| Abstract: | There are two opposite opinions as regards the mechanism and the path of downward oxygen transport in rice and other higher plants. Van Raalte (1940), Yamada (1952), and others maintain that an oxygen pressure gradient of decreasing magnitude from the stem base down to the root tip exists in the intercellular air spaces, which are interconnected throughout the cortex, and the oxygen transported therein is in free molecular form and moves about by diffusion along its own gradient. Recent diffusion experiments in plants by Barber (1962), employing radioactive O15 as indicator, gave direct confirmation of this hypothesis. The opposite view is held by Brown (1947), Soldatenkov (1963) and others. They consider that the passive diffusion of oxygen along its own gradient is inadequate to account for the actual amount transported downwards. The fact that downward oxygen transport in roots comes almost to a standstill, once the aerial part is removed while the cut end of the short stump is still left in air, casts doubts as to the validity of the diffusion hypothesis; and is in favour of their claim that in addition to, or in placement of, diffusion, active participation of living tissues in shoot is necessary to drive enough oxygen to meet the demand of roots. The oxygen in active transport is no longer in free gaseous state but is in dissolved or combined form (as in peroxides) and moves presumably along the vascular bundles in a way which is hitherto unrevealed but is apparently dependent upon the physiological activity of the conducting tissue. In our previous report (Lou et al 1964), we gave data based on quantitative measurement of the amount of oxygen transported downwards from aerial to submerged parts in intact seedlings with the respiratory hydrometer specially designed for the purpose. In seedlings of marshy plants (e.g. rice), it amounts to about 50% of the total oxygen absorbed by the aerial part; in water cultured seedlings of ordinary land plants (e.g. pea), 20%–30%. By deliberately blocking the alternative paths of oxygen transport in seedlings, one at a time, and measuring the downward oxygen transport accordingly in the same way as before, we should be able to decide which one of the two paths is mainly responsible for the transport. The blocking can be conveniently carried out at the upper end of the radical in a pea (or broadbean) seedling by surgical treatment (see Fig.1); either by ringing off the peripheral cortex where most of the air spaces reside; or by piercing through the central cylinder, within which the vascular bundles are confined. The treated radical is then submerged in water and ready for measurement. Without recourse to surgical treatment and mechanical injury, the air space in the cortex can also be blocked by displacing its air content with water through vacuum infiltration. The present investigation has shown that when the intercellular spaces in the cortex of the radical are blocked either by ringing or by infiltration, the aerial part of the treated seedling absorbs much less oxygen than the control as though its radical were completely severed (Table 2); or, in other words, the downward oxygen transport is effectively stopped by such a means. On the other hand, interruption of vascular bundles in the central cylinder only reduces the amount of oxygen in transport to less than one half, which can be accounted for by the combined effect of the reduced root activity due to shortage of food supply and the unavoidable partial disruption of the peripheral cortex. Besides taking actual measurement, downward oxygen transport in intact pea (or broadbean) seedlings can also be detected by simply noticing the growth rate of its radical. As is shown in this investigation, the radical ceases growing in still water, if the oxygen supply from its aerial part is interrupted. As a result of oxygen deficiency, the radical tip deteriorates in a few days. These effects can be easily realized by ringing off the cortex or by infiltrating its air spaces with water. That the peripheral ringing of the radical does no harm to its growth process is revealed by the fact that if air is bubbled through the water culture steadily, normal growth ensues. The above results leave no doubt that in seedlings of rice, pea, and broadbean, downward oxygen transport mainly takes place in the intercellular spaces in the radical cortex, and seems to have no concern with the activities of vascular bundle and cortex. Although there are evidence that rice roots may actively secrete oxygen in the form of peroxides to its immediate neighborhood (the rhizosphere), the actual amount and the distance traversed in such an active transport however, is very much limited and is insignificant as compared with that taking place in the intercellular spaces. |
