Alteration of Greenhouse Light Quality by Photoselective Covers to Produce Compact Plants

Nihal C. Rajapakse1, Teresa Cerny1, Shumin Li1, Ryu Oi2 and Sadayuki Esaki2,
1Department of Horticulture, Clemson University, Clemson, SC and
2Organic Performance Materials Laboratory, Mitsui Chemicals, Inc., Yokohama, Japan

Height control of greenhouse crops is an important practice that is achieved mainly by using chemical growth retardants.  Concerns for health hazards and environmental pollution, however, have led to regulations on the use of growth regulating chemicals in agriculture.  Several research teams around the world are investigating various non-chemical alternative height control methods, such as greenhouse temperature management and light manipulation.

A multi-disciplinary research team at Clemson University is currently investigating manipulation of light quality in the plant micro-climate to improve productivity and consumer acceptance of horticultural crops while minimizing chemical use.  We have shown that eliminating far-red (FR) light from the greenhouse by liquid CuSO4 spectral filters is effective in reducing height of a wide range of plants.  Consequently, greenhouse light quality manipulation can be used as an alternative to chemical growth regulators for producing short and compact plants.  Liquid filter technology, however, has limited value to a commercial grower because of the difficulties in handling, high construction cost, and phytotoxicity in the event of leakage.  Clemson University is collaborating with Mitsui Chemicals, Inc., Japan to develop and test photoselective plastic greenhouse covers that can absorb FR light and control plant height.

We have identified pigments that intercept red (elongation stimulating - tall type) or far-red wavelengths (elongation reducing - dwarf type) from the natural spectrum and are stable in polyethylene films.  Initial trials with dwarf type films focused on identifying a dye concentration that effectively filters out FR light from sunlight and reduce plant height while minimizing the reduction of photosynthetically active radiation (PAR).  Far-red intercepting films reduced height of bell pepper, watermelon, and chrysanthemum plants similar to liquid CuSO4 filters.  The height reduction increased as the dye concentration increased but the response varied with the species.  In general, watermelon plants showed the greatest height reduction followed by bell peppers and chrysanthemums.  Although filters with higher dye concentration were more effective in controlling height in some cases, the reduction in PAR can reduce the overall quality of plants.

Based on the initial findings, we have developed both tall and dwarf type photoselective films with a dye concentration that result in a 25% light reduction.  The crop response to photoselective films, film life, and alternative ways of using photoselective films will be discussed.