Effect of nitrates supplied with the transpiration flow on assimilate translocation

Solutions of nitrates (0.5% KNO3, 0.2% NH4NO3) or urea (0.15%) were fed under the pressure of 104 Pa to 50-60-cm-long detached shoots of common flax (Linum usitatissimum L.). One hour after the start of supplying the solutions, an assimilation clip chamber was fastened to the middle part of the shoot (C-14 source area), and (CO2)-C-14 was blown through in the light for 2.5 min. The analysis of distribution of C-14 among the labeled products of photosynthesis produced by source leaves showed that nitrates reduced the incorporation of the label into sucrose. At the same time, the ratio of labeled sucrose to labeled hexoses decreased, and the incorporation of the label into serine greatly increased. Urea did not produce such effects. The pattern of distribution of C-14 within the plant 3 h after the assimilation of (CO2)-C-14 points to the suppression of assimilate efflux from the leaves of plants fed with nitrates. In plants supplied with water or urea, 17-20% of labeled carbon was found below the C-14 source area of the shoot; in nitrate type of treatment, only 3-5% was found there. In plants supplied with nitrates, the cortex tissue below the source leaf contained more C-14 in proteins and less in low-molecular substances. In the wood tissue, such a correlation was not observed. When the shoot was supplied with water or urea, the content of C-14 in sucrose in the source leaves in 3 h declined from 55-60% to 38-42%. When the shoot was fed with nitrates, the share of label in sucrose increased from 50 to 62-73%. Autoradiography of the source leaves showed that, in plants supplied with water or urea, the label was predominantly accumulated in large vascular bundles, and in nitrate type of treatment, it was accumulated outside large bundles. Electron microscopy showed that, in nitrate plants, the companion cells of phloem endings were very much vacuolated.