The effect of boric acid (0-450 ppm) and sucrose (0-40%) on pollen germination and pollen tube growth
in Eucalyptus globulus, E. morrisbyi, E. ovata and E. tirnigera was examined in vitro. Over the con-
centrations tested, sucrose had by far the largest effect upon both pollen germination and tube lengths.
The optimum sucrose concentration for pollen germination (30%) and pollen tube growth (20%) differed
markedly with very little (<lo%) germination occurring in the absence of sucrose. The interaction of
sucrose and boric acid was significant. However, in general both pollen germination and pollen tube
growth were increased by the addition of up to 100 ppm boric acid, but above this level the response
plateauxed. The four species differed significantly in their pattern of response to both boric acid and
sucrose and the predicted optima derived from analysis of response surfaces differed between species.
The predicted sucrose concentration for optimal germination and growth of E. urnigera pollen was
consistently less than the other species and in terms of the optimal level of boric acid for pollen tube
growth species can be ranked in the order E. globulus > E. ovata > E. morrisbyi = E. urnigera. Pollen
germination and tube growth of all four species on a medium comprising 20% sucrose and 200 ppm
boric acid would not differ significantly from the observed maximum response of each species and this
could suffice as a generalised medium. However, if only percentage germination is to be assessed 30%
sucrose would be preferable. It is argued that subtle interspecific differences in optimal in vitro con-
ditions for pollen germination and pollen tube growth are likely to reflect differences in pollen physiology
which in vivo may have important implications for the success of hybridisation where pollen competition
occurs.