Influence of spectral quality on the growth response of intact bean plants to brassinosteroid, a growth-promoting steroidal lactone. II. Chlorophyll content and partitioning of assimilate

The chlorophyll content and partitioning of assimilate of bean ( Phaseolus vulgaris L. 'Pinto') plants were determined 6 days after treatment of the second internode (I₂ with 5 μg of brassinosteroid (BR), a growth-promoting steroidal lactone. Plants were grown for 6 days under equal levels (90 μmol s^-1 m^-2) of photosynthetic photon flux density (PPFD) provided by cool white fluorescent (CWF) or incandescent (INC) lamps and equal levels of far-red (28 W m^-2, 700–800 nm) radiation provided by the same INC or far-red (FR) fluorescent lamps. Brassinosteroid treatment had no appreciable effect on total biomass production but caused a decrease of 15–20% dry matter distribution in the upper portion of the shoot, a small (4%) but constant increase in dry matter in l₂ and a large (11–16%) increase in dry matter in the lower portion of the shoot (especially I₁). Treatment with BR increased assimilate accumulation in the primary leaves, especially under INC and FR lamps, and reduced dry matter in the trifoliate leaves. BR also caused a 16–21% reduction in total leaf area and even a greater reduction in area of the trifoliate leaves, but significantly increased specific leaf weight of the primary leaves and the first trifoliate leaf and the amount of dry matter in the lateral shoots under all radiation sources. In comparison to controls, BR treatment increased dry matter accumulation in the treated internode 3.3x under CWF and 1.6x under INC or FR. BR treatment also increased chlorophyll content in the primary leaves under all radiation sources and in the trifoliate leaves under CWF and INC lamps. These findings suggest a possible mobilization role of BR and establish the importance of adequate PPFD (and photosynthate) for maximum swelling and splitting response to brassinosteroid.