Northern hemisphere evergreen forests assimilate a significant fraction of global atmospheric CO2 but monitoring large-scale changes in gross primary production (GPP) in these systems is challenging. Recent advances in remote sensing allow the detection of solar-induced chlorophyll fluorescence (SIF) emission from vegetation, which has been empirically linked to GPP at large spatial scales. This is particularly important in evergreen forests, where traditional remote-sensing techniques and terrestrial biosphere models fail to reproduce the seasonality of GPP. Here, we examined the mechanistic relationship between SIF retrieved from a canopy spectrometer system and GPP at a winter-dormant conifer forest, which has little seasonal variation in canopy structure, needle chlorophyll content, and absorbed light. Both SIF and GPP track each other in a consistent, dynamic fashion in response to environmental conditions. SIF and GPP are well correlated (R2 = 0.62–0.92) with an invariant slope over hourly to weekly timescales. Large seasonal variations in SIF yield capture changes in photoprotective pigments and photosystem II operating efficiency associated with winter acclimation, highlighting its unique ability to precisely track the seasonality of photosynthesis. Our results underscore the potential of new satellite-based SIF products (TROPOMI, OCO-2) as proxies for the timing and magnitude of GPP in evergreen forests at an unprecedented spatiotemporal resolution.

Environmental stresses such as high light, low temperatures, pathogen infection and nutrient de¢ciency can lead to increased production of free radicals and other oxidative species in plants. A growing body of evidence suggests that plants respond to these biotic and abiotic stress factors by increasing their capacity to scavenge reactive oxygen species. E¡orts to understand this acclimatory process have focused on the components of the`classical' antioxidant system, i.e. superoxide dismutase, ascorbate peroxidase, catalase, monodehydroascorbate reductase, glutathione reductase and the low molecular weight antioxidants ascorbate and glutathione. However, relatively few studies have explored the role of secondary metabolic pathways in plant response to oxidative stress. A case in point is the phenylpropanoid pathway, which is responsible for the synthesis of a diverse array of phenolic metabolites such as £avonoids, tannins, hydroxycinnamate esters and the structural polymer lignin. These compounds are often induced by stress and serve speci¢c roles in plant protection, i.e. pathogen defence, ultraviolet screening, antiherbivory, or structural components of the cell wall. This review will highlight a novel antioxidant function for the taxonomically widespread phenylpropanoid metabolite chlorogenic acid (CGA; 5-O-ca¡eoylquinic acid) and assess its possible role in abiotic stress tolerance. The relationship between CGA biosynthesis and photosynthetic carbon metabolism will also be discussed. Based on the properties of this model phenolic metabolite, we propose that under stress conditions phenylpropanoid biosynthesis may represent an alternative pathway for photochemical energy dissipation that has the added bene¢t of enhancing the antioxidant capacity of the cell.

