Effects of Hormones on Morphogenesis and Cold Resistance in Berseem Clover (Trifolium alexandrinumL.)

Plants of berseem clover (Trifolium alexandrinum L.) cv. Tabor were raised under conditions inhibiting the acquisition of cold hardiness (non-hardened) or inducing cold hardiness (hardened). All non-hardened plants developed an elongated shoot and exhibited considerable frost sensitivity, as measured by the extent of the reduction in yield of variable chlorophyll fluorescence after exposure to sub-zero temperature. Hardened plants developed a shorter shoot, with fewer leaves and a greater percentage of dry matter in the root system. These parameters were associated with a marked increase in frost resistance. Exogenous application of ABA to plants effected similar morphological modifications in both hardening and non-hardening temperature regimes; plants developed a shorter primary shoot axis and leaves exhibited a marked increase in frost hardiness. In berseem clover ABA can thus substitute, at least partially, for the low temperature treatment required to induce cold hardiness. Spraying plants raised under hardening conditions with gibberellic acid reversed the effects of the hardening treatment, since they developed an elongated shoot and exhibited frost sensitivity comparable to non-treated plants grown under non-hardening conditions. It is concluded that these endogenous hormones are directly involved in triggering changes in morphogenesis which accompany physiological and metabolic events associated with the induction of plant cold hardiness.