Morphological and Physiological Responses to Short-Term Micronutrient Deficiency in the Root of Two Citrus Rootstocks

Citrus production is highly vulnerable to micronutrient deficiencies. To understand these effects, this study investigated how short-term deficiencies of iron (Fe), manganese (Mn), boron (B), zinc (Zn), and copper (Cu) influence root morphology and nutrient status in two major rootstocks: trifoliate orange (Ptr) and Carrizo citrange (Cit). Using a controlled hydroponic paper-bag system, seedlings were subjected to specific nutrient-deficient solutions. The results revealed distinct responses between rootstocks and micronutrient treatments. The total root length and root surface area in both rootstocks were significantly lower under Fe-deficiency treatment conditions than that of control, but the total root number of Fe-deficiency treated Ptr plants was higher than control. B-deficiency treatment severely inhibited taproot elongation and lateral root growth in Cit, while Ptr showed greater tolerance. The root morphological traits of Ptr and Cit seedlings were significantly decreased under Fe-deficiency and B-deficiency conditions at a root diameter of 0.0–0.5 mm. Mineral nutrient concentration and their correlation analysis revealed that a single micronutrient deprivation alters the balance of multiple nutrients in roots, resulting in divergent and rootstock-specific correlation patterns between Ptr and Cit. Dynamic analysis highlighted that lateral root growth rates were more sensitive to Fe-deficiency and B-deficiency than taproot growth rates, emphasizing the differential impact of micronutrient stress on root system components. This study provides insights for citrus micronutrient deficiency diagnosis by root morphological changes and optimizing fertilization strategies under micronutrient-limited soil conditions, especially in the optimization of fertilization time.