CA1321876C - Method and apparatus for stimulating plant growth - Google Patents

Abstract

ABSTRACT OF THE INVENTION
The present invention discloses a method and means for selectively simulating predetermined developmental responses in plants by the positioning of a reflective surface generally between the soil surface and the leaves of the plants. The reflective surface is of a predetermined color for upwardly reflecting light of a predetermined wavelength for absorbtion by the plants, such that the upwardly reflected light of the predetermined wavelength selectively stimulates the photoreceptor systems of the plants for stimulating predetermined developmental responses. A plastic mulch having an upper surface of the predetermined color is disclosed for selectively reflecting the predetermined waveband of light upwardly to the leaves of the plants.

Description

132~876 PATENT
Attorney Docket No.: CXU~78 TITLE: METHOD AND APPARATUS FOR
STIMULATING PLANT GROWTH
BACKGROUND OF THE ~NVENTION
This invention relates to a method and an apparatus fox stimulating certain plant developmental responses by providing colored soil coverings, which upwardly reflec~ light of a predetermined waveband for absorbtion by the plants. The upwardly reflected light selectively stimulates photoreceptor systems of the plants for enhancing the predetermined developmental responses.
The growth and development of plants are regulated by genetics and various components found in the growth environment. Plants have internal sensing systems that respond to different spectral balances or wavebands of light, i.e., coloræ, or to different ratios or combinativns of two or more wavebands. These internal sensing systems include photoreceptor systems which use the wavebands o~ light perceived by the plant to regulate plant developmental processes. The developmental processes generally invoIve the effect~
on the balance of natural growth regulators within the plant,~ and serve to alter or modify the partitioning of photosynthate among the various plant parts such as the roots, leaves, stemr fruit, etc.
Exposure of plants to An increased amount of light of a particular waveband may cause change~ in certain developmental ~haracteristics of the plants. For example, upon exposure of the plants to increased blue light, the growth of the length of the stems of the plants is suppressed, while the root growth of the plants is increased. Also, as is well known, through exposure of increased red or far-red light to the ' ~32~7 ~

plants, the photoreversible effects of the light acting through the photoreceptor system of the plants can be regulated to vary the time of flowering, fruiting, etc.
It is known that the effect of changes in lighting on many physiological processes of plants can be obtained in controlled enviro~ments equipped with special artificial light sources, filters, and timers.
However, such ~ystems may be costly, complicated, and do not lend themselves readily to both indoor and outdoor commercial growing operations.
Turning ~rom the effects of lighting on plants, another plant growth improvement means is mulch. Mulch commonly involves the use of a protective covering for soil, such as compost, straw, sawdust, paper, plastic or the like. Mulch is typically placed upon the ground beneath the plants and around the stems thereof for reducing evaporation, controlling soil temperature and weeds, for conserving water, for preventing erosion, etc. Black plastic sheeting ls commonly used as a mulch material, and in addition to controlling weeds, evaporation, erosion, etc., provides the sometimes desirable effect of warming the soil therebeneath.
The present invention includes providing a mulch with a reflective sur~ace means for reflecting light upwardly therefrom of a waveband which can alter the light of the growth environment, or microenvironment, of growing plants to su ficiently alter plant growth characteristics, such as ~hoot-to-root ratios, nodulation, yields, etc.
SUMMARY OF THE INVENTION
From the foregoing, it is a general object of this invention to provide both a method and a means for selectively enhancing specific pIant developmental responses.
It is a more specific object of the present ......... . . ..

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inventlon to provide a mulch having an upper surfa~e of a predetermined color for reflecting upwardly light o~
a predetermined waveband for selectively stimulat~ng the photoreceptor systems of a plant.
Another ob;ect of the present invention is to provide a method and a means for improving the qUantity and ~uality of plant products at a reas~nable cost without the use of applied chemicals and while conserving soil and water resources.
Still another object o~ the present invention is to provide a colored surface for natural t light-mediated regulation of plant growth and development for use in the vegetable, horticultural, and land reclamation industries.
Various combinations of the presently disclosed features may be provided in a given embodiment thereof in accordance with this invention. Generally, one such exemplary embodiment of the present invention includes a device for selectively stimulating predetermined developmental responses of plants in a plant growth medium. The device comprises a reflective medium for receiving light positionable generally above the plant growth medium, the reflective medium being for reflecting light o~ a substantially predetermined waveband only for absorbtion by the plants. The reflected light is of a predetermined waveband for selectively stimulating the photoreceptor systems of the plants, which results in stimulation o~
predetermined developmental responses in the plant. A
more specific embodiment of the reflective medium includes an upper surface of a plastic sheeting mulch having a pxedetermined surface color ~or upwardly reflecting light of the predetermined waveband.
Another aspect of the present invention include~ a method for selectively stimulating predetermined plant ,~

