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Pollen tube growth of ‘Tilton’ apricot (Prunus armeniaca L.) was reduced by exposure to SO2. An empirical model, based on modified spline functions, described the decrease in pollen tube growth due to increasing dose (hours exposure × concentration of SO2) expressed on logarithmic scale. The response curve, based on this analysis, was “S-shaped” with a decrease from 98.5% pollen tube growth (PTG) for unexposed pollen tubes, based on percent of the style that the longest pollen tubes had grown to 91.6% at ℓ n dose 4.0. These was then a very sharp decrease in PTG to about 45% at ℓn dose 5.7, then a gradual curvilinear response to <1% PTG at the maximum ℓn dose of 7.7. There data strongly indicate that there is a threshold response to SO2 with respect to PTG. Response of ‘Van’ PTG in ‘Napoleon’ sweet cherry (P.avium L.) styles was similar to apricot, but not as definitive because of greater within year variation and differences between years. In one year (1979), a spline function model of PTG in cherry suggested a threshold value at about ℓn dose of 2.1, while in another year (1978), there was a nearly linear decrease in PTG with increasing ℓn dose.

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