An interesting case of inhibition of leaf development by X-irradiation
--M. M. Johri and Ed Coe

While tracing the progression of internode differentiation after germination, an interesting case of irradiation damage was observed. Seeds heterozygous for anthocyanin (b, pl) and chlorophyll markers (wd) were germinated in peat pots and allowed to develop for 8 and 13 days after sowing (DAS). The entire seedling was x-rayed (1,500 R at the meristem level) and promptly planted in the field. Between 60 and 80% of the seedlings survived and developed into mature plants which, although morphologically similar in size and node number to unirradiated plants, showed some radiation-induced damage. The culm diameter and Apparent Cell Number (ACN) were reduced in the 7th and 8th internodes. These observations suggest that irradiation alters the fitness of cells. Those losing dominant markers seem to proliferate more rapidly than the neighboring cells (NB) and consequently give rise to wider sectors. At the same internode the NB cells seem to be less fit to divide and expand, otherwise it is difficult to explain reduced culm diameter.

A single instance of a leaf primordium failing to develop into the leaf was found. Among 82 plants developing from seedlings irradiated 13 DAS, one appeared to have missed the 12th leaf and two consecutive leaves were present on the same side of the plant (Fig. 1A). An examination of the internode above the 11th node showed the presence of a protuberance at the site corresponding to the midrib of the 12th leaf (Fig. 1B). Whether the protuberance represents the aborted midrib or the axillary bud related to node 13 is difficult to decide. Since a sector for the loss of Pl extended from the protuberance basally to node 11, it is likely to represent an aborted midrib. It appears the leaf primordium just after initiation is very susceptible to radiation damage. The absence of leaf and axillary bud was accompanied by a reduction of internode length. The length of the 12th internode was reduced by 33% and of the 13th by 66%. It is possible that a developing leaf or an axillary bud provides hormonal factors for the elongation of adjacent internodes.

Fig. 1. A. Nodes 10-15 in a plant from a seedling irradiated 13 DAS. Note the two consecutive leaves on the same side of the plant. B. Removal of leaves shows the presence of a protuberance (white pointer) at the location of 12th leaf. A sector for the loss of Pl (black arrow) extends from the protuberance to node 11. Numbers 10-15 refer to respective nodes. 

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