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~L323L~6 developmental responses in plants being grown in a growth mediu~. ~he method comprises providiny a preselected reflective surface that reflects light therefrom of a predetermined waveband for stimulating a predetermined plant developmental response. The method further includes positioning the reflected medium in a location wh~re upon receipt of light thereon, light of substantially the predetermined waveband only is reflected onto the plants ~or absorbtion thereby, whereby the predetermined plant developmental responses of the plants are stimulated. More speci~ically, the positioning of the reflective medium may include placing a mulch generally between the growth medium and the leaves of the plants which has a predetermined upper surface color for reflecting light of substantially the predetermined waveband. The predetermined waveband of light is selected for providing a predetermined stimuli to the photoreceptor systems of the plants, the predetermined stimuli causin~ stimulation of the predetermined developmental responses in the plants.
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Th~ foregoing as well as other object~ of the pres~nt invention will be mor~ apparent from the following detailed description of a preferred embodiment of the invention, including the best mode thereof, when taken together with the accompanying drawings, in which:
Figure 1 is a perspective view of mulch constructed in accordance with the present invention being used in c~njunctlon with tomato plants;
Figure 2 is a sectional view along lines 2-2 of Figure 1 illustrating light being reflected o$f of a contoured upper surface of a mulch material; and Figure 3 is a perspective view of an alternate . .

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embodiment of the presen~ invention and includes a ~oil covering having colored exterior surfaces.
DESCRIPTION_OF THE PREFERRED EMEI l~IMENT
Referring to the drawings in detail, whexein like re~erence characters represent like elements and/or features throughou~ the various views, the reflective medium or surface of the present invention is designated generally in Figure 1 by the reference character 10. Reflective surface 10 includes plastic sheeting 12 which serves as a mulch or soil covering for soil 14~ Soil 14 is formed in the shape of a mound 16 and includes a row, generally 18, of tomato plants ZO staked up by stakes 22. Twine wrappings 24 hold tomato plants 20 to stakes 22 in a conventional manner.
It ls to be understood that the present invention is not limited to use with tomatoes only, but could be used in con;unction with a variety of plants such as , peas, corn, soybeans, etc. The present invention cou~d also be used with bushes, shrubs, or trees, and in particular, fruit trees. Tomato plants 20 are shown in the Figures for use with the present invention for illustrative purposes only.
As shown in Figures 1 and 2, tomato plants 20 extend upwardly throu~h mulch 12 at openings 26~ As best shown in Figure 2, the stem 28 of a tomato plant 20 extends through opening 26 and terminates at the surface 30 of soil 14. Below surface 30 of soil 14, the roots 32 of tomato plant 20 extend downwardly into soil 14.
The upper surface 34 of plastic sheeting 12 is provided with a predetermined color. Alternatively, plastic sheet 12 could be entirely of the predetermined color throughout such that upper surface 34, lower surface 36 and body portion therebetween are all of the predetermined color. The predetermined color is chosen ... . .. .

