SOURCE-SINK PHYSIOLOGICAL CHANGES AND METABOLIC ADJUSTMENTS TRIGGERED BY WATER DEFICIT IN TWO SOLANUM LYCOPERSICUM GENOTYPES

Drought stress limits the growth and yield of crops, affecting the source-sink relationships. A complex and dynamic signalling network, in which hormones, reactive oxygen species (ROS) and sugars are mainly involved, is activated by plants to cope with water shortage. Tolerant plants have developed several efficient adaptive mechanisms to establish new cell metabolism homeostasis avoiding and/or reducing permanent impairments triggered by drought. The Southern Italy landrace of tomato (Solanum lycopersicum L.) "Ciettaicale" and the well-known tomato cultivar "Moneymaker" were compared evaluating their biometrical and metabolic responses to 20 dayswater deficits (high and moderate) under controlled growth chamber conditions. In order to investigate the drought induced adaptive mechanisms of the two tomato genotypes, we evaluated in vivo chlorophyll a fluorescence, gas exchanges and leaf water potential (LYw), together with the analyses of non-structural carbohydrates in source and sink organs and with the hormonal and antioxidative responses. A decline in actual photochemical efficiency of photosystem II in the light (FPSII) associated with an enhancement of non-photochemical fluorescence quenching (NPQ) were more pronounced in Moneymaker than Ciettaicale stressed plants, despite the maintenance of high potential efficiency of PSII photochemistry (Fv/Fm) in any case. Gas exchanges revealed higher water use efficiency (WUE) in Ciettaicale comparing to Moneymaker thanks to more efficient CO2 assimilation capacity. Higher sucrose levels observed in Moneymaker leaf and root tissues comparing to Ciettaicale ones indicated a potential role of these compounds as osmoprotectant or a reduced export demand from sink organs. Similarly, the elevated levels of hydrogen peroxide (H2O2), lipid peroxidation and abscisic acid (ABA) in Moneymaker leaf and root tissues supported the hypothesis of a drought induced alteration of source-sink relationships. In addition, Ciettaicale showed higher root metabolic activity, as supported also by elevated indole-3-acetic acid (IAA) levels. Changes in the antioxidant pool, including enzymes and compounds like ascorbate peroxidase, ascorbate and phenols, and compatible osmolites, such as proline, played a key role to counteract redox and osmotic pressure in both genotypes. Overall, while Moneymaker showed a survival strategy leading to the accumulation of metabolites to maintain more negative LYw but at same time showing the disruption of source-sink balance, Ciettaicale increased WUE and maintained carbon translocation from source to sink supporting the investment in root metabolism and growth to escape from an environmental osmotic pressure.