Plant Responses to Liquid Spectral Filters

Height and internode length

Reports from Norway indicated that liquid CuSO4 filters reduced plant height and internode length of chrysanthemum, tomato, and lettuce plants. Green and yellow filters increased plant height of these crops compared to natural light. In chrysanthemum and tomato, lateral buds were stimulated by CuSO4 filters but inhibited by green and yellow filters. Poinsettia and two cultivars of chrysanthemum, ‘Spears’ and ‘Yellow Mandalay’, grown under CuSO4 filters had reduced heights and internode lengths compared to control plants grown under natural light or water filters. A later study evaluated the influence of concentration of CuSO4 (4%, 8%, and 16%) in the filter on the growth of ‘Bright Golden Anne’ chrysanthemums. Increasing the concentration of CuSO4 in the filter from 4% to 16% reduced PPF by 26% to 47%, respectively, compared to control. Average plant height and internode length were reduced by about 35% regardless of the concentration, suggesting that concentrations as low as 4% CuSO4 could be effectively used. Height reduction under CuSO4 filters was caused mainly by the decrease in internode length, as the number of nodes was not altered. In all these studies, the PPF had been adjusted to be the same among all treatments.

In addition to height reduction, plants grown under CuSO4 filters had more leaf chlorophyll, darker green leaves, and were compact than control plants, similar to plants treated with chemical growth regulators. Subsequent studies revealed that a wide range of plants respond to CuSO4 filtered light (Table 1). Azalea and bulbs such as tulip, hyacinth, and daffodil did not respond to CuSO4 filtered light.

Table 1. Plant height reduction in response to CuSO4 filtered light.

Positive response No response
Ageratum Easter lily Azalea
Geranium Poinsettia Tulip
Impatiens Lettuce Hyacinth
Pansy Chrysanthemum Narcissus
Pepper Miniature roses  
Petunia Exacum  
Salvia Vinca  
Tomato Marigold  

In chrysanthemums, CuSO4 filtered light reduced height by Å30% in short photoperiod-grown (fall and spring) plants but in long photoperiods (summer), plant height reduction was Å20%. A similar response was observed with miniature roses grown in short and long photoperiods.

Water use

Work with chrysanthemum ‘Bright Golden Anne’ indicated that plants grown under the CuSO4 filter had about 37% less cumulative water use than control plants. However, water loss rate per unit leaf area was similar between plants grown under CuSO4 and control filters suggesting this reduction in cumulative water loss was due smaller plant size. Plants grown under the CuSO4 filter had lower stomatal density compared to control plants. Light transmitted through the CuSO4 filter did not alter the size of individual stomata. Total number of stomata and total stomatal pore area per plant was about 50% less in plants grown under the CuSO4 filter than those of control plants due to less leaf area.

Flower development

The influence of filtered light on flower development and flower quality varied with plant species, cultivar, and growing season. In ‘Meijikatar’ miniature roses, CuSO4 filters slightly accelerated (2-3 d) anthesis of early spring grown plants but slightly delayed (2-3 d) anthesis of late spring- and summer-grown plants. In ‘Bright Golden Anne’ chrysanthemums, CuSO4 filters delayed anthesis by 7 d in early fall-grown (September) plants and by 13 d in late fall-grown (December) plants. Spectral filters did not affect total number of flowers, but plants grown under CuSO4 filters produced smaller flowers than control in both miniature roses and chrysanthemums. In ‘Spears’ chrysanthemums, CuSO4 filters promoted earlier flowering under non-inductive natural long days compared to control plants. However, under artificial short days, CuSO4 did not affect the time to flower in ‘Spears’ chrysanthemums. In ‘Nellie White’ Easter lilies, CuSO4 filters did not delay anthesis or reduce flower size.

Dry matter accumulation and partitioning

Total shoot dry weight of chrysanthemums decreased when plants were grown under CuSO4 filters. Shoot dry matter partitioning was also affected by CuSO4 filters; reduced stem dry matter accumulation and increased leaf dry matter accumulation. This suggests that the translocation of photosynthates may be affected by light quality under CuSO4 filters. The dry weights per unit leaf area and the unit length of stem were reduced by light transmitted through the CuSO4 filter.

Carbohydrate status

CuSO4 filters also reduced both leaf and stem total soluble sugars (sucrose, glucose, and fructose) and starch concentrations in miniature roses and chrysanthemums. However, the magnitude of reduction varied with the growing season; greater in spring than in fall. For example, CuSO4 filters reduced leaf soluble sugar concentration by approximately 54% in spring-grown chrysanthemum plants but only 29% in fall-grown plants. The reduction in carbohydrate pools may be a result of reduced photosynthesis or increased respiration of plants grown under CuSO4 filters. Our preliminary work with chrysanthemums indicated that the rate of photosynthesis was lower in plants grown under CuSO4 filters than under control filters but rate of respiration was not different.

Postharvest quality

The reduced carbohydrate levels of plants grown under CuSO4 filters could lead to adverse effects on postharvest longevity. Work with miniature roses indicated that postharvest quality was reduced and leaf yellowing increased in plants grown under CuSO4 filters compared to control plants. In Easter lilies and chrysanthemums, CuSO4 spectral filters reduced flower shelf life by 3 to 4 d compared to control plants. Plants subjected to 4 oC storage for 1 week before being placed in an interior environment exhibited even less shelf life.