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to alter the spectral composition of upwardly xeflected light such that light o$ a predetermined waveband is reflected upwardly, indicated by dotted arrows 40 in Figure 2, to act through photore~eptor systems within plant 20 for producing selected developmental responses. The term waveband as used herein includes the spectral balance and the spectral distribution of the upwardly reflected light. The developmental responses of the plants may include altered flowering, fruiting, shoot-to-root growth rates, nodulation of legume roots, etc. Natural light, artificial light, or filtered light is indicated by arrows 42 as incident light rays which impinge upon plant 20 and reflective surface 10 from directly overhead. It is to be understood, however, that reflective surface 10 will serve to reflect light upwardly into plants 20 even when the incident light is not directly above plants 20, but is angled with respect thereto~ Also, reflective surface 10 could be oriented with respect to the plants such that it extended upward abou~ the peripheries of the plants, or even above the plants, iP, for example, an upwardly directed light source was used.
As discussed later, attachment of an appropriately colored upper surface 34 to plastic sheeting 12 allows for a controlled reflection of a specific waveband of light and ratios of wavebands of light to be reflected upwardly from reflective surface 10 to the leaves 44 of the plants ~0 for eliciting desired pl nt responses such as, altered shoot-to-root ratio growths, altered nodulation of legume roots, and altered yields of crops such as tomatoes. It is to be understood that reflective surface 10 could be provided as a patterned surface (not shown) of different colors andior designs for reflecting several specific wavebands of light at : 7 ~,187~

specific ratios with respect to one another. Such would allow for several plant developmental responses to be stimulated simultaneousl~.
Figure 3 illustrates ~n alternate embodimen~ of the present invention and includes a ~pigmented substance having a predetermined color as d.iscussed above which can ~e applied directly to soil surface 30, or a ground covering mulch, generally 48, suc'h as synthetic fiber, straw, sawdust, plastic or paper materials, or the like. The predetermined color of the pigmented substance allows for the selected waveband or spectral composition of upwardly reflected light therefrom to act through the pigment systems of the plants 20 similarly as reflective surface io of plastic sheeting 12. In one possible application, the wheat stubble remaining in a wheat field after harvesting of wheat could be painted or dyed with a pigmented substance of a color for stimulating certain developmental responses of soybean plants, which could be subsequently grown in the field.
An important feature of the present inven~ion is the provision of a reflective surface of a particular color for reflecting light of a desired waveband or spectral composition upward to stimulate specific plant developmental responses. For example, a blue upper surface 34 has been provided as reflective surface 10 for stimulating growth of roots 32 relative to the growth o~ the shoots, generally 45, of tomato plants 20. In another example, a red upper surface 34 has been provided reflective surfacP 10 for increasing the yield of tomatoes 46 of.tomato plants 20.
As discussed above 9 a specific surface color can be applied to upper surface 34 of plastic sheeting 12, or, plastic sheeting 12 could be a predetermined color throughout. Alternately, other mulches could be - :.; .

