280
RUSSELL
of radioactivity (6). Even after several weeks the major part of the deposit
may remain superficial so that it can be removed by washing with water.
Leaching in rain has been shown to be the major cause of loss under normal circumstances (32), but not the onJy one. Moorby & Squire (33) have
shown that measurable, though small, quantities of strontium 89 which have
been deposited as fine sprays on leaves can become air-borne if the plants
are kept in dry conditions. The radioactivity is contained in fine particles
(ca. 4.) and the maximum loss may not take place until 2 to 3 weeks after
contamination. It has been suggested that losses under these conditions are
due to the dehiscence of fragments of cuticle.
The assessment of the relative importance of foliar and plant-base entry
in permanent pastures first received attention in the United Kingdom. The
suggestion that plant-base entry (or stem-base entry as it was first called}
might be important was made in 1957 to account for the fact that in hill
areas, strontium 90 was transferred considerably more readily to the bones
of animals, and to milk, from slowly growing hill pastures than in the lowlands (3+). The difference could not be explained in terms of enhanced
absorption from the soil due to its low calcium status; the entrapment of
strontium 90 at the plant-base appeared to be the probable explanation
because of the considerable development of “mat,” consisting of prostrate
stems and surface roots, which is characteristic of the hill pastures. The
hypothesis was readily compatible with observations that the major part of
the cumulative deposit was in the mat zone and not in the underlying soil.
It appeared that strontium 90 might remain readily available for plant-base
absorption for a considerable period since, in the early years of world-wide
fallout, the levels of strontium 90 in biological materials from some hill
areas rose in a similar manner to the cumulative deposit (34). Experimental studies have since provided considerably stronger evidence for the
importance of plant-base absorption under these circumstances. Middleton
& Squire (35) applied strontium 90 as a spray to turves from various
types of pasture which had been transplanted to boxes. Within a relatively
short period, the levels of strontium 90 on herbage from lowlands swards
fell to those expected as a result of absorption from the soil. In contrast,
turves from heavily matted upland pastures showed for many years a
markedly greater transfer to herbage and this could not be inhibited entirely
by the application of lime. Ellis & Newbould (36) compared relationships
between the levels of strontium 90 in soil and the contamination of pastures in a number of areas with the extent to which strontium 89 was
ee ee
absorbed from the underlying soils by grass grown under laboratory con-
ditions. Their results indicate that the extent of direct contamination is
markedly greater, presumably due to plant-base uptake, in hill areas. Other
studies point to the same conclusions (37).
The interpretation of the mechanism whereby strontium 90 enters by
direct contamination into rapidly growing lowland pastures has proved
more difficult. There is strong evidence that, when the rate of fallout was
relatively high, direct contamination was the major route of entry and the
TOT re on, Sacer Seee
.
1