Rapid Hormetic Responses of Photosystem II Photochemistry of Clary Sage to Cadmium Exposure

Abstract

Five-day exposure of clary sage (Salvia sclarea L.) to 100 μM cadmium (Cd) in hydroponics was sufficient to increase Cd concentrations significantly in roots and aboveground parts and affect negatively whole plant levels of calcium (Ca) and magnesium (Mg), since Cd competes for Ca channels, while reduced Mg concentrations are associated with increased Cd tolerance. Total zinc (Zn), copper (Cu), and iron (Fe) uptake increased but their translocation to the aboveground parts decreased. Despite the substantial levels of Cd in leaves, without any observed defects on chloroplast ultrastructure, an enhanced photosystem II (PSII) efficiency was observed, with a higher fraction of absorbed light energy to be directed to photochemistry (ΦPSΙΙ). The concomitant increase in the photoprotective mechanism of non-photochemical quenching of photosynthesis (NPQ) resulted in an important decrease in the dissipated non-regulated energy (ΦNO), modifying the homeostasis of reactive oxygen species (ROS), through a decreased singlet oxygen (1O2) formation. A basal ROS level was detected in control plant leaves for optimal growth, while a low increased level of ROS under 5 days Cd exposure seemed to be beneficial for triggering defense responses, and a high level of ROS out of the boundaries (8 days Cd exposure), was harmful to plants. Thus, when clary sage was exposed to Cd for a short period, tolerance mechanisms were triggered. However, exposure to a combination of Cd and high light or to Cd alone (8 days) resulted in an inhibition of PSII functionality, indicating Cd toxicity. Thus, the rapid activation of PSII functionality at short time exposure and the inhibition at longer duration suggests a hormetic response and describes these effects in terms of “adaptive response” and “toxicity”, respectively.

Hormetic Responses of Photosystem II in Tomato to Botrytis cinerea

“…In plant cellular metabolism, and especially in chloroplasts, reactive oxygen species (ROS) are continuously produced at basal levels that are incapable to cause damage, as they are being scavenged by different antioxidant mechanisms [9][10][11][12]. However, under most biotic or abiotic stresses an increased production of ROS occurs that can lead to oxidative stress if it is not scavenged by enzymatic or non-enzymatic antioxidants [11][12][13][14][15]. However, ROS derived from the chloroplasts also play a role in plant resistance against B. cinerea [15].…”

“…ROS, such as superoxide anion radical (O 2 •− ), hydrogen peroxide (H 2 O 2 ), and singlet oxygen ( 1 O 2 ), produced in chloroplasts play dual roles as they generate oxidative stress and also confer essential biological function as redox signaling in response to biotic and abiotic stress conditions [14][15][16]. The role of chloroplast antioxidants, that often have overlying or interrelating functions, is not to totally eliminate ROS, but rather to achieve a suitable balance between production and removal so that to counterpart photosynthetic function, permitting an effective diffusion of signals to the nucleus and adjusting a plethora of physiological functions [13,16,17]. Leaf image analysis is also used as an objective and accurate alternative method to quantify ROS production at the affected leaf areas, and has proven to be a useful tool in plant science [15].…”

“…Plants respond to biotic and abiotic stresses by a plethora of mechanisms [13,40]. A low-level of stress or a short exposure duration has been frequently referred to stimulate plant performance [13,20,[40][41][42][43][44].…”

“…A low-level of stress or a short exposure duration has been frequently referred to stimulate plant performance [13,20,[40][41][42][43][44]. This enhancement can be achieved through a basal level of ROS [9,10,13,16,45], that is being regulated by the photoprotective mechanism of photosynthesis, the non-photochemical quenching (NPQ) [23,43,46]. NPQ under stress conditions decreases electron transport rate (ETR) to avoid ROS formation [35,47].…”

“…ROS production that can occur through PSII damage can prevent the repair of PSII reaction centres [35,48]. Dose-response studies are proposing hormesis as a widespread phenomenon usual in nature and independent of the kind of stressor, the physiological process or the organism it occurs [13,40,43,44,49].…”