When growing vegetables at this time of year,and as the days lengthen and the temperature fluctuates, you can encounter the problem of bolting , plants going to flower and seed before producing any leaves. Asian greens ,Brassicas for example , such as this Chinese Broccoli are particularly susceptible to this. In this case I left the seedlings for too long in the tray, they became drawn and stressed from not enough moisture and nutrients. After planting them out they went into transplant shock and the low night temperatures accelerated their demise.
Joy Larkcom goes into more detail about bolting in her excellent book, Oriental Vegetables The complete guide for the gardening cook. Her suggestion of growing seedlings in a module so that individual plants are able to develop a good root system is one way of overcoming the problem as well as germinating seeds at at reasonably high temperature of 18C degrees -25C. Low temperatures in the early stages of growth is the single most important factor leading to the initiation of flowers instead of leaves. Young plants need to clock up a certain number of heat units to prevent bolting : once this has been done they can be subjected to lower temperatures without damage.