17 

 et al . , 1980), and cotton (Castro and Rossetto, 1979). Castro and 

 Rossetto (1979) reported that gibberellic acid treatment lowered the 

 osmotic potential of cotton plants such that it interfered with aphid 

 feeding. Thus, gibberellic acid increased activity of hydrolytic enzy- 

 mes results in conditions leading to increased cellular growth by (1) 

 increased cellular osmotic concentration which permits water to enter 

 the cell more rapidly thus diluting the sugars and causing cell expansion 

 (Kato, 1956; Audus, 1972; Cleland et al . , 1968; Salisbury and Ross, 

 1978; de la Guardia and Benllock, 1980); (2) increased cellular plasti- 

 city which allows the cell walls to stretch in response to the change in 

 osmotic potentials (Addicott, 1970; Stuart and Jones, 1977; Molz and 

 Boyer, 1978); and/or (3) increased availability of hexose molecules which 

 provide energy for respiration and the formation of pectin and 

 hemicellulose, the cell wall maxtrix polysaccharides and cellulose, the 

 microfibril fraction of the cell wall (Cleland et al . , 1968; Kaufman, 

 1974; Salisbury and Ross, 1978). 



Kaufman (1974) and more recently Rappaport (1980) reviewed control 

 points for the physiological activity of gibberellins. Figure 1, as 

 taken from Kaufman (1974), provides a schematic representation of mecha- 

 nism of gibberellic acid in growth metabolism at the cellular level as 

 discussed above. This scheme has the following basic features: 



(a) Under natural conditions photosynthetically produced sugars 

 function as the substrate for tissue growth. 



(b) Sugars enter the tissue at rates determined by transport 

 mechanisms and membrane permeability and become incorporated into a 

 pool of simple or phosphorylated sugars. 