Colorado 80309-0334 l h e protective role of leaf antioxidant systems i n the mechanism of plant acclimation to growth irradiance was studied in Vinca major, Schefflera arboricola, and Mahonia repens, which were grown for several months at 20, 100, and 1200 pmol photons m-* s-'. As growth irradiance increased, several constituents of the "Mehler-peroxidase" pathway also increased: superoxide dismutase, ascorbate peroxidase, glutathione reductase, ascorbate, and glutathione. This occurred concomitantly with increases i n the xanthophyll cycle pool size and in the rate of nonphotochemical energy dissipation under steady-state conditions. There was no evidence for photosystem II overreduction in plants grown at high irradiance, although the reduction state of the stromal NADP pool, estimated from measurements of NADP-malate dehydrogenase activity, was greater than 60% in V. major and S. arboricola. Ascorbate, which removes reactive O, species generated by O, photoreduction in the chloroplast and serves as a reductant for the conversion of the xanthophyll cycle pigments to the de-epoxidized forms A plus 2, generally exhibited the most dramatic increases in response to growth irradiance. We conclude from these results that O, photoreduction occurs at higher rates i n leaves acclimated to high irradiance, despite increases in xanthophyll cycle-dependent energy dissipation, and that increases in leaf antioxidants protect against this potential oxidative stress.Plants acclimated to high irradiance use various mechanisms to protect the photosynthetic apparatus against the deleterious effects of excess light absorption. Much attention has been focused on elucidating the role of the xanthophyll cycle in the dissipation of excess excitation energy in the light-harvesting antennae . The associated increases in the xanthophyll cycle pool size, V plus A plus Z, and the capacity for nonphotochemical energy dissipation, NPQ, is a well-characterized and apparently fundamental acclimatory response to high PFDs (Bjorkman and Demmig-Adams, 1994; DemmigAdams and Adams, 1994). Xanthophyll cycle-dependent energy dissipation lowers the photon efficiency of PSII, thus providing a mechanism to balance the synthesis of ATP and NADPH with the rate at which these metabolites can be utilized in photosynthesis (Foyer, 1993 , 1995) or serve a regulatory role by augmenting the transthylakoid proton gradient (Schreiber and Neubauer, 1990). Although the magnitude of O,-dependent flux in vivo remains controversial, direct measurements using MS suggest that in C, plants up to 25% of the total noncyclic electron transport is consumed by this process at light saturation (Canvin et al., 1980;Badger, 1985;Osmond and Grace, 1995). Protection against the possible toxicity of ROS is afforded by an integrated system of enzymatic and nonenzymatic antioxidants that are concentrated in the chloroplast (Asada, 1994). The superoxide anion, the initial product of photosynthetic O, reduction, is either dismuted by SOD or reduced by ascorbate to H,O,, which is t...

The effect of an acclimation to light stress during the growth of leaves on their response to high photon flux densities (PFDs) was characterised by quantifying changes in photosystem II (PSII) characteristics and carotenoid composition. During brief experimental exposures to high PFDs sun leaves exhibited: (a) much higher levels of antheraxanthin + zeaxanthin than shade leaves, (b) a greater extent of energy dissipation in the light-harvesting antennae, and (c) a greater decrease of intrinsic PSII efficiency that was rapidly reversible. During longer experimental exposures to high PFD, deep-shade leaves but not the sun leaves showed slowly developing secondary decreases in intrinsic PSII efficiency. Recovery of these secondary responses was also slow and inhibited by lincomycin, an inhibitor of chloroplast-encoded protein synthesis. In contrast, under field conditions all changes in intrinsic PSII efficiency in open sun-exposed habitats as well as understory sites with intense sunflecks appeared to be caused by xanthophyll cycle-dependent energy dissipation. Furthermore, comparison of leaves with different maximal rates of electron transport revealed that all leaves compensated fully for these differences by dissipating very different amounts of absorbed light via xanthophyll cycle-dependent energy dissipation, thereby all maintaining a similarly low PSII reduction state. It is our conclusion that an increased capacity for xanthophyll cycle-dependent energy dissipation is a key component of the acclimation of leaves to a variety of different forms of light stress, and that the response of leaves to excess light experienced in the growth environment is thus likely to be qualitatively different from that to sudden experimental exposures to PFDs exceeding the growth PFD.

To investigate if Eucalyptus species have responded to industrial-age climate change, and how they may respond to a future climate, we measured growth and physiology of fast-(E. saligna) and slow-growing (E. sideroxylon) seedlings exposed to preindustrial (290), current (400) or projected (650 lL L À1 ) CO 2 concentration ([CO 2 ]) and to current or projected (current 1 4 1C) temperature. To evaluate maximum potential treatment responses, plants were grown with nonlimiting soil moisture. We found that: (1) E. sideroxylon responded more strongly to elevated [CO 2 ] than to elevated temperature, while E. saligna responded similarly to elevated [CO 2 ] and elevated temperature; (2) the transition from preindustrial to current [CO 2 ] did not enhance eucalypt plant growth under ambient temperature, despite enhancing photosynthesis; (3) the transition from current to future [CO 2 ] stimulated both photosynthesis and growth of eucalypts, independent of temperature; and (4) warming enhanced eucalypt growth, independent of future [CO 2 ], despite not affecting photosynthesis. These results suggest large potential carbon sequestration by eucalypts in a future world, and highlight the need to evaluate how future water availability may affect such responses.