We have studied the cytological behavior of the mutant floury-a line 79-7660 subjected to the lysine-threonine 2.5 mM strong inhibitory medium. Each experiment consisted of the culture of 15 excised mature embryos of the floury-a line in tubes with the Murashige-Skoog medium containing lysine-threonine 2.5 mM, and 15 embryos in lysine-threonine free medium (control). After 10 days of culture at 28 C in the dark, two embryo derived seedlings were taken from the 5 repetitions of the experiment carried out. Thus, a total of 20 (10 treated, 10 control) seedlings were analyzed cytologically. Root tips from the differentiated seedlings were prefixed at 4 C in saturated paradichlorobenzene for 4 hours, transferred to Farmer's solution, fixed for 24 hours at room temperature, and stored in 70% ethanol. Chromosome screening was made in metaphase cells from squashed root tips stained with acetic carmine and observed by optic microscopy.
We have not obtained good photographs due to the low quantity of metaphase cells found. Such low numbers could probably be due to the dark culture of the "in vitro" material.
As shown in Figure 1 (A and B), either in the treated or in control derived root tips, we have found normal chromosome number and normal pairing. All the cells have 10 pairs of chromosomes with normal meiotic behavior. At anaphase I, the two members of the bivalent also separated normally. We have found an increase in the nuclear and cell size. Such kind of phenomenon was similar to that produced by colchicine. Nuclear division was not observed in such cells. Probably the increase in the nuclear size could be due to an increase in the DNA content. In all the cells studied, the phenomena observed were the same. Then, apparently the increase in the nuclear and cell size and the lack of nuclear division were not due to the lysine-threonine effect, because such phenomena were observed either in treated or in control derived root tips.
In conclusion, up to now we have not found in the floury-a root tip cells any effect directly associated with the exogenous supply of lysine-threonine.
Miguel Angel Rapela and Maria del Carmen Molina
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