OXYGEN DIFFUSION IN PEA

S ummary The apical oxygen concentrations in the primary roots of agar‐grown pea plants were assayed polarographically with the roots immersed in anaerobic media, their laterals present or excised, and with or without the sub‐apical regions jacketed in a solid agar. Oxygen concentrations declined for several days following emergence as a function of accumulated demands of primary and lateral Touts for oxygen, internal diffusive resistance, and radial oxygen leakage from both primary root and laterals. Under the combined influence of all these factors (unjacketed roots with laterals), primary root apices could become totally anoxic when only 4 to 6 d old. Total apical anoxia did not occur however if lateral roots were excised, Or if sub‐apical regions of the roots remained encased in the solid agar. In these roots the decline in apical oxygen concentration continued for about 7 d of growth, after which the trend was reversed, and at 40 d apical concentrations as high as 7% were recorded. The anoxic state, which after 4d had characterized unjacketed roots with laterals, was also alleviated m approx. 10 d; at 40 d apical oxygen concentrations had risen to 3.5 to 4.5%. Respiratory rates in the primary root declined both with distance from the apex and with time. IT was concluded that it was chiefly these effects which accounted for the subsequent elevation of the oxygen concentrations in the primary‐ apex. It is pointed out that the oxygen demands of flooded soil might even lead to apical anoxia m the roots of 2‐d‐old plants. However, it is also tentatively suggested that, since respiratory decline can restore a better balance between oxygen demand and internal supply in pea roots, the ageing pea plant may become eventually somewhat less sensitive to soil waterlogging.