November 2021 - 42

technically speaking
BY ERIK RUNKLE
The Buzz of Secondary Metabolites
The diversity of crops grown in
greenhouses and indoor vertical farms
continues to expand. One broad segment that
is increasing dramatically is the production
of consumable crops. This includes a broad
range of vegetables, fruits, culinary herbs
and medicinals. While these crops are still
predominantly grown in fields, yield and
quality can be greatly increased when grown
in controlled environments (although
often with higher costs, too). Crop
quality can refer to a lot of different
plant parameters, but one of the more
important ones for ingested crops
is the concentrations of secondary
metabolites.
Secondary metabolites are
compounds that plants produce that
are not directly involved in growth
and development. These compounds
are sometimes referred to as natural
products, specialized metabolites, or
secondary products, and vary widely
from one crop to another. In contrast,
primary metabolites are compounds
that plants directly use during their life cycle, for
example carbohydrates and proteins, and are common
Erik Runkle is
professor and
floriculture
Extension specialist
in the department
of horticulture at
Michigan State
University. He can be
reached at runkleer@
msu.edu.
Figure 1. We commercially produce many edible specialty crops specifically for their
secondary metabolites, which influence their taste, color, aroma, and nutritional or
bioactive content.
from one species to another.
There are hundreds of thousands of secondary metabolites.
Their concentrations influence leaf and flower color (e.g.,
anthocyanins and carotenoids), nutrition (e.g., vitamins A and E),
taste (e.g., glucosinolates and essential oils), and smell (volatile
organic compounds).
Some secondary metabolites have medicinal properties [e.g.,
codeine and cannabidiol (CBD)], whereas others can be toxic.
Some of these toxic compounds are used to control weeds,
insects and pathogens. Regardless of their use, secondary
metabolites can be modified through breeding activities as well
as by plant culture and the environment.
Different forms of plant stress generally increase the
concentrations of secondary metabolites. Some stressors
include a high light intensity (especially UV and blue light),
low temperature, poor nutrition, insect feeding and pathogen
infection. The increase in some of these metabolites serve to
protect plant tissue, and can be accompanied by increases in
the nutritional or medicinal quality as well as flavor intensity.
In the commercial production of some specialty crops,
the environment is sometimes manipulated to increase the
concentrations of secondary metabolites. For example, delivery
of UV, blue, and/or a high light intensity, particularly at the end
of production, can increase leaf color, nutritional value, and the
42 NOVEMBER 2021 GPNMAG.COM
intensity of taste and smell of leafy greens and herbs. Growth
regulators and biostimulants can also regulate secondary
metabolites, including disease resistance and plant flavor, but
responses are variable from one species and variety to another.
The term " plant resilience " is sometimes used to describe
an increased tolerance to stressful conditions. For example,
plants generally perform better during shipping and in the
retail environment when they have been " hardened off, " such
as exposure to more light, lower temperatures, or both. One
of the reasons for this improved performance is the increased
biosynthesis of secondary metabolites that protect plants from
stressors.
While many secondary metabolites have a high value, in some
cases there can be too much of a good thing. For example, the
taste of some culinary herbs (such as basil) and leafy greens
(such as kale) can become undesirably strong when grown
under very high light levels (Figure 1). In addition, the sudden
introduction of stress can inhibit growth, damage plant tissue,
or both. Therefore, a managed and deliberate stress, especially
toward the end of a production cycle, can yield the best results
from growth and plant quality perspectives.
However, whether growing flowers, foliage, or food, quality
and value usually increase with their concentrations of
secondary metabolites. That is why there's an increasing buzz
about secondary metabolites as our understanding of their value
- and ways to regulate their concentrations - increase.
http://www.GPNMAG.COM
November 2021

Table of Contents for the Digital Edition of November 2021
