Phase transition and its regulation in the development of horticultural plants

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Changes in leaf shape, leaf curvature, the number of leaf primordia, and the allometric relationships between leaf and stem growth were monitored in order to clarify the time of occurrence of phase transition. Two phases were distinguished in the relationships between the rate of leaf primordia initiation and the rate of development in leaves larger than 1 cm. The phase transition occurred earlier in crisphead lettuces than butterhead lettuces.Bags were placed over the younger leaves when the leaf 15 became longer than 1 cm (15th-leaf stage). The dark grown leaves curled inwards and became narrow. At the 26th-leaf stage, shading did not affect leaf breadth; and on leaves at position 25 and higher, the basal parts of midribs became bent in both shaded and control plants. Therefore, shading did not enhance the heading process. Linear relationships could be found between logarithms of stem dry weight and those of stem length before flower bud initiation.However, the relationship became curvilinear if all plots were combined.Until flower bud initiation, the linear relationships were found between logarithms of stem length and those of stem diameter, and between logarithms of dry weight of stems and those of leaves.The results of the present study suggest that lettuce plants pass through several different phases during its development. As a result of the sequence of these phase transitions, lettuce plants showed distinct changes in morphology, such as head formation and bolting.