Salt stress and tomato: role of seed priming and acclimation in the amelioration of stress response

The lack of rainfall, high temperatures, and poor snowfall during the winter seasons, are the main factors that lead to drought, related to climate changes with dramatic consequences, for agriculture and for the supply of drinking water. Besides the limited water availability, it has been detected an enhancement of salts in the water, due to the seas flowing back into the branches of river deltas and, in coastal areas, it infiltrates increasingly low aquifers due to excessive drawing from irrigation wells. It is well known that plants irrigated with saline solutions undergo osmotic and oxidative stresses, which affect their growth, photosynthesis and yield. Therefore, the use of saline water for irrigation, in addition to the increasing soil salinity, is one of the major threats to crop productivity worldwide. Among the horticultural crops, tomato (Solanum lycopersicum L.) requires a lot of water for cultivation, although high humidity leads to the higher incidence of pests and diseases and fruit rot. Dry climates are therefore preferred for tomato production. To improve crop tolerance to abiotic stresses seed priming and acclimation, can represent useful tools. Seed priming is a pre-sowing treatment, which consists of soaking the seeds in a priming agent, followed by drying the seeds to avoid rootlet emergence. A priming agent can provoke abiotic stress to seed, inducing a cross-tolerance to different stresses. While, an acclimation to stress can be obtained through a gradual exposure of the plant to stressful conditions, leading the plant to an adaptation to stress, therefore to a better performance. Based on the economic importance of tomatoes, an experimental greenhouse trial was conducted on salt-sensitive cherry tomato (cv. Principe Borghese), to determine the effects of combined treatment of seed priming and acclimation on salt stress response. Chemical priming of seeds was performed by treating seeds with polyamines (PAs): 2.5 mM putrescine (PUT) 2.5 mM spermine (SPM) and 2.5 mM spermidine (SPD). Germinated seeds, primed and non-primed (controls), were sown in non-saline soil; after 2 weeks, the plants were irrigated with saline and non-saline water for 4 weeks. At the end of the growth period, physiological and biochemical analyses were performed. The positive effects of combined treatments were evident when primed plants were compared to non-primed plants, grown under the same conditions. An increase in antioxidant activity and reducing power of primed plants was observed, using the DPPH, FRAP and PFRAP assay. PAs increased the activity of various antioxidant enzymes, including superoxide dismutase (SOD), polyphenol oxidase (PPO), and peroxidase (POD), in particular ascorbate peroxidase (APX). In conclusion, priming with PAs improved tolerance to salt stress and enzymatic antioxidant responses in all salt-exposed plants, reducing the negative effects of salinity on growth. PAs have been proved to be beneficial for detoxification of reactive oxygen species (ROS) and activation of the antioxidative machinery, ameliorating abiotic stress tolerance of tomato. These results may open new perspectives and strategies to increase tolerance to salt stress in sensitive species