Nitrate Transport Rate in the Xylem of Tomato Plants Grafted onto a Vigorous Rootstock

Vigorous interspecific rootstocks increase nitrogen (N) uptake in tomato plants but limited information is available on xylem transport rate. Non-grafted and self-grafted tomato plants cv. Attiya and plants grafted onto an interspecific hybrid, Kaiser, were grown under growth chamber conditions and subjected to two light levels, 400 or 800 mu mol PAR m(-2) s(-1). Plant water uptake, xylem sap NO3- content, and stem hydraulic conductance (k(s)) were measured after two weeks of growth. Xylem vessel number and diameter were evaluated in cross-sectional stem cuts and the theoretical xylem hydraulic conductance (k(h)) was calculated. Only the light level modified the xylem NO3- content. Grafting reduced k(s) by 84% in comparison to non-grafted plants. The water uptake rate and xylem sap NO3- content were 4.02 +/- 0.66 g H2O kg(-1) DW h(-1) and 12.78 +/- 1.16 mM, respectively, across all grafting treatments. The rootstock has a higher k(h) because the vessel diameter is 79.3 +/- 14.4 mu m while in non-grafted plants it is 62.0 +/- 10.1 mu m. Nitrate concentration and transport rate changes accordingly to the plant's growth rate. The vigorous rootstock relies on larger vessels to supply the required amounts of N.