Physiological, biochemical and molecular responses to drought conditions in field-grown grafted and ungrafted citrus plants

The physiological, biochemical and molecular alterations developed by plants under soil water restriction, especially rootstock, is important to elucidate the mechanisms associated to drought tolerance. In order to verify the influence of rootstock and grafting on drought tolerance of citrus plants, we examined water status, photosynthesis, oxidative stress and gene expression in grafted and ungrafted citrus plants subjected to different soil water regimes (well-watered, moderate and severe drought and rehydrated) under field conditions. Analysis were performed in roots and leaves of six plant combinations: ungrafted 'Itangpur Santa Cruz' lime (RL) and 'Sunki Maravilha' mandarin (SM), RL and SM grafted onto themselves (RL/RL and SM/SM) and 'Valencia' sweet orange (VO) grafted onto RL or SM (VO/RL and VO/SM). Drought stress reduced chloroplastid pigment contents and limited the photosynthetic rate, mainly in RL/RL plants. In contrast, the lowest leaf osmotic potential, the best osmotic adjustment and the greater phenotypic plasticity were observed in SM, SM/SMand VO/SM plants. Phenotypic plasticity and the main components of multivariate analysis showed that the physiological variables, such as Fv/F0, F0/Fm and Fv/Fm, were more responsive to soil water deficit. An increase in the superoxide dismutase (SOD) activity and transcript abundance of mitochondrial SOD [Mn-Fe], especially in the scion onto RL, and chloroplastidic SOD [Cu -Zit], inthe scion onto SM, was also observed under water deficit. Besides, an induction of transcription of genes coding for catalase (CAT X2) and ascorbate peroxidase (APX) was also observed, respectively, in leaves and roots of plants subjected to water deficit. However, such transcription increment did not affect the activity of these enzymes, especially in RL/RL plants. RL plants were more effective in soil water extraction than SM plants, evidenced mainly by the maintenance of leaf gas exchange and increase in the leaf relative water content after rehydration. The grafted plants showed alterations in their responses to the stress factor, expressed by a reduction of photosynthetic rates, changes in pigment contents, activity of antioxidant enzymes and expression of genes associated to the antioxidant metabolism, mainly in RL/RL plants. Plants with 'Valencia' scion, regardless of the rootstock studied, showed similar physiological, biochemical and molecular responses when exposed to soil water deficit. Multivariate analysis showed that Fv/F0, F0/Fm and Fv/Fm were the most important physiological variables for discriminating citrus plants subjected to control and soil water deficit conditions.