ABSTRACT
Cytokinins are a major group of plant growth regulators that modulate a number of physiological and biochemical processes (Mok and Mok, 1994). They markedly affect flowering, fruit set, ripening, leaf senescence, seed germination, and stomatal function (Davies, 1987; Skoog and Miller, 1957) and, as a result, are being used as bioregulators in commercial applications. The combination of endogenous and exogenously applied growth hormones is likely to induce physiological changes that also affect plant responses to pathogens and pests (Bailiss, 1977; Balazs and Kiraly, 1981; Faccioli et al., 1984; Hallahan et al., 1992; Hedin and McCarty, 1994; Hedin et al., 1988; Mills et al., 1986; Nicholson, 1992; Orr and Lynn, 1992; Plich, 1976; Thomas and Blakesley, 1987). On several occasions, cytokinin applications have been shown to suppress virus-and fungus-induced hypersensitive responses that are generally considered to be part of the disease resistance response in plants (Balazs and Kiraly, 1981; Beckman and Ingram, 1994). However, an overwhelmingly larger number of other reports correlate cytokinin applications with enhanced pathogen resistance. In one of the first reports, tissue culture-derived Solanum tuberosum plants became more resistant to the fungal pathogen Phytophthora infestans when certain combinations of Cytokinins and auxins were added to the medium (Ingram, 1967). Foliar applications of cytokinins have also been shown to be effective against viral infection. Symptom development was reduced in tobacco mosaic virus (TMV) infected Nicotiana tabacum, and cytokinin analysis of TMVresistant and -susceptible tomato cultivars revealed a strong correlation between resistance and higher cytokinin concentrations (Balazs and Kiraly, 1981; Li and Qiu, 1986). Although cytokinin influence on insect infestations has only been addressed indirectly, results from field trials using commercial formulations of natural cytokinins suggest that the overall increase in yields may in part have been due to reduced insect populations (Blanco-Montero and Ward, 1995; Hedin and McCarty, 1994).
