Stomatal conductances of Vicia faba leaves were recorded over a day. Coordinately, (a) guard cells dissected from leaflets were assayed, providing total sucrose content, and (b) guard cells dissected from rinsed epidermes were also assayed, providing symplastic sucrose content. Compared with that of pre-dawn samples, apoplastic sucrose content increased 4.8 – 5.2 × (2 experiments), reaching 1,130 – 1,300 fmol·guard-cell-pair−1 at midday, when conductance was highest (ca. 0.13 mol·m−2·s−1); symplastic sucrose content increased 2.5 – 3.5 ×, reaching 350 – 390 fmol·guard-cell-pair−1. Thus, there is a correlation between transpiration and guard-cell sucrose content, particularly that portion localized to the apoplast. Moreover, apoplastic sucrose is apparently a source of guard-cell nutrition and, possibly, osmoticum.
