Elucidation of Physio-Biochemical Changes in <i>Citrus</i> spp. Incited by <i>Xanthomonas citri</i> pv. <i>citri</i>

India is one of the leading citrus−producing countries, next to China and Brazil. Despite the sizeable production, especially of lemons and limes, India contributes meagerly to the world’s citrus market. Of the various factors responsible for the restricted quantum of citrus export, citrus canker (<i>Xanthomonas citri</i> pv. <i>citri</i>) is one of the leading serious causes and directly impacts the appearance of the fruits. Considering the extent of damage, the present study aimed to understand the impact of <i>Xanthomonas citri</i> pv. <i>citri</i> (Xcc) on the physio−biochemical responses in contrasting <i>Citrus</i> spp. Two genotypes, each of three citrus species, namely <i>Citrus aurantifolia</i> (Pusa Udit and ALC−35), <i>C. limon</i> (Kagzi Kalan and Konkan Seedless), and <i>C. paradisi</i> (Redblush and Marsh Seedless), were artificially inoculated with Xcc (10⁸ to 10⁹ cfu/ mL) by the pinprick method. The physio−biochemical changes in the host were evaluated after 48 h post inoculation (hpi). The chlorophyll content (total, a, and b) degradation and reduction in leaf gas exchange parameters, such as photosynthetic rate (<i>A</i>), transpiration rate (<i>E</i>), stomatal conductance (<i>g_s</i>), and intrinsic water−use efficiency (iWUE), were measured to a greater extent in susceptible than resistant genotypes. The microscopic observations also evidenced higher stomatal density with larger stomatal areas in susceptible genotypes, favoring the easier penetration of Xcc in host tissues than resistant species or genotypes. The higher activities of various antioxidant enzymes, <i>viz</i>., superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and glutathione reductase (GR), the contents of soluble protein, and phenolics were measured in resistant genotypes in contrast to susceptible genotypes. The activities of phenyl ammonia lyase (PAL) and polyphenol oxidase (PPO) were also higher in resistant genotypes, whilst the levels of ROS (superoxide radical and hydrogen peroxide) production were enhanced in susceptible genotypes. Based on the host–pathogen interaction, the order of resistance in a descending manner was found as Kagzi Kalan, Marsh Seedless, Redblush, Konkan Seedless, Pusa Udit, and ALC−35. Further, the results will enhance the understanding of the pathogen mechanism during pathogenesis in resistant and susceptible <i>Citrus</i> species.