Nitrogen Deficiency Induced Starch Accumulation in Maize Leaves Is Not Due to Plasmodesmata Occlusion.

Under nitrogen (N) deficiency, maize leaves exhibit higher starch concentrations than under N-sufficient conditions. Thus, photosynthate availability per se may not be the factor responsible for yield reductions under N-deficient conditions, rather impaired sugar mobilization and translocation out of leaves likely contributes to reduced yields. To test whether the sugar export pathways in maize leaves are occluded under N deficiency, tissue samples from ear leaves were examined by transmission electronic microscopy (TEM).  In addition N, sugars, and starch concentrations were assayed. To this end, ear leaf samples of N deficient and N sufficient plants were collected at silking and 21 days after silking (DAS).  Consistent with other reports, N-deficient ear leaves had lower N but higher starch concentrations than those from N sufficient plants, both at silking and 21 DAS. The TEM analysis revealed normal appearing plasmodesmata of both N-deficient and N-sufficient ear leaves, lacking occlusions between mesophyll cells and bundle sheath cells, and between bundle sheath cells and vascular parenchyma cells. The lack of plasmodesmata occlusions indicates that the symlastic pathway of sugar export from leaves is open.  Additionally, the size of starch granules in N-sufficient leaves was similar to those in N-deficient leaves, but more starch granules per chloroplast and smaller chloroplasts were observed in N-deficient leaves, at least partially explaining their higher starch concentrations.

Key words: carbon, nitrogen, plasmodesmata, transmission electronic microscopy, maize