Rol-Transformed Kalanchoë Shows Altered Root Allocation and Photosynthetic Responses to Nitrogen Deficiency

Ri/rol transformation by Rhizobium rhizogenes is well known to alter plant architecture, including shoot compactness, branching and root development. However, its significance for plant responses to nitrogen limitation remains unclear. Here, we investigated developmental, physiological and hormonal responses of an interspecific Kalanchoë laciniata × K. blossfeldiana hybrid to nitrogen deficiency using complementary pot and hydroponic experiments. Nitrogen deficiency strongly reduced leaf area and total biomass in both wild-type (WT) and rol-transformed (Rol) plants. Nevertheless, rol transformation consistently produced a more compact, highly branched phenotype, with increased leaf number, greater side-shoot formation, higher specific leaf area, lower leaf dry matter content, and, in hydroponics, a striking increase in root mass allocation. Rol plants also maintained higher chlorophyll and carotenoid concentrations than WT and showed altered carbon partitioning, with lower soluble sugars but a higher relative proportion of sucrose. Under nitrogen deficiency, Rol plants maintained positive net photosynthesis and higher electron transport, whereas WT plants showed near-complete suppression of CO₂ assimilation despite high intercellular CO₂ concentration, pointing to strong non-stomatal limitation. Rol plants were characterized by lower ABA and PA concentrations, reduced auxin pools and enhanced leaf cytokinin status. Under low N, greater root allocation in Rol plants was not accompanied by reduced overall growth when root expansion was unrestricted, suggesting compensation through higher specific leaf area and maintenance of photosynthetic function. The strong shift towards root investment highlights a potential role of rol transformation in applications where belowground biomass and root development are especially relevant.