Growth,root morphology and boron uptake by citrus rootstock seedlings differing in boron-deficiency responses

Boron (B) is an essential microelement for higher plants and has important physiological functions in plant growth and development. Citrus plants are frequently exposed to B-deficiency, but knowledge regarding the effects of B-deficiency on rootstock growth, root morphology and genotypic variations in citrus is limited. To evaluate the variations in plant-growth parameters in response to B-deficiency, five citrus rootstocks seedlings that grown in modified 1/2-strength Hoagland's solution that contained 0.25 μM H₃BO₃ (moderate B-level, Control) or 0 μM H₃BO₃ (B-deficiency, −B) were investigated before and 90 days after treatment. There are significant genotypic variations in plant-growth parameters (dry mass, leaf area and seedling height), root-morphological traits and B-concentrations. B-deficiency inhibited plant dry mass and leaf area expansion of Fragrant citrus, Sour orange and Trifoliate orange seedlings significantly (P < 0.05), whereas no effect was found on the total dry mass of Carrizo citrange and Red tangerine seedlings. Further, B-deficiency reduced the root number, root length and root surface area in Fragrant citrus and Trifoliate orange significantly (P < 0.05); however, no significant effect was detected on these root morphological traits of Carrizo citrange and Red tangerine (P > 0.05). In addition, B-deficiency reduced the B-concentration in the leaves of all the five genotypes and the B-concentration in the roots of Fragrant citrus and Sour orange, none effect was observed on the B-concentration in stems of all studied genotypes and roots. However, there were significant relationships between the root-morphological traits and B-uptake efficiency. These results showed that Carrizo citrange and Red tangerine are B-efficient, Trifoliate orange is the moderate B-efficient genotypic rootstocks, whereas Fragrant citrus and Sour orange are B-inefficient genotypic rootstocks. In addition, Trifoliate orange was the moderate B-efficient rootstocks of five genotypes. The root-to-shoot dry mass ratio (R/S) and longer root numbers were higher in the Carrizo citrange and Red tangerine than that of Trifoliate orange, Fragrant citrus and Sour orange genotypes. That is to say, maintaining higher R/S and numerous longer lateral roots is very important in improving the B-efficiency and thus contribute much to the resistance of seedlings to B-deficiency in the Carrizo citrange and Red tangerine genotypes.