Fractional induction is the summation of inductive cycles despite the intercalation of non-inductive cycles. Plants reported to show fractional induction include the LDP sugar beet, Hyoscyamus niger, Plantago lanceolata, Lolium temulentum and the SDP Bidens tripartita, Glycine max cv 'Biloxi', Chenopodium amaranticolor, Impatiens balsamina, Caryopteris clandonensis (Lang, 1965, 1980; Vince-Prue, 1975) and Lemna paucicostata (Oota, 1984).

In Hyoscyamus, summation occurred when 1 LD was alternated with 1 SD and flower initiation was delayed, at most, by the number of days corresponding to the total number of intercalated SD (Lang, 1980). However, summation frequently fails unless more than a certain number of consecutive inductive cycles is given (Vince-Prue, 1975). In Kalanchoe, cycles of 1 SD/1 LD were not effective in inducing flowering, but cycles of 2 SD/2 LD were. Similarly in Glycine 'Biloxi', cycles of 1 SD/1 LD were ineffective, but pairs of SD could be partly summated even with as many as 12 intercalated LD; the biggest delay occurred with the first intercalated LD, and subsequent ones had little further effect (Fig. 6.1). Although summation is possible in both LDP and SDP, it has been suggested that, in general, the inhibitory effect of intercalated non-inductive photoperiods is greater in the latter (Lang, 1980);

Number of consecutive LD

FIG. 6.1. Effect of the number of consecutive intercalated LD on flowering in Glycine max Biloxi. After Carr, 1955.

Number of consecutive LD

FIG. 6.1. Effect of the number of consecutive intercalated LD on flowering in Glycine max Biloxi. After Carr, 1955.

in Hyoscyamus, for example, six cycles of 1 SD/1 LD gave flowering about as rapidly as 6 LD whereas, in the SDP Kalanchoe, a single LD interrupting 2 X 6 SD reduced flowering by more than 50%. Where summation occurs, the number of intervening non-inductive cycles does not seem to be very important provided that an adequate number of consecutive inductive cycles is given (see Fig. 6.1). Other examples include Impatiens balsamina, where SD cycles were summated with as many as 16 intercalated LD and Caryopteris (which is day-neutral for initiation but requires 11 SD for development to anthesis), where up to 30 LD could interrupt the SD treatment without altering the number of inductive cycles required. In okra, summation requires three consecutive SD; this could be carried over during four intercalated LD but the summation effect was partially lost when more than four consecutive LD cycles were given (Nwoke, 1986). Thus, it seems that, after an adequate number of inductive cycles, the resulting change is relatively stable and can be maintained for some time (the precise duration depending on the plant); it can then be added to when subsequent inductive cycles are experienced.

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