Effect of Temperature on the Internal Morphology and Development of Leaves in Chinese Cabbage (Brassica campestris L.)

Abstract

The process of leaf development in relation to temperature was investigated morphogenetically to clarify the mechanism of the “small head” formation in Chinese cabbage. Seedlings of ‘Nozaki-Kohai No.3’ were grown in a phytotron kept at day-night temperatures of 30-25°C (high), 23-18°C (medium), and 15-10°C (low). For histological observations, leaves and shoot apices collected at intervals of 7 days were fixed and sectioned by the paraffin microtechnique.
In leaf 5 (numbered from the base) the thickness of the midrib was greatest at the medium temperature and smallest at the high temperature. The number of rows of parenchymatous cells along the dorsiventral axis in the midrib (hereafter referred to simply as “the number of parenchymatous cells”) was greater at lower temperatures. The proportion of the number of parenchymatous cells on the adaxial side to that on the abaxial side of the midrib was also greater at lower temperatures. The length of the parenchymatous cells was greater at higher temperatures. The thickness of the midrib vein was greater at lower temperatures. The thickness of the lamina was greater at lower temperatures.
Increase in the width of successive leaf primordia in plastochrones was more rapid at higher temperatures, whereas in a certain number of days after the initiation of leaf primordia, it differed little between the medium and high temperatures.
At the lower nodes, leaf primordia grew upright, but at progressively higher nodes, hyponastic curvature was observed first at the tip, next in the upper half, and finally in the whole of leaf primordium. Consequently, the leaf primordia folded over the shoot apex, forming the “small head”. This process progressed faster at lower temperatures.
The increase in the number of parenchymatous cells in successive leaf primordia was more rapid at higher temperatures. It differed little between the abaxial and adaxial sides at the lower nodes, whereas, at the higher nodes, it was greater on the adaxial side. In addition, it ceased earlier at the high temperature, so that the difference in cell number between the abaxial and adaxial sides was smaller than at medium and low temperatures.
There appears to be a correlation between the hyponastic curvature of leaf primordia becoming more pronounced and the difference in the rate of increase in the number of parenchymatous cells becoming greater as the plants grew. This correlation was also evident in relation to temperature. Therefore, the hyponastic curvature of leaf primordia (future midribs) is thought to be caused by the relative retardation of cell elongation on the adaxial side in the state where the rate of increase in the number of parenchymatous cells was greater on the adaxial side.
Hyponasty of the lamina was also found in its young stage, and this hyponastic growth was necessary in the formation of the folded leaves. The mechanism of lamina hyponasty was not clarified in this report, but it is possible that the difference in duration of cell division in the various tissues of the lamina, which is seen in many dicotyledonous plants, causes the hyponasty of the lamina in Brassica plants.