Contractile growth of the hypocotyl in white clover seedlings (Trifolium repens L.) : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science at Massey University
White clover (Trifolium repens L.), like many epigeal emerging legumes, displays a phenomenon known as contractile growth. This process occurs in the weeks following seedling emergence and involves the longitudinal shortening and radial expansion of tissues in the hypocotyl and upper primary root. During routine trials at AgResearch Grasslands, Palmerston North, it was noted that some seedlings of cv. Grasslands Huia displayed an aberrant growth form characterized by a failure to display normal contractile growth. These seedlings were intermated for one to two generations to form the 'non-contractile' populations used in this thesis. The 'contractile' populations used were formed through the random selection and intermating of plants which showed normal contraction. The non-contractile phenotype was found to occur naturally in populations of white clover at a frequency of 2-22%. When compared to tissue from normal contractile seedlings, hypocotyl tissue from non-contractile seedlings showed reduced radial expansion which in turn led to decreased contraction. Since the plant hormone ethylene has been implicated in the promotion of radial expansion in many plant tissues and systems (Abeles 1973 b), the effects of applying silver thiosulphate and the ethylene-releasing compound Ethrel, were examined in contractile and non-contractile seedlings. Endogenous ethylene production by contractile and non-contractile seedlings was also examined. Seedlings from the contractile line showed an overall higher production of ethylene during early seedling growth than did seedlings from the non-contractile line. When treated with silver thiosulphate (which blocks ethylene perception), hypocotyls from seedlings of the contractile line failed to display contractile growth and hypocotyls from seedlings of the non-contractile line displayed non-contractile growth to a greater degree. When treated with Ethrel hypocotyls from seedlings of the non-contractile line displayed contractile growth but hypocotyls from seedlings of the contractile line failed to display contractile growth. These results indicate that ethylene is required for normal contractile growth of the hypocotyl in white clover seedlings, and that non-contractile growth of the hypocotyl is caused by a decreased production of, rather than a decreased sensitivity to, the plant hormone ethylene.