The Response of the Primary Root Meristem ofZea maysL. to Various Periods of Cold

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Abstract
Primary roots of maize (Zea mays L.) grown in nutrient solution at 5°C elongate at about 1% of the rate found at 20°C. The apical meristem becomes shorter and shows little proliferative activity at 5°C, but following transfer to 20°C mitoses increase in frequency and the meristem regrows to its original length. Both the amount by which the meristem shortens and the time for its complete regrowth are related to the period spent at 5°C. The shortening of the meristem suggests that at the lower temperature the balance which normally exists between cell production and differentiation is altered, the latter continuing at a relatively faster rate than the former. A new, steady-state balance between the two processes is re-established during the recovery period. The meristem recovers as a result not only of its own mitotic activity but also through stimulation of cell division in the quiescent centre. The degree to which the quiescent centre is activated, as judged by its mitotic index and the number of nuclei labelled by feeding with tritiated thymidine, increases as the duration of the preceding cold treatment increases. The close relationship between proliferative activity in the quiescent centre and the minimum length of the meristem following the cold treatment suggests that there is communication between these two zones which co-ordinates their respective rates of cell production and helps to maintain a normal meristem structure. The results emphasize the importance of the quiescent centre as a reservoir of cells that can re-establish a meristem rendered non-functional through the impact of unfavourable environmental conditions.

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