The gibberellin-deficient dwarf2 mutant of sunflower shows a high constitutive level of jasmonic and salicylic acids and an elevated energy dissipation capacity in well-watered and drought conditions

Understanding the role of a low gibberellin (GA) level in counteracting water stress is crucial to improve crop productivity under future climate change scenarios. To date, there have been only a few studies on the physiological characterisation of severe dwarf mutants with deficient GA biosynthesis grown in soil subject to water shortage. Moreover, it is not yet clear whether already in well-watered conditions, the strong reduction in GA observed in these genotypes directly and/or indirectly affects photosynthetic performance, the content of stressrelated hormones and the expression of genes associated with chloroplast functions that could contribute to confer tolerance to subsequent stress events. The drought tolerance responses of the sunflower mutant dwarf2 (dw2), deficient in GAs, were studied in both detached leaves left to dehydrate and in potted plants by watering interruption. In the absence of stress, dw2 plants showed higher constitutive endogenous levels of salicylic and jasmonic acids compared with wild type (WT) plants, together with thicker and less expanded leaves, lower stomatal conductance, higher photochemistry activity of photosystem II and higher photosynthetic capacity per unit leaf area. During water deprivation, dw2 plants maintained a better leaf water status and photosynthetic performance compared with WT plants, associated with a peculiar pattern of transcription for genes related to energy conversion processes. The drought tolerance of dw2 plants appeared to be related to the combination of several structural, photosynthetic and hormonal features together with the up-regulation of genes encoding proteins related to Rubisco subunits and photosystem II repair cycle, avoiding photoinhibition and photodamages to the photosynthetic apparatus.