Rootstock characteristics contributing to nitrogen use efficiency and morpho-physiological and biochemical responses of grafted tomato under drought stress conditions

Among the abiotic stresses, it is not just only the nitrogen that is the limiting factor in crop growth and yields, but also due to the shortage and uneven distribution of water resources, drought stress has become a most critical circumstances restricting sustainable crop production. The aim of this study was to determine genotypic differences in the Nitrogen Use Efficiency (NUE) of some local and hybrid tomato (Lycopersicum esculentum L.) genotypes and to assess whether grafting with rootstocks could improve the NUE and drought tolerance of susceptible tomato scions. Plants were grown hydroponically under high N (3.0 mM) supply treated with drought stress (10% (w/v) PEG 6000) by using an aerated Deep-Water Culture technique in a fully automated growth chamber of Georg-August University, Agricultural Faculty in G & ouml;ttingen, Germany. The hydroponic experiment was arranged in a completely randomized block design with three replications for ten weeks between July-September 2022. Two tomato genotypes [Goldene K & ouml;nigin F1 (German hybrid variety) and Iraq 5 (obtained from Iraq Crop Research Institute)] were used as scion and two pre-selected tomato genotypes [Karahidir (N-inefficient Turkish local variety) and ALT (N-efficient Ghanaian local variety obtained from Ghana Research Institute] were used as rootstock materials in this study. Although growing under high N supply, shoot growth, as well as root morphological, leaf physiological and biochemical responses and NUE of both grafted and non-grafted (scions) tomato plants were significantly adverse affected by drought stress. On the other hand, the grafted plants usually exhibited highest growth performance than the non-grafted control plants under drought stress indicating that N-efficient and drought tolerant rootstocks significantly contributed to growth and biomass production and thus NUE of susceptible tomato scions. Among the two graft combinations significant genotypic variation existed in drought stress under high N supply. Previously as 'N-efficient' characterized tomato root stock genotype ALT showed higher drought tolerance in the graft combination of Iraq 5/ALT than the 'N-inefficient' characterized genotype Karahidir (KH) in the graft combination of Goldene K & ouml;nigin (GK)/KH. Two non-grafted susceptible scion genotypes varied significantly under drought stress. The scion genotype Iraq 5 usually showed a higher drought tolerance than the scion genotype GK under high N supply. Both rootstocks (ALT and KH) have high rootstock potential for tomato, and their significant contributions to salt tolerance were closely associated with inducing physiological and biochemical responses of scions under high N supply. These traits could be useful for the selection and breeding of drought-tolerant rootstocks for sustainable high-input agriculture in the future. However, to recommend these tomato rootstocks for using in a low-input agriculture, the experiment should be conducted also under a low N condition. Therefore, a next hydroponic study will be conducted by grafting same tomato scions (Iraq 5 and GK) onto the same rootstocks (ALT and KH) under low N (0.3 mM) supply which will be treated with drought stress in the near future.