Organisation of apical meristems

The shoot apex of rice, Oryza sativa CV Balilla was studied with a view to obtaining information about the meristematic activity of the different regions. The vegetative state only was studied and the rice was grown for 4-5 days at 30anddeg;C or 35anddeg;C under 4cm water in darkness, intermittent white light or continuous white, red, far-red or blue light. The part of the shoot apex studied was that above the level of the youngest leaf primordium and it was divided into six regions which took into account the tunica and corpus and regions of leaf initiation.
The mitotic index was found every 2hrs. throughout 24hrs. in plants growing in continuous darkness, in a 12hr. day with white light of 10,000 lux and in continuous white light of 2,400 lux. The mitotic index varied little and no pronounced synchrony was evident under any of these conditions. The mitotic index of the flank and corpus regions was higher than that of the summit and sub-summit regions.
The meristematic activity of the regions was investigated further using colchicine to accumulate metaphases. Colchicine blocks mitosis at metaphase, but does not affect the rate of entry of the cells into mitosis. The gradient of the increase in metaphases over 3-4 hours was used to calculate the cell-doubling time in each region and the effects of different environments on this were found. Cells of the summit region were found to divide at least 7 times more slowly than cells of the flanks and corpus and intermediate rates occurred in tie sub-summit regions. In 10,000 lux white light and at 30anddeg;C, the cell-doubling time for flanks and corpus averaged 13 hours which is shorter than any reported for equivalent regions in other plants. Cell-doubling times for the summit were over [illegible] hrs. In experiments where the intensity of white light varied during the growth of the seedlings a different relationship between the regions was found. The cells of the flanks and corpus divided more slowly and only about twice as fast as in the summit.
The effect on cell-doubling times of excision on the shoot was studied and the immediate result was to speed up cell cycles in the leaf primordium and corpus. Excision has often been used to facilitate entry of a chemical into the shoot apex and it may have given a misleading picture of the situation in undisturbed shoot apices.
Nuclear DNA synthesis in the regions was studied using tritiated-thymidine. In 1,000 lux continuous white light the flanks and corpus showed a gradual increase in labelling throughout the feeding which lasted for 72hrs. while the sub-summit and summit regions showed a negligible or low level of labelling in the same period of time. The level of DNA synthesis taking place in the regions thus paralleled that of meristematic activity found by using colchicine. The supply of tritiated thymidine to plants grown in continuous white light at 10,000 lux produced a non-linear low labelling pattern which could not be easily explained. Many attempts were made to overcome this lack of success involving excision, and surgery of various kinds, use of (6-3H) thymidine as well as (methyl-3H) thymidine and experimentation with red, far-red and blue light. However, none of those succeeded and so a pulse-labelling experiment was not attempted.
The effect of an acute dose of X-rays was studied by finding the mitotic index immediately afterwards, andfrac12;hr. afterwards and 24hrs. afterwards and by studying the level of metaphases after colchicine in control and irradiated plants. A dose of 1, 2, 4 and 8 k.rad had a differential effect on the regions which is interpreted as a reflection of their different levels of meristematic activity. No stimulation of division of the summit was found which is the reverse of what has been found in the quiescent centre of roots after acute X-irradiation, although this region in often equated to the summit of the shoot.
A study of cell pattern throughout a plastochron suggested on the basis that evidence of cell division and cell size are related, that meristematic activity in the tunica is highest in the flanks and that, as the activity progresses round the circumference of the apex in the formation of the leaf primordium, which sheaths the stem, cell division also takes place for a short distance toward the summit. In this way, preparation for formation of a leaf primordium occurs sometimes before it appears as a bump. The summit is considered to be carried forward passively and not to contribute a significant number of cells to the rest of the meristem. This interpretation supports the view that the summit is not the site of apical initials in higher plants as considered by the orthodox view.
A study of the effect on cell kinetics of red and far-red light by using accumulation of metaphases with colchicine revealed a change in response to far-red light with the age of the seedling. Forty hrs. red and far-red light given to dark-grown 3andfrac12; day-old seedlings caused increased meristematic activity to take place in all regions but 16hrs. far-red light reduced activity and 16hrs. red light was stimulating. Forty hrs. far-red light after 2andfrac12; days dark gave high rates similar to 4andfrac12; day-old plants given 16hrs. far-red. Comparison of these rates on the basis of the age of the seedling at the end of the treatment revealed that plants which are 5 days old had similar rates after 16 or 40hrs. far-red, but plants which were 4 days old had higher rates after 40hrs. far-red than after 16hrs. far-red. Variation of response to far-red with age of the seedling has been found before and the link between phytochrome and leaf initiation is discussed. All regions of the apex, including the summit respond to light by a speeding-up of mitotic cycles but the summit is still at least seven times slower than the flanks and corpus.
Finally, consideration is given to the significance of the results in rice and the mandeacute;ristandegrave;me d'attente controversy. Cell divisions in the summit may vary in importance in different apices but in the absence of the possibility of proving presence or absence of apical initials the controversy is put aside. The shift of growth balance in apices of plants subjected to different conditions is brought out and the importance of the summit to growth of the shoot apical meristem and the nature of its influence is discussed.