Michael J. Kasperbauer

Plastic Mulch Color Effects on Reflected Light and Tomato Plant Growth

Abstract Effects of plastic mulch color on upwardly reflected light and on the growth of tomato plants were investigated. Plants grown in sunlight over black polyethylene mulch had fewer axillary shoots (branches) and were taller than plants grown over white polyethylene mulch. The black surface reflected less total light and less blue light, but a higher ratio of far-red (FR) relative to red (R) light. In a controlled environment, tomato plants that were exposed to brief periods of FR at the end of the daily photosynthetic period were taller than those that received brief periods of R. The effect of FR on plant height could be reversed by R and implies phytochrome involvement. Differences in the light spectrum reflected from the plastic, and the similar differential responses to mulch color and light treatments, suggest that tomato plants grown in plastic mulch culture may respond to relatively small changes in light environment induced by the surface color of the mulch.

Light Reflected from Red Mulch to Ripening Strawberries Affects Aroma, Sugar and Organic Acid Concentrations¶

Abstract Strawberry (Fragaria ananassa) fruit size and flavor are important to both growers and consumers. Plastic mulches are frequently used in raised-bed culture to conserve water, control weeds with less herbicides, keep fruit clean and produce ripe berries earlier in the season. The most commonly used plastic mulch color is black. We hypothesized that changing mulch color to reflect more far-red (FR) and red light (R) and a higher FR/R photon ratio would keep those benefits and improve berry size and flavor by altering phytochrome-mediated regulation of pathways in ripening berries. Size and chemical composition of berries developed in sunlight over a specially formulated red plastic were compared with those that developed over standard black plastic mulch. Berries that ripened over red were about 20% larger, had higher sugar to organic acid ratios and emitted higher concentrations of favorable aroma compounds. We conclude that FR and the FR/R ratio in light reflected from the red mulch on the soil surface acted through the natural phytochrome system within the growing plants to modify gene expression enough to result in increased fruit size and improved concentrations of phytonutrient, flavor and aroma compounds.

PHYTOCHROME REGULATION OF MORPHOGENESIS IN GREEN PLANTS: FROM THE BELTSVILLE SPECTROGRAPH TO COLORED MULCH IN THE FIELD*

Development of the theory and use of photon ratios reflected from colored mulches to regulate plant morphogenesis in the field utilized information obtained on the Beltsville Spectrograph, in controlled environments and in field plant spacing studies. An upwardly reflected far‐red to red (FR/R) ratio higher than the ratio in incoming sunlight resulted in larger shoots and a higher shoot/root biomass ratio. A reflected FR/R ratio lower than the ratio in incoming sunlight resulted in larger roots and a lower shoot/root biomass ratio.

Light Reflected from Colored Mulches to Growing Turnip Leaves Affects Glucosinolate and Sugar Contents of Edible Roots

Abstract— Plastic mulches are widely used to conserve water and control weeds with less applied herbicides in production of food crops. Both yield and quality are important and can be affected by reflected blue (B), red (R) and far‐red (FR) light combinations received during growth and development. Photosynthate allocation among growing plant parts and flavor of edible roots were studied in turnip (Brassica rapa L.) grown in trickle‐irrigated field plots with blue, green and white mulches. The blue and green mulches reflected different amounts of B, but they both reflected FR/R ratios higher than the ratio in incoming sunlight. The white mulch reflected more photosyn‐thetic light and a lower FR/R ratio than the blue or green mulches. Plants grown with blue and green mulches did not differ significantly in leaf length, root size and shoot/ root biomass ratio. Those grown with white had shorter leaves and larger roots. Taste testers found that plants grown with blue mulch developed roots with a sharp flavor, and roots from plants grown with green mulch had a mild flavor. Those grown with white had a less distinct flavor. Roots grown with blue mulch had the greatest concentrations of total glucosinolates (GSL) and ascorbic acid. Reducing sugar concentrations were higher in roots grown with green than in those grown with blue mulches. The comparison of chemical composition of roots from plants grown with blue versus green mulches is important because the main difference was the amount of reflected B, suggesting that B influenced an enzyme involved in the pathway from glucose to GSL. We conclude that the spectrum of light reflected from mulch on the soil surface can influence not only shoot/root biomass ratio but also flavor‐related chemical composition of field‐grown food crop plants.

COTTON SEEDLING MORPHOGENIC RESPONSES TO FR/R RATIO REFLECTED FROM DIFFERENT COLORED SOILS AND SOIL COVERS

Cotton (Gossypium hirsutum L.) seedlings were morphologically responsive to red (R) and far‐red (FR) light (low and high FR/R photon ratios, respectively) from time of emergence. Controlled environment R and FR treatments confirmed phytochrome involvement in biomass allocation among new leaf, stem and root growth. Seedlings that received the high FR/R photon ratio developed longer and heavier stems, leaves with lower specific weights, less massive roots and higher shoot/root biomass ratios. Other cotton seedlings were grown in sunlight over different colored soils or painted panels to measure morphological effects of FR/R photon ratio in upwardly reflected light. Morphological responses to a given FR/R ratio were the same whether the reflection was from natural soil or from a painted surface. Seedlings that received the higher FR/R photon ratios in upwardly reflected light developed longer stems, leaves with lower specific weights, less massive roots and higher shoot/root biomass ratios. The potential use of colored mulches in agriculture was discussed.

MULCH SURFACE COLOR AFFECTS ACCUMULATION OF EPICUTICULAR WAX ON DEVELOPING LEAVES

Abstract— Leaf waxes are important to plant growth because they impede water loss and may influence entry of light. Leaf wax quantity and composition were studied in Capsicum annuum plants in trickle‐irrigated field plots covered with white, black or red plastic mulches. The quantity of reflected blue light (BL) was greatest over white and about the same over black versus red surfaces; and reflected far‐red to red ratios (FR/R) were about the same over white versus black and higher over red. The greatest quantity of total epicuticular wax developed on leaves of plants grown over white mulch (443 μg/cm2) while plants grown over black and red mulches had 229 and 227 μg/cm2, respectively. When individual lipid classes were expressed as percentages of the totals, esters constituted the highest percentages over white and black surfaces while secondary free fatty alcohols dominated over red. The higher ester content of epicuticular wax on plants grown over black (versus red) was almost exactly equalled by increased contents of free fatty alcohols and free fatty acids in the epicuticular wax of plants grown over red mulch. We conclude that the total wax concentration on leaves of field‐grown plants was influenced by quantity of BL, and the percentage of individual components was influenced by the FR/R ratio.

Row orientation affects fruit yield in field-grown okra.

ABSTRACT Effects of row orientation on fruit yield of okra (Abel-moschus esculentus [L.] Moench) were studied. Plants in north-south (N-S) rows produced more fruit than those in east-west (E-W) rows. Amounts of photosynthetic (400–700 nm) and morphogenic light (far-red to red light ratio [FR/R]) received by plants grown in N-S and E-W rows were also determined. The mean amounts of photosynthetic light received by plants grown in E-W and N-S oriented rows were similar. Reflected light received by plants in N-S rows had a higher FR/R ratio than that received by plants in E-W rows. We conclude that row orientation affected yield of okra by influencing the morphogenic light received by plants.