Growth and Development of Horticultural Crops (1+1)

Plant hormones and flowering

• A number of lines of evidence point to a role for plant hormones, especially gibberellic acid, in the induction of flowering, both in photoperiodic responses and in vernalization. It has been known for many years that the application of exogenous plant growth hormones to vegetative plants can trigger reproductive development in some species. For example, auxin applications are used commercially to induce flowering in the pineapple (Ananas comosus) and in the litchi tree (Litchi chinensis), although in pineapple the effect of auxin on flowering results from the stimulation of ethylene production. In many species the requirement for vernalization can be overcome by treatment with GA.
  
• In Thlaspi the activity of a key enzyme of GA biosynthesis is induced in the stem apex (but not the leaves) by low temperatures. Further evidence for the role of GA in the induction of flowering comes from measurements of GA content and enzymes involved in GA biosynthesis in extracts prepared from spinach (Spinacia oleracea). Spinach plants maintained under short days remain vegetative and have a rosette growth habit. Extracts prepared from these plants suggest that a critical step of GA biosynthesis is inhibited. When exposed to long days, the content of active GA increases, the stems elongate and flowers are produced. It appears that the synthesis of a number of enzymes of GA biosynthesis is under photoperiodic control.