Inside chloroplasts, several abiotic stresses (including drought, high light, salinity, or extreme temperatures) induce a reduction in CO₂ assimilation rates with a consequent increase in reactive oxygen species (ROS) production, ultimately leading to leaf senescence and yield loss. Photo-oxidation processes should therefore be mitigated to prevent leaf senescence, and plants have evolved several mechanisms to either prevent the formation of ROS or eliminate them. Technology evolution during the past decade has brought faster and more precise methodologies to quantify ROS production effects and damage, and the capacities of plants to withstand oxidative stress. Nevertheless, it is very difficult to disentangle photo-oxidative processes that bring leaf defence and acclimation, from those leading to leaf senescence (and consequently death). It is important to avoid the mistake of discussing results on leaf extracts as being equivalent to chloroplast extracts without taking into account that other organelles, such as peroxisomes, mitochondria, or the apoplast also significantly contribute to the overall ROS production within the cell. Another important aspect is that studies on abiotic stress-induced leaf senescence in crops do not always include a time-course evolution of studied processes, which limits our knowledge about what photo-oxidative stress processes are required to irreversibly induce the senescence programme. This review will summarize the current technologies used to evaluate the extent of photo-oxidative stress in plants, and discuss their advantages and limitations in characterizing abiotic stress-induced leaf senescence in crops.

Although plant responses to drought stress have been studied in detail in several plant species, including CAM plants, the occurrence of stress memory and possible mechanisms for its regulation are still very poorly understood. In an attempt to better understand the occurrence and possible mechanisms of regulation of stress memory in plants, we measured the concentrations of phytohormones in Aptenia cordifolia exposed to reiterated drought, together with various stress indicators, including leaf water contents, photosynthesis and mechanisms of photo- and antioxidant protection. Results showed that plants exposed to drought stress responded differently if previously challenged with a first drought. Gibberellin levels decreased upon exposure to the first drought and remained lower in double-stressed plants compared with those exposed to stress for the first time. In contrast, abscisic acid levels were higher in double- than single-stressed plants. This occurred in parallel with alterations in hydroperoxide levels, but not with malondialdehyde levels, thus suggesting an increased oxidation state that did not result in oxidative damage in double-stressed plants. It is concluded that (i) drought stress memory occurs in double-stressed A. cordifolia plants, (ii) both gibberellins and abscisic acid may play a role in plant response to repeated periods of drought, and (iii) changes in abscisic acid levels in double-stressed plants may have a positive effect by modulating changes in the cellular redox state with a role in signalling, rather than cause oxidative damage to the cell.

Red (retro)-carotenoids accumulate in chloroplasts of Buxus sempervirens leaves during the process of winter leaf acclimation. As a result of their irregular presence, different leaf colour phenotypes can be found simultaneously in the same location. Five different colour phenotypes (green, brown, red, orange, and yellow), with a distinct pattern of pigment distribution and concentration, have been characterized. Leaf reddening due to the presence of anthocyanins or carotenoids, is a process frequently observed in plant species under photoinhibitory situations. Two main hypotheses have been proposed to explain the function of such colour change: antioxidative protection exerted by red-coloured molecules, and green light filtering. The potential photoprotective role of red (retro-) carotenoids as light filters was tested in Buxus sempervirens leaves. In shade leaves of this species the upper (adaxial) mesophyll of the lamina was replaced by the equivalent upper part of a different colour phenotype. These hybrid leaves were exposed to a photoinhibitory treatment in order to compare the photoprotective effect exerted by adaxial parts of phenotypes with a different proportion of red (retro)-carotenoids in the lower mesophyll of a shade leaf. The results indicated that the presence of red (retro)-carotenoids in the upper mesophyll did not increase photoprotection of the lower mesophyll when compared with chlorophyll, and the best protection was achieved by an upper green layer. This was due to the fact that the extent of photoinhibition was proportional to the amount of red light transmitted by the upper mesophyll and/or to the chlorophyll pool located above. These results do not exclude a protective function of carotenoids in the upper leaf layer, but imply that, at least under the conditions of this experiment, the accumulation of red pigments in the outer leaf layers does not increase photoprotection in the lower mesophyll.