PHYSIOLOGICAL RELATIONSHIPS BETWEEN PHYTOCHROME EFFECTS ON INTERNODE EXTENSION GROWTH AND DRY MATTER ACCUMULATION INLIGHT-GROWN MUSTARD

The physiological relationships between the effects of phytochrome photoequilibrium (Pfr/P) on internode extension growth and dry matter accumulation were investigated in white light (WL)-grown Sinapis alba L. seedlings. After 11 days under continuous WL, the seedlings were exposed: (a) to pulses of light providing different Pfr/P, followed by 24 h darkness (D); (b) to pulses of light providing different Pfr/P, followed by 3 h D and 24 h continuous WL; (c) to continuous WL with or without supplementary far-red light (to reduce Pfr/P); or (d) to pulses of light providing different Pfr/P followed by D, in factorial combination with either water or a saturating (0.2 M) sucrose solution applied to one of the leaves. In D (“a” and “d”) low, compared to high Pfr/P increased both internode extension growth and dry weight to the same extent. Under WL (“b” and “c”) low PfrlP promoted internode extension growth but had no proportional effects on internode dry weight. Sucrose promoted internode extension growth with a lag of at least 8 h (compared to the rapid effect of low Pfr/P) and did not reduce the effect of low Pfr/P. These results indicate that Pfr/P effects on internode extension growth are not the consequence of changes in photoassimilate translocation from the leaves. Under WL, PfdP effects on internode length occur partially at the expense of internode dry matter per unit length.