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provided which are the select~d predetermined color throughout or which had a pigmented substance of the selected predetermined color applied to the exterior surfaces thereof. For example, a granular fertilizer of the selected predetermined color throughout could be deposited on a surface 30 of soi] 14 for reflecting a predetermined waveband of light upwardly to stimulate specific plant developmental res~)onses. Further, the outer surfaces of the granular fertilizer could be provided with a paint ~r dye of t:he predetermined color for yielding similar results~ Still further, a pigmented substance could be applied directly to surface 30 of soil 14 for also providing a reflective surface for reflecting the specific waveband or wavebands of light.
Reflective surface 10 could be used in conjunction with spaced plants or row crops, an~ could be applied to the surface of a penetrable material ~or improving survival and growth of broadcast seedings such as those used on road banks or other erosion control projects.
The spectral composition of reflected light, as indicated by arrows 40, is substantially controlled by the color combinations and glossiness of the various reflective surfaces and materials therefor. The materials used for providing the suitable reflective surface 10 can be used in conjunction with other soil and water conservation systems for substantially tailor-making plants for specific needs.
Certain physiological aspects of plants allow the successful results made possible by reflective sur~ace 10. Plants are adapted for growing under natural sunlight or artificial light and include internal mechanisms for allowing them to adjust to even relatively minor changes in light conditions. Plants contain delicate environment sensors which can regulate . !, ', ' , . ' 132~87~
physiological and developmental processes that lead to better adaptation to what the plants peraeive as their growth conditions. For example, genetically identical plants will develop guite differently if they are grown in a crowded population density as opposed to a sparse population density. Hence, the plants are able to sense their surrounding conditions and adjust their development accordingly.
Plants include photoreceptor systems which include a pigment known as phytochrome which can measure the amount of competition for light by other nearby plants.
This is accomplished by the photochrome sensing the amount of far~red light reflected by o~her green leaves of the nearby plants. Accoxdingly, the development of the plants is affected dramatically by shifts in the spectral balance of the wavelengths of light received in their total light environment. However, plants cannot differentiate as to whether the source of the shifted spectral balance of light is a reflec~ion from competing vegetation, artificial light and special filter systems in controlled environments, or light reflected upwardly from the soil surface, or from reflective surfaces of a predetermined color, as disclosed by the present invention.
Plants contain various photoreceptors, each photoreceptor having its own particular function in relation to the light absorbtion spectrum. Plants cannot perceive the source of an altered light spectral distribution, which would conse~uently affect the light absorbtion by the various photore~eptors and thus, the developmental characteristics ~f the plant~ related to the function of the photoreceptors. Since plants, particularly seedlings, are extremely sensitive to even relatively small changes in the spectral distribution of light, a soil surface covering of a predetermined : . . -,~ .~ - . .` . . .
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13~76 color may modify the light microenvironment, through effects on spectral distribution of reflected light received by the plants, in a manner sufficient to alter the growth and development of the plant. The difference in intensity of a predetermined wavelength, or color, of light, or in a ratio of two or more wavelength ranges or wavebands, of light can have a very pronounced effect on the plant.
By reflecting more light for absorb~ion by the plant, the amount of photosynthate produced by the plant can be .increased. By reflecting li~ht o~ a particular waveband, the utilization of the photosynthate by the plant can be affected. Therefore, different colors of reflective surfaces provided to a medium such as soil will allow specific wavebands of light to be reflected, for allowing specific tailoring of plants or plant products through a selected natural regulation of photosynthate partitioning among the shoots, fruit or seed, and roots of the plants. Also, the metabolic products within the plant can be altered by shifts in light spectral distribution, which can ultimately affect consumer acceptability of the plant products such as vegetables and fruits.
In order for light to be effective in plant development, it has to enter the tissue of the plant and be absorbed by a photoreceptor. Thus, two factors are important in relation to light and resulting plant development. The first factor involves the guantity and wavebands of light that reaches the lea~ surface.
The second factor involves the light transmission characteristîcs of the leaf surface itself.
The leaf surface of plants can selectively filter out some wavebands, or colors, of light and transmit other wavebands. Also, the transmission characteristics of the lower leaf surface differs . .

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somewhat from the transmission characteri$tics of the upper leaf surface of some plants. Fundamentally, even if a leaf surface is capable of transmitting a certain waveband of light, the plant will not respond physiologically unless the waveband of light is actually received at the surface.
Experiments have been conduct:ed for evaluating the effects of use of a reflective surface of a predetermined color provided in accordance with ~he present invention. Several of the experiments which ~ hJ
have been conducted in accordance with the present invention by applicants are summarized as follows~
EXPERIMENT A ~C~
A plant bed system in a greenhouse was used in this ~;~ ter 6 experiment for facil~tating accurate biological and physical measurements, as well as for minimizing environmental extremes. Six plant beds ~3.4 m x 1.0 m x 0.25 m) were built and placed in a glass greenhouse and filled with soil. Lime and fertilizer were applied to the soil according to soil test recommendations, Trickle irrigation tubing and plastic mulch were placed over the soil prior to transplanting.
Several colors of plastic mulch were evaluated.
Treatments were established by uniform painting of a black polyethylene mulch surface with the appropriate colorecl paint. Plots were 1.6 m in length.
Experimental plots were arranged in a randomized complete block design with 3 replicates. Red, silver, and white paints were utilized to produce colored plastic mulches.
The tomato (Lycopersicon esculentum L.) cultivar Mountain Pride was used in ~his experiment. Five to , six week old transplants were placed in the beds at a;
30.5 cm ~pacing (one row per bed~. Pruning of laterals was not practic:ed and insecti ide and fungicide sprays were not needed.
Half plots were harvested at 22 and 50 days after transplant~ng. Total plant biomass and fruit and flower numbers were recorded. Analysis of the variance between the plants was used to determine significant effects and least significant difference (LSD~
calculations to compare means.
Environmental measurements of light reflection off the plastic and soil temperature was recorded during the s~udy. Light in~ensi~y and ~uality (spectral distribution) o~ reflected light were measured on a representative cloudless day at solar noon with a LiCor LI-1800 Spectroradiometer, manufactured by LI-COR, of Lincoln, Nebraska. Light intensity measurements are ~ ~
reported as photosynthetic photon ~l~x density (PPFD) ~ 8 and ~uality as the quantum ratio of ~ ~to r ~ ~D/,~
light. Soil temperatures at a ~.5 cm depth were , pe ~
measured continuously in all plots for a three week 6/~ 7 period with a Campbell CR7 Data Logger, manufactured by Campbell Manufacturing of hogan, Utah, e~uipped with fixed thermocouplers.
Fresh weight, dry weight and leaf area of tomato plants harvested at a preflowering physiological stase of development (22 days after transplanting) were affected by mulch color treatments (Table 1). Plants grown with black or white~;mulch produced more leaves and petioles, and had ~ stems than plants with ~/~r~
silver or red mulch. Plants in the white mulch ~e~
treatment also had greater amounts of lateral leaves (suckers) than those in the other treatments. Plants in the darker ~olored mulches (red or black) tended to have longer internode lengths at the lowex nodes than plants in the lighter colored mulches (white or silver).

