Rosemary Trial field, RHS Wisley Gardens

Research

My PhD focuses on improving Rosemary aroma and taste, in collaboration with The Royal Horticultural Society and Vitacress.

Rosemary, Latin name Rosmarinus officionalis, is a perennial evergreen shrub commonly used for culinary purposes and whose essential oil has antioxidant, antimicrobial and antiviral activities. The characteristic taste and aroma of Rosemary is due to the production of essential and volatile oils by the plant. Yet the production of these oils varies between varieties of Rosemary and the production of essential oils can be influenced through growing conditions. Different Rosemary varieties will produce different quantities of essential and volatile oils. Therefore, the selection for rosemary varieties with improved essential and volatile oil production could enhance the taste of commercially grown Rosemary. Enhancing the growing conditions could also increase essential oil production of commercial Rosemary varieties. This project will focus on the selection of Rosemary varieties, the improvement of growing conditions and gene expression to improve the aroma and taste of Rosemary.

Selecting Aromatic Rosemary

The selection process begins with examining the morphological traits of different Rosemary varieties. Traits such as height, growth form and colour are examined to select varieties that would be good for commercial horticulture and the amateur gardener. Rosemary shrubs usually grow as a prostrate form or an upright form. The upright form would be more appealing to commercial horticulture, as upright plants can be easily transported, while prostrate plants are a good option to gardeners who desire a low-growing variety. Other traits such as speed of growth and ease of cutting establishment are important to commercial growers who mass produce Rosemary pots in a short timescale. The most important aspects of Rosemary that everyone wants is a strong aroma and taste.

Left image: Rosemary variety ‘Sorcerer’s Apprentice’ has a low growing prostrate form with small, dark green leaves forming dense ground cover.
Right image: Rosemary variety ‘Brevifolia’ has an upright form that is taller than most upright varieties with long leaves. The leaves closer to the ground are red and orange in colour giving this variety a unique colouring.

The leaves are covered in small glands called trichomes that store and release volatile oils. The quantity of glandular trichomes on the surface of the leaves are used as a measure of strength of aroma. The more glandular trichomes present on the leaf surface, the more volatiles that can be released into the air. Volatile oils are composed of many different types of aromatic compounds, each compound requires a unique series of steps to be produced. The unique steps in the pathway will modify the original sugars from photosynthesis to the final product; the aromatic compound. The aromatic compounds are then blended together into the volatile oil. Looking at the composition and production of volatile oils can be helpful to select varieties with a strong aroma and an increased production of volatile oils. Taste-testing different varieties will be used to select Rosemary with a strong aroma and pleasant taste.

Improving Growing Conditions

Environmental factors have been known to influence the production of volatile oils and improving certain conditions could help to increase volatile oil production in Rosemary. Conditions such as light, temperature, nutrient availability and the presence of plant promoting soil microbes contribute to promoting faster plant growth and increased production of volatile oils. Different light conditions can change the leaf morphology, aroma compounds and soluble phenolics in Basil. Essential oil composition in Rosemary also changes with different light conditions.

One of the main changes that can be made is to the soil and fertilizers. Organic seaweed fertilizers have been known to increase the availability of nutrients for foliage growth. The more leaves a Rosemary plant has, the more it can photosynthesise and the more volatile oil it can produce. Another change to the soil includes the introduction of plant growth-promoting bacteria and a type of fungus, called mycorrhizal fungi, that associate with the plant roots. The plant-microbe interaction increases nutrient uptake from the soil and promote foliage growth. The increased plant growth can allow for greater production of volatile oils.

In Summary

Rosemary is a popular herb in the UK with 5 million pots, bunches and packs of Rosemary being sold each year. By selecting for varieties with better aromatic traits this can improve the quality of Rosemary being produced in the UK. The improvement of Rosemary aroma and taste can be achieved by examining morphological and genetic traits, enhancing growth conditions and the addition of soil microbes to boost volatile oil production. This project will aim to examine what characteristics makes a Rosemary plant a good volatile producer and can volatile oil production be improved through modification of growth conditions.