A NEW MECHANISM FOR THE REGULATION OF STOMATAL APERTURE SIZE IN INTACT LEAVES - ACCUMULATION OF MESOPHYLL-DERIVED SUCROSE IN THE GUARD-CELLWALL OF VICIA-FABA

At various times after pulse-labeling broad bean (Vicia faba L.) leafl ets with (CO2)-C-14, whole-leaf pieces and rinsed epidermal peels were harvested and subsequently processed for histochemical analysis. Cell s dissected from whole leaf retained apoplastic contents, whereas thos e from rinsed peels contained only symplastic contents. Sucrose (Suc)- specific radioactivity peaked (11 GBq mol(-1)) in palisade cells at 20 min. In contrast, the C-14 content and Suc-specific radioactivity wer e Very low in guard cells for 20 min, implying little CO2 incorporatio n; both then peaked at 40 min. The guard-cell apoplast had a high maxi mum Sue-specific radioactivity (204 GBq mol(-1)) and a high Suc influx rate (0.05 pmol stoma(-1) min(-1)). These and other comparisons impli ed the presence of (a) multiple Suc pools in mesophyll cells, (b) a lo calized mesophyll-apoplast region that exchanges with phloem and stoma ta, and (c) mesophyll-derived Suc in guard-cell walls sufficient to di minish stomatal opening by approximately 3 pm. Factors expected to enh ance Suc accumulation in guard-cell walls are (a) high transpiration r ate, which closes stomata, and (b) high apoplastic Suc concentration, which is elevated when mesophyll Suc efflux exceeds translocation. The refore, multiple physiological factors are integrated in the attenuati on of stomatal aperture size by this previously unrecognized mechanism .