4 – Secondary Plant Products and Cell and Tissue Differentiation

To test the predictions that plants will have a larger flavonoid concentration in a future world with a CO 2 -enriched atmosphere, wheat (Triticum aestivum L. cv. Yecora Rojo) was grown in a field experiment using FACE (free-air CO 2 enrichment) technology under two levels of atmospheric CO 2 concentration : ambient (370 μmol mol -1 ) and enriched (550 μmol mol -1 ), and under two levels of irrigation: well-watered (100% replacement of potential evapotranspiration) and half-watered. We also studied the effects of CO 2 on the concentration of total non-structural carbohydrates (TNC) and nitrogen (N), two parameters hypothesized to be linked to flavonoid metabolism. Throughout the growth cycle the concentration of isoorientin, the most abundant flavonoid, decreased by 62% (from an average of 12.5 mg g -1 on day of year (DOY) 41 to an average of 4.8 mg g -1 on DOY 123), whereas the concentration of tricin, another characteristic flavone, increased by two orders of magnitude (from an average of 0.007 mg g -1 of isoorientin equivalents on DOY 41 to an average of 0.6 mg g -1 of isoorientin equivalents on DOY 123). Although flavonoid concentration was dependent on growth stage, the effects of treatments on phenology did not invalidate the comparisons between treatments. CO 2 -enriched plants had higher flavonoid concentrations (14% more isoorientin, an average of 7.0 mg g -1 for ambient CO 2 vs an average of 8.0 mg g -1 for enriched CO 2 ), higher TNC concentrations and lower N concentrations in upper canopy leaves throughout the growth cycle. Well-irrigated plants had higher flavonoid concentrations (11% more isoorientin, an average of 7.1 mg g-' for half watered vs an average of 7.9 mg g -1 for well-watered) throughout the growth cycle, whereas the effect of irrigation treatments on TNC and N was more variable. These results are in accordance with the hypotheses that higher carbon availability promoted by CO 2 -enrichment provides carbon that can be invested in carbon-based secondary compounds such as flavonoids. The rise in atmospheric CO 2 may thus indirectly affect wheat-pest relations, alter the pathogen predisposition and improve the UV-B protection by changing flavonoid concentrations.

Free‐air CO2 enrichment of wheat: leaf flavonoid concentration throughout the growth cycle

A recent review of climate patterns in Southern Germany has suggested significant increases in ultraviolet (UV) radiation due to decreases in cloud coverage and in cloud frequency which compound the effects of stratospheric ozone depletion. Whether such UV radiation increases result in UV damage of higher plant leaves depends partly on the capacity of UV-absorbing hydroxycinnamic acids and flavonoids located in the plant epidermis to screen out UV radiation. Epidermal UV screening is most often assessed from UV absorbance of whole-leaf extracts but in the present work, this method is critically examined. In grapevine (Vitis vinifera L.), hydroxycinnamic acid as well as mono-hydroxylated and ortho-dihydroxylated flavonoid concentrations increased in parallel with fluorometrically detected adaxial epidermal UV absorbance but only the latter class of flavonoids was associated with epidermal UV absorbance in barley (Hordeum vulgare L). For both species, curvilinear relationships between epidermal and total phenolic UV absorbance were established: initial slopes of the curves differed markedly between species. Modelling suggested that curvilinearity arises from UV-transparent epidermal areas located between vacuoles which are particularly UV-absorbing due to high levels of phenolics. The species-dependent differences were related to allocation of high amounts of phenolics in the mesophyll and abaxial epidermis in barley but not in grapevine. Both factors, optical heterogeneity and variable distribution of phenolics, severely restrict the use of phenolic absorbance to estimate true epidermal screening.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2005.01302.x

Origins of non‐linear and dissimilar relationships between epidermal UV absorbance and UV absorbance of extracted phenolics in leaves of grapevine and barley

The induction of defense compounds in oats (Avena sativa) in response to invasion by parasitic nematodes and to application of the wound hormone methyl jasmonate was examined. Oats cv. Quo...

https://apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO.2004.94.11.1207

Inducible Flavone in Oats (Avena sativa) Is a Novel Defense Against Plant-Parasitic Nematodes.

Primary leaves of oats (Avena sativa L.) have been used to study the integration of secondary phenolic metabolism into organ differentiation and development. In particular, the tissue-specific distribution of products and enzymes involved in their biosynthesis has been investigated. C-Glucosylflavones along with minor amounts of hydroxycinnamic-acid esters constitute the soluble phenolic compounds in these leaves. In addition, considerable amounts of insoluble products such as lignin and wall-bound ferulic-acid esters are formed. The tissue-specific activities of seven enzymes were determined in different stages of leaf growth. The rate-limiting enzyme of flavonoid biosynthesis in this system, chalcone synthase, together with chalcone isomerase (EC 5.5.1.6) and the terminal enzymes of the vitexin and isovitexin branches of the pathway (a flavonoid O-methyltransferase and an isovitexin arabinosyltransferase) are located in the leaf mesophyll. Since the flavonoids accumulate predominantly (up to 70%) in both epidermal layers, an intercellular transport of products is postulated. In contrast to the flavonoid enzymes, L-phenylalanine ammonia-lyase (EC 4.3.1.5), 4-coumarate: CoA ligase (EC 6.2.1.12), and S-adenosyl-L-methionine: caffeate 3-O-methyltransferase (EC 2.1.1.-), all involved in general phenylpropanoid metabolism, showed highest activities in the basal leaf region as well as in the epidermis and the vascular bundles. We suggest that these latter enzymes participate mainly in the biosynthesis of non-flavonoid phenolic products, such as lignin in the xylem tissue and wall-bound hydroxycinnamic acid-esters in epidermal, phloem, and sclerenchyma tissues.

Bernhard Effertz

Papers

Entwicklungs- und lichtabhängige akkumulation von C-glycosylflavonen im haferkeimling (Avena sativa L)

Tissue specific variation of C-glycosylflavone patterns in oat leaves as influenced by the environment

Dynamik der Flavonoid‐Akkumulation in Primärblattgeweben von Avena sativa L.

14C-Phenylalanine Incorporation into C-glycosylflavones of Developing Primary Oat Leaves