Remote monitoring of dynamic canopy photosynthesis with high time resolution light-induced fluorescence transients

Abstract

Understanding the net photosynthesis of plant canopies requires quantifying photosynthesis in challenging environments, principally due to the variable light intensities and qualities generated by sunlight interactions with clouds and surrounding foliage. The dynamics of sunflecks and rates of change in light intensity at the beginning and end of sustained light (SL) events makes photosynthetic measurements difficult, especially when dealing with less accessible parts of plant foliage. High time resolved photosynthetic monitoring from pulse amplitude modulated (PAM) fluorometers has limited applicability due to the invasive nature of frequently applied saturating flashes. An alternative approach used here provides remote (<5 m), high time resolution (10 s), PAM equivalent but minimally invasive measurements of photosynthetic parameters. We assessed the efficacy of the QA flash protocol from the Light-Induced Fluorescence Transient (LIFT) technique for monitoring photosynthesis in mature outer canopy leaves of potted Persea americana Mill. cv. Haas (Avocado) trees in a semi-controlled environment and outdoors. Initially we established that LIFT measurements were leaf angle independent between ±40° from perpendicular and moreover, that estimates of 685 nm reflectance (R685) from leaves of similar chlorophyll content provide a species dependent, but reasonable proxy for incident light intensity. Photosynthetic responses during brief light events (≤10 min), and the initial stages of SL events, showed similar declines in the quantum yield of photosystem II (ΦII) with large transient increases in 'constitutive loss processes' (ΦNO) prior to dissipation of excitation by non-photochemical quenching (ΦNPQ). Our results demonstrate the capacity of LIFT to monitor photosynthesis at a distance during highly dynamic light conditions that potentially may improve models of canopy photosynthesis and estimates of plant productivity. For example, generalized additive modelling performed on the 85 dynamic light events monitored identified negative relationships between light event length and ∆ΦII and ∆electron transport rate using either ∆photosynthetically active radiation or ∆R685 as indicators of leaf irradiance.

Genotype Specific Photosynthesis x Environment Interactions Captured by Automated Fluorescence Canopy Scans Over Two Fluctuating Growing Seasons

“…Canopy structure affects physiological processes directly, e.g., the leaf angle distribution affects the light penetration into the canopy leading to variation in F q '/F m ' and NPQ. In addition, F q '/F m ' values differed in the upper compared to the lower canopy and were affected by steep leaf angles (Rascher and Pieruschka, 2008;Wyber et al, 2018). Our data support this conclusion: variability in F q '/F m ,' NPQ, and PRI were higher in leaves with natural orientation than in leaves with a fixed leaf angle (Supplementary Figure 7).…”

Dynamic non‐photochemical quenching in plants: from molecular mechanism to productivity

“…However, the requirement for full dark adaptation of leaves for up to 1 h or more and subsequent light application limits the spatial range and frequency (Murchie and Lawson, ; Logan et al , ; Murchie et al , ). There are possibilities using monitoring systems, proxies like laser‐induced fluorescence and spectral reflectance which are gaining credence (Raesch et al , ; Alonso et al , ; Wyber et al , ). The new pNPQ technique (Ruban and Murchie, ), reviewed in Ruban () offered a way of measuring NPQ without the need for dark adaptation.…”

Measuring the dynamic photosynthome

“…Further technical development enabled this instrument to observe fluorescence signals from up to 50 m distance in a fast, non-invasive way to better understand photoprotection in arabidopsis ( Kolber et al , 2005 ), to monitor the dynamics of winter hardening ( Pieruschka et al ., 2007 , 2014 ; Rascher and Pieruschka, 2008 ) and to monitor the seasonal dynamics of photosynthetic adaptation in different barley varieties ( Raesch et al , 2014 ). A new, lighter and more integrated LIFT instrument has been developed using light-emitting diodes (LEDs) at 470 nm wavelengths with maximal operating distance of a few metres ( Osmond et al , 2017 ; Wyber et al , 2017 ). The nature of LIFT means that it could track dynamic shifts in PSII efficiency and NPQ quite easily in a remote setting and at high spatial scale, which would be a significant advance.…”