Sucrose and Starch Content Negatively Correlates with PSII Maximum Quantum Efficiency in Tomato (Solanum lycopersicum) Exposed to Abnormal Light/Dark Cycles and Continuous Light

Light is most important to plants as it fuels photosynthesis and provides clues about the environment. If provided in unnatural long photoperiods, however, it can be harmful and even lethal. Tomato (Solanum lycopersicum), for example, develops mottled chlorosis and necrosis when exposed to continuous light. Understanding the mechanism of these injuries is valuable, as important pathways regulating photosynthesis, such as circadian, retrograde and light signaling pathways are probably involved. Here, we use non-targeted metabolomics and transcriptomics analysis as well as hypothesis-driven experiments with continuous light-tolerant and -sensitive tomato lines to explore the long-standing proposed role of carbohydrate accumulation in this disorder. Analysis of metabolomics and transcriptomics data reveals a clear effect of continuous light on sugar metabolism and photosynthesis. A strong negative correlation between sucrose and starch content with the severity of continuous light-induced damage quantified as the maximum quantum efficiency of PSII (Fv/Fm) was found across several abnormal light/dark cycles, supporting the hypothesis that carbohydrates play an important role in the continuous light-induced injury. We postulate that the continuous light-induced injury in tomato is caused by down-regulation of photosynthesis, showing characteristics of both cytokinin-regulated senescence and light-modulated retrograde signaling. Molecular mechanisms linking carbohydrate accumulation with down-regulation of carbon-fixing enzymes are discussed.