Plant-Environment Interactions, 431-434 (2000)
Robert E. Wilkinson

Phytochrome in Crop Production
M. J. Kasperbauer


Development of colored mulch technology was a natural progression from the research with plants growing in sunlight over panels with different surface colors, as described above. Use of exterior enamels to provide the different panel surface colors was an economical and convenient approach for obtaining a range of reflection spectra for small plots. Because the visible and FR parts of the spectrum were both important for plant growth, it was necessary to know the reflection spectrum for each batch of paint before we could interpret the plant growth responses. The approach with painted panels was to allow plants to grow in summer sunlight for photosynthesis and to use a reflected FR/R photon ratio to act through the natural photomorphogenic pigments (primarily phytochrome) within the growing plant to regulate partitioning of the photoassimilate to developing roots, shoot, and fruit. The working hypothesis (based on previous observations of seedling growth responses to FR at 750 to 760 nm on the Beltsvillc Spectrograph, experiments in controlled environments, reflection from nearby growing plants, upward reflection from different colored dead plant residue, and reflection from pointed panels) for use of different colored panels in sunlight was that an upwardly reflected FR/R photon ratio higher than the ratio in incoming sunlight would signal the plant to allocate more of its new resources to shoot (including fruit) growth, while a FR/R ratio lower than that in the incoming sunlight would favor root growth.

In 1986 D. R. Decoteau, who was a new horticulturist at the Clemson Pee Dee Research Center at that time, asked if he could join in for a field test with trickle-irrigated tomato (Lycopersiron esculentum Mill.). It was an ideal choice, because we had already tested R-FR photoreversible control of allocation of photosynthate in tomato and other food-crop seedlings in a controlled environment. In that experiment, all seedlings were of the same age and received the same amount of photosynthetic light. Nevertheless, those that received a brief exposure to FR (a higher FR/R photon ratio) at the end of each day had larger shoots and a higher shoot/root biomass ratio than those that received R (a low FR/R ratio). Shoots of seedlings that received a brief exposure to R immediately after the FR remained smaller and appeared the same as those that did not receive the FR treatment. This strong photoreversible control of seedling morphogenesis by phytochrome indicated a high probability that sun-grown tomato plants would be responsive to the FR/R photon ratio reflected from the soil surface.

46. DR Decoteau, MJ Kasperbauer, PG Hunt.
J Am Soc Hort Sci 114: 316-220 (1989)

The experiment that contributed greatly to early stages of the colored mulch technology was relatively simple. Standard black plastic mulch was placed over trickle-irrigation tubes in raised-bed field plots. A range of upwardly reflected spectra was obtained by painting some of the plastic with exterior enamel. Subplots were painted red or white and some were left as unpainted black (controls). These colors were selected because black plastic mulch (over trickle-irrigation tubes) was widely used in commercial tomato production to conserve water, control weeds using less herbicides, and keep fruit clean. Red and white were used because of our previous experiments with small painted insulation panels (discussed above). The red paint that we used reflected a higher FR/R photon ratio than was present in incoming sunlight at the same time and place, whereas the white paint reflected much more photosynthetic light than the red paint but a FR/R photon ratio very similar to the ratio in the incoming sunlight. Soil temperature was cooler under white-painted plastic but very similar below red and black. The basic experiment was conducted for 2 years and in two locations. The early-crop tomato yields were 12 to 20% higher over red than over the standard black (control) (46). Early crop yields over the white surfaces were lower than those over black or red. In follow-up experiments, we found that yields sometimes differed over different hatches of red paint. All of these observations contributed to the development of the colored mulch technology.

Patent applications were filed and the technology was licensed by a major manufacturer of plastic mulch. The next step was the development of a "theoretically ideal" reflection spectrum for yield of tomato, strawberry (Fragaria x

<missing 432-433>

that potential is attained. Light is a component of the environment that follows a generally predictable pattern year after year at a given geographic location. Light involvement in photosynthesis is well known and widely studied. However, photomorphogenesis is involved in the allocation and use of the products of photosynthesis in a manner that favors survival of the plant as it proceeds through its life cycle. Knowledge of the natural regulatory systems involved in photomorphogenesis is important in developing innovative strategies for crop improvement.

Phytochrome is an important photomorphogenic pigment system that signals seedlings when other plants are nearby and they must adapt to the competition; it also tells grown plants when to flower, so that the seed will have time to ripen before adverse weather sets in. Knowledge of phytochrome action in regulation of photoperiodic control of flowering has resulted in development of cyclic lighting, which is now used internationally to control time of flowering in the floral industry at a fraction of the cost of continuous lighting to extend photoperiod. Awareness that the phytochrome system in growing plants (especially seedlings) responds to FR reflected from nearby growing plants and that an increased FR/R photon ratio acts through the natural phytochrome system within the plant to allocate more growth to shoots is important in developing new field-crop management systems. For example, plant spacing, row orientation, and even the color of soil and dead plant residue on the soil surface can reflect morphogenic light patterns that affect yield and quality.

The accumulated information on phytochrome regulation of morphogenesis in controlled environments as well as the phytochrome-regulated growth response to FR reflected from nearby growing plants has led to development of colored mulch technology. Although other photoreceptors are involved in affecting some flavor and nutrient components in food crops grown over colored mulches, the FR/R photon ratio reflected from mulch on the soil surface to sun-grown plants can have a major impact on the allocation of new growth among developing roots, stems, leaves, fruit, and seed. An objective of the colored mulch technology is to retain the water-conservation, soil-warming, and weed-control benefits of standard black plastic mulch and to add the yield- and quality-enhancing benefits of reflected morphogenic light at little added cost to the grower. Enhanced yield of tomato and strawberry have already been documented over the red selective reflective mulch versus standard black plastic mulch, as have some effects on the flavor and nutrient quality of food crops. Many other experiments on yield and quality of shoot and root crops are in progress with red and a range of other colors versus standard black plastic mulch. The colored mulch technology has advanced during the last 15 years from a laboratory theory to reality in improving crop yield and quality, with worldwide implications.