interference by fermentative activity, and a close correlation links the ATP/ADP ratio and the respiratory rate of excised maize root tips over the whole range of O(2) tensions tested.Using this correlation, a method is proposed for the quantitative determination of the relative cellular respiratory rate permitted by O(2) transport from the aerial part of young maize seedlings along the seminal root placed in an anoxic environment.Data are presented which demonstrate the preeminent part played by the cortical air spaces in O(2) transport. Their contribution to respiration was high in the first few centimeters nearest the seed and decreased rapidly as the distance from the aerated source increased. It is concluded that O(2) transport might contribute to the survival or to adaptive responses of root tissues in flooded soils but that the ventilation of the apical growing zone was inadequate to sustain the growth.