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13~1876 Table l. Influence of mulch color on the growth and development of tomato plants 22 day~ ater transplanting.
~._ Mulch Color Biomass Production Leaf Area/Plant Leaves & Petioles Stem Main Lateral (g DW/plant) (cm~) Black 33.4 0.58 795.04 46.77 Red 20.8 0.38 545.19 15.62 Silver 21.8 0.43 605.87 15.76 White 32.7 0.53 735.04 111.27 Ta~le 2. Influence of mulch color on main stem inter-node lengths of tomato plant~ ~2 days after transplanting.

Mulch Color Internodes Length (mm) Black 26.1 29.1 38.4 40.9 46.7 Red 28.7 2708 27.5 36.6 30.7 Silver 17.1 19.9 22.0 33.1 ~5.
White 15.4 19.2 21.6 29.8 35.8 LSD (0.05) _ 8.5 4.3 5.2 8.4 10.3 At a later harvest (50 days after transplanting),~
plants in the wh~te mulch treatment had fewer flowers than plants in the other treatments ( Table 3 ~ . The greatest number of fruits were observed on plants .in the silver treatments, while the largest fruits were measured in the black mulch treatments.

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- ~2187~-Table 3. Influence o~ mulch color on fruit an~ flower production ~$ tomato plants 50 days after tran~planting.
___ Flower_ no. ~ Ave E~ no.
plant plant fruit plant t Black 18.5 1616.00 87.72 16.0 Red 19 . 7 122B .18 58 . 5018 . O
Silver 29.1 ~745.67 64.39 16.2 White 22.0 1585.00 78.68 5.1 LSD (0.05) 7.3 nsd 29~82 10.7 ~ he color o~ the mulch lnfluenced the plant light envir~nment. The lighter color mulches reflected more total light; but a lower ratio of far-red relative ~o red light (Table 4). Increase in light intensity can affect plant development and yield through increases in the photosynthetic rates, and the ratio of far-red relative to red light is important in phytochrome regulation of plant physiological processes and ~an affect internode lengths and stem elongation, chloroplast ultrastructure, photosynthetic efficie~cy, and photosynthate partitioning among leaves, stems and roots.
Table 4. Light reflection upward to selec~ed points : above the plastic.
_ Distance Above Plastic (cm) Light Parameter Mulch Color lO 20 30 Photosynthetic Pho~g3LI_~y~ y~ (umol m~2s~l)a Red 167 lS1 175 5ilver 737 411 385 Whlte 794 594 537 Far-Red/Red Ratio Relative to Ratio in Direct Sunliqht Black l.01 1.061.06 Red 1.15 1.131.12 Silver 0.96 0.96 0.97 White O.9S 0.970.97 e' .;.

