Physiological Basis of Light Quality Responses

Gibberellins (GAs) are a group of plant growth hormones involved in a wide range of plant processes such as germination, cell division, cell elongation, flowering and fruit set and development. Endogenous gibberellins play an important role in the control of stem elongation and internode length. Chemical growth retardants reduce plant height by suppressing the production of natural gibberellins and there are similarities between the effects of chemical growth regulators and CuSO4 spectral filters. Therefore, it is possible that GA biosynthesis or its action may be suppressed under CuSO4 spectral filters.

Stem elongation in response to changes in light quality may be mediated by changes in GA level or sensitivity to GA. In efforts to understand the physiological basis for growth control by spectral filters, we applied 50 mgL-1 (ppm) GA3 (Pro-Gibb) on the first day of spectral filter treatment or weekly to chrysanthemum plants grown under control or CuSO4 spectral filters. Both single and weekly applications of GA3 reversed the plant height reduction caused by CuSO4 filters, but the weekly applications were more effective than the single application. We also applied 3500 mgL-1 of daminozide (B-Nine), a known gibberellin biosynthesis inhibitor, weekly to chrysanthemum plants grown under control and CuSO4 filters. Daminozide treatment reduced plant height under both CuSO4 and control filters but the effect was greatest under the control filter.

The level of GA-like substances in apical regions is known to be high in plants treated with far-red light. Exposure to end-of-day far-red light reversed the reduction of plant height and internode length caused by the CuSO4 filters to a level comparable with plants that received no end-of-day far-red treatment under control filters. Exposure to end-of-day red light reduced height and internode length of chrysanthemum plants grown under control filters but had no effect under CuSO4 filters. Exposure to end-of-day far-red did not significantly alter height and internode length under control filters. Observations with exogenous GA application and with end-of-day exposure to red or far-red light suggest that reduction of gibberellin levels by CuSO4 filter may be, at least partially, responsible for plant height reduction.

Gibberellin biosynthesis is a complex process that involves several enzymes and intermediate gibberellins. The current research focuses on quantifying the endogenous gibberellin levels (GA19, GA20, and GA1) and on investigating the responses of spectral-filter-grown chrysanthemum plants to intermediate gibberellins (GA19 and GA20) in the GA biosynthetic pathway. Our quantification studies indicate that GA19 levels (inactive) were higher and GA1 (active) levels were lower in CuSO4 filter grown plants than in control plants. The response of CuSO4 filter grown plants to exogenous GA19 was lower than control plants. These preliminary observations suggest that the conversion of GA19 to GA20 may be reduced under the CuSO4 filters.