Effects Of Boron Deficiency On Physiology And Biochemistry In Citrus

Citrus (Citrus sinensis (L.) Osbeck) seedlings grown in pots containing sand were fertigated for 14 weeks with nutrient solution at a B concentration of 0, 2.5, 5, 10, or 25μmol/L every other day. Effects of boron deficiency growth, root morphology and structure, mineral nutrition, photosynthesis, photosynthetic enzymes, carbohydrates, and active oxygen metabolism were investigated in this study.1 Effects of B deficiency on growth and contents of nutrient elements in citrus seedlingsAll growth was normal in appearance and did not differ very much among different treatments except that 0μmol/L B treated trees showed visible symptoms of B deficiency. Also, leaf B levels were in the normal range except for the plants without supplying B. Based on these results, plants without supplying B and those treated with 2, 5, 10, or 25μmol/L B every other day are considered as B-deficient plants and B-sufficient ones, respectively. Boron deficiency symptoms in the shoots firstly appeared at the apex and in the actively growing leaves 9 weeks after B treatment. Boron deficiency symptoms included dieback of terminal growth, yellow, water-soaked spots in young leaves and deformation of these leaves, thickening and yellowing of mature and aging leaves, enlargement, splitting, and corking of leaf veins. Boron deficiency symptoms in roots included enlargement of tips, deformation of cells and tissues, unevenness and crack of surfaces. Because leaf and stem dry weights decreased to a larger extent than root dry weight in response to B deficiency, root/shoot ratio was greater in B-deficient plants than in B-sufficient ones.Boron deficiency decreased B, K, Ca, and P contents of citrus leaves, but had less effect on K, Ca, and P contents of roots. Boron deficiency did not affect Mg content of citrus roots, stems, and leaves. Because B deficiency decreased N content of citrus leaves and had little effect on their C content, C/N ratio was higher in B-deficient leaves than in B-sufficient leaves.2 Effects of B deficiency on photosynthesis in citrus leaves Contents of Chl a, Chl b, Chl, and Car, and Chl a/b ratio were higher in B-deficient leaves than in B-sufficient ones, whereas Car/Chl ratio was higher in B-deficient leaves than in B-sufficient ones. Boron-deficient leaves had decreased CO₂ assimilation, stomatal conductance, and activities of photosynthetic enzymes (Rubisco, GAPDH, stromal FBPase), but increased intercellular CO₂ concentration compared with B-sufficient ones. Boron-deficient leaves had increased hexoses (glucose, fructose) and starch contents, but decreased sucrose content compared with B-sufficient ones. It is consluded that CO₂ assimilation may have been feedback-regulated by the excessive accumulation of hexoses (glucose, fructose) and starch in B-deficient leaves.3 Effects of B deficiency on active oxygen species metabolism in citrus leavesBoron-deficient leaves had increased superoxide anion and H₂O₂ profuction rates, APX, MDAR, GR, POD, and SOD activities expressed on a leaf basis, and AsA and GSH contents expressed on a leaf Chl, whereas leaf-area based activities of MDAR and CAT activities were lower in B-deficient leaves than in B-sufficient ones. Compared with B-sufficient leaves, B-deficient leaves had higher MDA content and electrolye leakage. In conclusion, although antioxidant systems are up-regulated in B-deficient citrus leaves, this up-regulation can not provide enough protection to B-deficient leaves against photooxidative damage under hight light.