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: ~321876 a PPFD of direct sunlight was 1673 umol m~2s~l0 The warmer root zone temperatures were recorded under the darker color mulch trea~ments (Table 5). The warmest mean soil temperatures were recorded in the black and the coolest ln the white mulch treatments.
While soil temperatures can influlence tomato growth and yield, their role in this study is not well understood since the average recorded soil tlemperatures of the individual color treatments ware ~within a range previously reported for maximal growth of tomato.
Table 5. Mean soil temperatures under plastic mulch treatments.
. .,_ Mulch Color R~ot-Zone Tem~ratures (~c)d Black 23.7 Red 23.6 Silver 23.2 White 22.9 a Measured continuously over a 3-week period at a 2.5 cm soil depth.
EXPERIMENT B
This experiment was conducted at the Pee Dee Research and Education Centex near Florence, South Carolina, using a commercial cultivar (Mountain Pride3 and plant spacing commonly used in this geographic area. Seeds were sown in a commercial potting soil in a greenhouse on March 14, 1986. Uniform seedlings were transplanted to field plots on April 25, 1986. There, were 4 soil surface colors and 4 replicates. Each treatment consisted of 8 tomato plants spaced 18 inches apart in rows that were 60 inches apart. The plants were staked, tied and pruned as by commercial growers of the area. The soil surface colors were obtained by using commercially available black plastic mulch with the various colors painted on the upper surface.
Surface colors were black, red, white, and silver. All plots were irrigated and ~ertilized at the s~me rates and schedules. Tomatoes were harvested and graded according to commerclal procedures of the area.
As shown in the table below, the results are given for tomato productivity over black, red, white and silver surfaced plastic mulch.
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_ Yield/Acre Surface Color Market ble Culls of Mulch No. 1 No. 2 ~ . _ _ _ , ~ wt) Black (control) 229 108 11 Red 297 88 7 White 214 98 33 Silver 184 99 19 From the table, it can be noted that yield and quality o~er the red ~urface were considerably higher than those over the other colors. The red sur~ace reflected very low in the blue and high in the red part of the light spectrum. Although white and silver reflected more total light than black, all three had about the same red/blue ratio in the reflected light.
Seasonal changes and the amount and intensity of sunlight received b~ ~he plants could account for the difference between the results of Experiment A, wherein the silver-surfaced mulch produced higher yields, and Experiment B, wherein the red-surfaced mulch produced higher yields.
From the forgoing examples, it can be seen that a soil surface cover having an upper surface of a predetermined color can significantly effeGt plant growth characteristi~s, In choosing the proper color ~or reflective surface 10, the desired spectral distribution of color waveband or wavebands of light to which the plant is to be subjected is selected for , : ~ , . ` : `, ~ `

selectively stimulating the photoreceptor systems of the plant to cause the occurrence of pre~elected plant developmental resp~nses. Upon selection of the waveband or wavebands o~ light which are to be used, a medium is chosen having a surface color or pattern of colors which reflects light primarily in a chosen waveband or wavebands. The deterlmination of Which waveband or wavebands of light are reflected from a colored surface can be made using a conventional light meter. Once a medium has been chosen having a surface of the re<~uired color or color combinat~ ons, the medium could be put into production with same for yielding large quantities of the medium for use in a commercial growing operation. One ~uch medium disclosed by the present invention could be plastic sheeting mulch of the desired color or combination of colors.
While one preferred embodiment of the inven~ion has been described using specific terms, such description is for present illustrative purposes only, and it is to be understood that changes and variations to such embodiment, including but not limited to the substitution of equivalent features or parts, and the reversal of various features thereof, may be practiced by those o~ ordinary skill in the art without departing from the spirt and scope of the following cl~ims~

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Claims (10)

1. A method for selectively stimulating a predetermined plant developmental response in at least one plant being grown in a growth medium, the method comprising:
providing a preselected reflective medium that reflects light therefrom of a predetermined waveband with a ratio of far-red to red light pre-selected for stimulating a predetermined plant developmental response; and positioning said reflective medium in a location whereupon receipt of light thereon, light of substantially said predetermined waveband only is reflected onto the plant for absorption thereby, whereby said predetermined plant developmental response of the plant is stimulated.

