The use of supplementary lighting in protected ornamental and edible crops: beyond the maximisation of biomass

HPS lamps are extremely efficient in terms of light produced for a given input of electricity, and there is little doubt that they are the optimum lamp for supplementary lighting where plants are (i) grown long-term under supplementary lighting and (ii) the final mass of foliage or fruit are the key elements in determining the value of the crop. On the other hand, supplementation using HPS does has effects on plant habit, such as increased stem elongation, which may be undesirable in crops such as bedding or pot-grown herbs. The undesirable effects of HPS on plant habit is largely a function of the imbalance between red and blue/UV in the output of this type of lamp. There is considerable evidence that using MH lamps instead of HPS lamps produces a more compact plant habit that more closely meets the needs of commercial bedding production. In principle, replacing HPS with MH might deliver improved plant habit, but this needs to be assessed very carefully taking account of the economics of using the two different lamp types.

In principle, the undesirable effects of HPS on plant habit could be corrected by adding blue or UV light. Specific additions of blue light has been shown to improve plant habit under HPS lamps, and might use highly efficient blue light emitting diodes (LEDs). Equally, in many plants a lack of UV light causes increases in stem elongation and leaf area, but suppresses branching. While specific UV supplements to improve plant morphology is technically feasible using readily available equipment, and is likely to be energy-efficient, using UV might also pose major health and safety issues that would require careful assessment in any commercial use. Practical systems for specific additions of blue or UV light in commercial crops have not been developed.

In herbs grown in the glasshouse under low-light conditions in winter, conventional supplementary light would be expected to increase the contents of many flavour or pharmacological compounds. However, it is certainly not the case that all compounds in all species will respond in the same way. Also, since there may be trade-offs between growth and the production of flavour compounds, simple supplementary lighting may be a relatively inefficient route to specifically.

Specifically manipulating the quality of light may be a more specific tool to manipulate the chemical composition of plant tissues. In particular specific supplementation using blue or UV light might be used to regulate growth (see 4) and stimulate the synthesis of a range of flavour compounds. There is limited experimental work confirming that such approaches can be effective and need relatively low intensities of added light. However, responses are likely to be cropspecific, and there is insufficient evidence on which to base any suggestion for practical exploitation of this approach. The use of UV sources in a commercial setting will require especially careful assessment. increasing the concentration of flavour compounds. On the other hand, supplementary lighting will deliver a balance of increased growth and modified tissue chemistry, both of which may be commercially desired.

In crops grown with poor natural light, conventional supplementary lighting and/or specific supplements such as UV light are likely to result in increased leaf thickness, greater leaf strength and increases in the contents of anti-oxidant compounds Together, these changes would be expected to increase shelf-life in herbs grown under protection, although key questions such as the appropriate irradiance, duration or dose lighting, and when it needs to be applied for optimum effect of shelf life in specific crops, remain unknown.

In salad crops, especially lettuce, there is a specific problem of high nitrate concentrations when crops are grown under poor light in winter. There is no doubt that light is a key factor influencing foliar nitrate concentrations in lettuce, and in other salads such as spinach, and any measure taken to improve light levels within protected lettuce crops will have a proportional benefit in reducing nitrate concentrations in winter.

This project is a review of the current understanding of how lighting might be used to address specific quality issues in both ornamental and edible protected crops.

The project remit did not include either photoperiodic lighting or supplementary lighting aimed simply to increase crop biomass and yield by increasing total light interception. However, the review did consider the relative properties of highpressure sodium (HPS) and metal halide (MH) lamps.