Dry seeds heterozygous for four plant-color markers (A a B b Pl pl R-r r-g) were x-rayed (8000 r), grown, and scored for sectors in the plants at flowering and at maturity. For numerical data, we determined numbers of tassel branches affected, fractions of sheath and culm perimeters, and lengthwise extent in nodes. From these fractions and percentages, apparent cell numbers (ACN) have been calculated.
Among over 500 plants examined, 45 had tassel sectors, of which 10 had sectors in the body of the plant as well, 6 with coincidental (i.e., separate) sectors and 4 with possible continuity of the sector from the remainder of the plant into the tassel. Sectored main spikes were usually divided evenly in half, so main spikes were scored as equivalent to two branches; most single branches were undivided. The average affected portion of the tassel was 34.8% (modal class 40-50%); ACN for the tassel in this material is 2.88 cells (1/0.348). Most tassels (56%) were represented by about 2 cells, the rest by 3-4 cells (24%) or more.
In the body of the plant, sectors could be scored in the sheath and
culm beginning with the 7th leaf; 94 sectors were found, on which perimeter
measurements gave the following ACN values for each node:
The average extent of sectors (number of contiguous
nodes affected) ranged from 2 to 8.5:
From these numbers it is evident that the ACN in the embryo drops from about 32 cells spanning 2 to 3 nodes to about 16 cells spanning 4 to 8 nodes, at about the 15th node. Ears were distributed mostly at nodes 12 to 14--i.e., just below the change point.
At nodes bearing palpable ears 21% of sectors crossing that node entered the husks; all 23 husk sectors were continuous with sectors covering one or more nodes of the culm and sheaths. ACN for the husks is 4.52 cells, a subset of the 32 cells spanning the node at this level. Only one cob sector was found in the test population; this sector was continuous with a body sector covering 4 nodes, divided the shank and husks equally, and included the vast majority (97.5%) of the cob (all but 16 of about 640 floral units). ACN for the cob must be quite small (not much more than 2 cells) and is probably a subset of the husk cells.
From these data, the meristem in this material, for
the plant body from about leaf 7 upward, can be represented diagrammatically
as a stack, roughly as follows:
|Top 4-5 nodes||16|
|2-3 nodes (ears)||32 (4-8)|
These cell groups are determined as compartments in the embryo, though presumably with some degree of plasticity. Those cells destined to elaborate the ear clearly do not extend into tassels and vice versa; thus, non-concordance of mutational events (see Neuffer, MNL 49:126, 1975) is the situation for seed treatment of maize with mutagenic agents, and recessive mutations would not be expressed in the selfed progeny ("M2") from treated seed.
E. H. Coe, Jr. and M. G. Neuffer
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