2. A method for selectively stimulating a predetermined plant developmental response in at least one plant being grown in a growth medium, the method comprising:
providing a preselected reflective medium that reflects light therefrom of a predetermined waveband for stimulating a predetermined plant developmental response;
and positioning said reflective medium in a location whereupon receipt of light thereon, light of substantially said predetermined waveband only is reflected onto the plant for absorption thereby, whereby said predetermined plant developmental response of the plant is stimulated, wherein said providing of said preselected reflective medium includes placing a mulch generally between the growth medium and the leaves of the plant having a predetermined upper surface color for upwardly reflecting light of substantially said predetermined waveband, said predetermined waveband of light being selected for providing a predetermined stimuli to a photoreceptor system of the plant for causing stimulation of said predetermined developmental response in the plant.

3. A method as defined in claim 1, wherein said providing of said preselected reflective medium includes applying a pigmented substance of a predetermined color to the surface of the growth medium below the leaves of the plant, the pigmented substance giving the growth medium surface a predetermined color which upwardly reflects light of substantially said predetermined waveband, said predetermined waveband of light being selected for providing a predetermined stimuli to a photorecptor system of the plant for causing stimulation of said predetermined developmental response in the plant.

4. A method as defined in claim 1, wherein said providing of said preselected reflective medium includes applying a pigmented substance of a predetermined color to an upper surface of a growth medium cover positioned generally above growth medium, the pigmented substance giving the upper surface of the growth medium cover a predetermined color for upwardly reflecting light of substantially said predetermined waveband, said predetermined wavelength range of light being selected for providing a predetermined stimuli to a photoreceptor system of the plant for causing stimulation of said predetermined developmental response in the plant.

5. A method as defined in claim 1, wherein said providing of said preselected reflective medium includes placing plastic sheeting having an upper surface of a predetermined color generally between the growth medium surface and the leaves of the plant, such that said upper surface of said predetermined color upwardly reflects light of substantially said predetermined waveband, said predetermined waveband of light being selected for providing a predetermined stimuli to a photoreceptor system of the plant for causing stimulation of said predetermined developmental response in the plant.

6. A device for selectively stimulating a predetermined developmental response of at least one plant in a plant growth medium, the device comprising:
a reflective medium positionable generally above the plant growth medium for receiving light, said reflective medium being adapted for reflecting light therefrom of a predetermined waveband of light for absorbtion by the plant, said predetermined waveband of light having a ratio of far-red light to red light preselected for selectively stimulating the photoreceptor system of the plant, which results in stimulation of said predetermined developmental response in the plant.

7. A device for selectively stimulating a predetermined developmental response of at least one plant in a plant growth medium, the device comprising:
a reflective medium positionable generally above the plant growth medium for receiving light, said reflective medium being adapted for reflecting light therefrom of a predetermined waveband of light for absorbtion by the plant, said predetermined waveband of light being for selectively stimulating the photoreceptor system of the plant, which results in stimulation of said predetermined developmental response in the plant, wherein said reflective medium includes an upper surface of a plastic sheeting mulch having a predetermined surface color for upwardly reflecting light of substantially said predetermined waveband only.

8. A device for selectively stimulating a predetermined developmental response of at least one plant in a plant growth medium, the device comprising:
a reflective medium positionable generally above the plant growth medium for receiving light, said reflective medium being adapted for reflecting light therefrom of a predetermined waveband of light for absorbtion by the plant, said predetermined waveband of light being for selectively stimulating the photoreceptor system of the plant, which results in stimulation of said predetermined developmental response in the plant, wherein said reflective medium includes a growth medium covering having exterior surfaces of a predetermined color for upwardly reflecting light of substantially said predetermined waveband only.

9. A mulch for plants which is positionable generally between the soil surface and the leaves of the plants, the mulch comprising:
a sheeting material positionable of the surface of the soil generally beneath the leaves of the plants, said sheeting material having an upper surface of a predetermined color for receiving light, said upper surface of said predetermined color reflecting upwardly light of substantially a predetermined waveband for absorbtion by the plants, whereby the photoreceptor systems of the plants are selectively stimulated by said light of said predetermined waveband upon absorbtion thereof for causing a predetermined developmental response in the plants.

10. A mulch as defined in claim 9, wherein said sheeting material is a plastic of said predetermined color.