Granata, Angelo (2025) Pre-sowing treatments and seed priming techniques in edible plant species. [Tesi di dottorato]

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Abstract

Today, one of the major challenges is to harmonize the growing demand for food due to increasing world population, with the need to preserve the environment and its biodiversity, including the agricultural ecosystems. In this perspective, pre-sowing treatments of the seeds represent an elective choice since they aim to increase germination performance, improving productivity even on poor soils or starting from seeds of non-optimal quality. In this doctoral thesis, seeds of edible leafy species (rocket, valerian salad, and lettuce) cultured in hydroponics were subjected to pre-sowing physical treatments (low-dose X-ray irradiation) to improve germination and increase biomass production compared to non-irradiated control plants. In Eruca sativa several parameters showed a hormesis-like response; for instance, a higher biomass production (shoot weight and leaf number) and phenol content were observed under the lower dose of X-rays, whereas higher-dose-treated seeds and control seeds produced plants not significantly different from each other. Although a significantly decreasing pigment content was observed while growing the stress level, rubisco and stomata number had a different trend. Radiation also induced a genotoxic effect in 15-day old seedlings, but a recovery of the genome template stability occurred in the successive weeks. In Lactuca sativa and Valerianella locusta radiation stress induced a significant increase of the fresh weight at 35 days culture, although the mean germination time (MGT) significantly decreased only in lettuce. In both species antioxidant content (which was intrinsically lower in lettuce compared to lamb’s lettuce) was substantially stable during growth, maintaining a constant concentration during plant weight gain; phenol content showed instead a certain increase under radiation stress. Overall results suggest that seed treatment by high energy radiation could be a valuable option to increase the yield of ready-to-eat crop species cultured in hydroponics. The second part of this PhD thesis focused on classical methods of seed priming (i.e., method based on water absorption) tested in some neglected cultivars of sweet pepper (Capsicum annuum) cultured in Campania region (Southern Italy). The seeds of these cultivars were treated with hydropriming and halopriming with different concentrations of KNO3, both carried out at three duration times (for a total of 12 treatments in addition to control). In a first set of experiments the different priming methods were tested to select the best ones based on germination performance, seedling growth, and antioxidant production. These experiments highlighted that hydropriming for 24 h and KNO3 6% for 96 h were overall the best tretaments, despite some cultivar-specific differences. These treatments increased seedling weight, development germination index (DGI), simple vigor index (SVI), and reduced the MGT. Based on these results I hypothesized that seed priming can differently modulate the expression of key genes involved in pre-germinative metabolic pathways. Hence, the expression of 4 genes, two stimulating giberellin and amylase production (pro-germinative genes) and two related to ABA production (i.e., hormon inhibiting seed germination) were estimated by RT-PCR in control seeds, and in seeds subjected to hydro/halo priming. The results confirmed that the priming treatments differently modulated gene expression and related responses, depending on the cultivar and the pre-sowing treatment. Moreover, the joint evaluation of gene expression, the level of antioxidant and hydrogen peroxide, and MGT and DGI indices, can help gaining a more complete knowledge, not only limited to germination, but also including plant development, to find out a proper pre-sowing treatment. Finally, a part of the thesis work (carried out at the royal Holloway University, London) was dedicated to investigating the effectiveness of selected seed priming on aged or low-quality seeds. For this purpose, seeds were ranked into quality categories by X-ray analysis, evaluating the part occupied by the embryo and the endosperm. Four groups were thus identified (1 to 4, with the first indicating the best quality). Seeds were subjected to halopriming, artificial aging and post-aging halopriming. In all categories and treatments, weight, electrical conductivity, antioxidant activity, germination performance and early growth stages of the seedlings were measured. The results indicate that priming is effective even on aged and low-quality seeds (group 2). Therefore, priming can be useful to reinvigorate old seeds and to recover seeds with low germination potential.

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