Chlorophyll fluorescence as an indicator of forage plant stress

Authors: Goslee, Sarah

Submitted to: American Forage and Grassland Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 1/3/2022
Publication Date: 1/18/2022
Citation: Goslee, S.C. 2022. Chlorophyll fluorescence as an indicator of forage plant stress[abstract]. American Forage and Grassland Conference Proceedings. P.1.

Interpretive Summary: No Interpretive Summary is required for this Abstract Only. JLB.

Technical Abstract: Chlorophyll fluorescence (CF) is an effective measure of plant stress in other systems, and is fast and non-destructive, requiring only a few minutes per sample and conducted completely in the field. As a measure of plant stress in forages, it might provide both an indication of forage production and even quality, and also a rapid protocol for quantifying species response to growing conditions in different field trials. This technique has rarely been used in pastures, so a pilot study was developed to investigate the potential use of CF in these systems. A field trial of forage species mixtures was implemented at Penn State’s Rock Springs Experimental Farm, University Park, PA, USA. Orchardgrass, white clover, and chicory were planted in autumn 2019, in three blocks of eight plots (three single species, three pairs, and all species in equal proportions, and with orchardgrass dominant [more similar to customary forage mixtures]). Chlorophyll fluorescence was measured with a hand-held instrument (Pocket Pea, Hansatech Instruments Ltd, UK), on five dark-adapted leaves of each planted species per plot. Measurements were taken four times a year for two years: at the beginning of the growing season, and approximately every eight weeks thereafter, dependent on growth rates. Samples were taken for biomass production and forage quality, but pandemic restrictions on lab access have delayed analysis. The most common CF measurement, Fv/Fm, is the ratio of the difference between dark-adapted (minimum) and brightly-lit (maximum) fluorescence (Fv) and the maximum value (Fm). An unstressed plant will generally have a Fv/Fm of around 0.8. A mixed-effects model of Fv/Fm on treatment and sampling date, with random effects of treatment nested within block and date nested within year, identified a strong species effect, a weaker treatment effect, and no effect of sampling date. However, 2021 was much wetter than 2020 during the growing season, resulting in a very different seasonal pattern; when years were analyzed separately season had a stronger effect due to timing and amount of precipitation. Measuring CF on forage species is a new area of research. The measurements were highly variable within a plot, suggesting that more samples could improve results. The differences across species, across treatments, and across years and dates, however, demonstrate that chlorophyll fluorescence has the potential to be a useful indicator of stress in forage plants, and that further investigation is justified.