Differences in mineral nutrients content in xylem and phloem sap between cotton cultivars with different boron efficiency

Boron deficiency is one of the most widespread of all micronutrient deficiencies. Plant response to boron deficiency varies widely among species or genotypes. In our past experiment, the differences were observed in boron content in shoots and distribution in different organs between B-efficient cultivar 9901 and B-inefficient one 9903. It has been proved that the phenomenon was in connection with different capacity of boron uptake between two cotton cultivars under boron deficiency. Bellaloui et al (1998) and Takano et al (2002) reported genotypic differences of boron efficiency in some species was attributed to different boron transport from roots to shoots. To further elucidate the mechanism of cotton genotypic differences, the transport of B and other mineral nutrients in xylem and phloem in two cotton cultivars with different B efficiency was studied under boron deficiency. The cotton seedlings were cultured in Hoagland solution, the 5-leaf-old seedlings were treated by 3 boron levels: 0, 0.002, 0.50 mg B per liter. The phloem and xylem exudation sap was collected after two weeks and the content of some mineral nutrients in the sap was determined by ICP-OES. Almost no boron was detected in cotton phloem sap at sufficient or insufficient boron supply, suggesting that B mobility in cotton was restricted. Boron concentration and exudation rate in xylem sap of two cotton cultivars were decreased at boron deficiency, but the trend was more obvious in B-inefficient cultivar. The B concentration in xylem sap of both cotton cultivars was lower than that of nutrient solution under sufficient boron supply (0.5 mg/L), but the contrary trend was found at a lower B supply (0.002 mg/L). B concentration in xylem sap for the efficient cultivar was up to 32 fold higher than in the nutrient solution and compared to 20.5 fold for the inefficient one. These suggest that there may be two different mechanisms for B translocation into xylem: a passive diffusion process plays an important role at sufficient B supply, but a concentration mechanism is involved at low B concentration. It seems that the latter mechanism works in B efficient (cotton) cultivar. The content of K, Mn, Cu and Zn in xylem sap in two cotton cultivars was increased at B deficiency, (but) with greater intensity in B efficient cultivar. The exudation rate of K, Mg, Ca, Mn, Cu and Zn in xylem sap of both (cotton) cultivars was cut down at B deficiency, the degree of reduction in B inefficient cultivar being greater than in B (efficient) one. When B was extremely insufficient, the exudation rate of K, Mg, Mn and Cu in phloem sap of B efficient (cotton) cultivar was raised, but depressed in B inefficient one. The exudation rate of Ca and Zn in phloem sap of both (cultivars) was decreased, but to greater degree in B inefficient one.