Why are there “dew drops” at the tips of leaf veins?
❦ Have you ever seen clear orbs of water glisten along a leaf edge? You may have mistaken them for dew drops, which are caused by moisture from the air condensing on cool surfaces. But these drops are only found at the edges of leaves and if you look around- they won’t be found on dead leaves. So what are they?
❦ Plants use a plumbing system of xylem tubes to move water and nutrients. During the day, transpiration (water evaporation) from leaves creates a vacuum that pulls the column of water up from the roots to the leaves. At night, the stomata (leaf pores) close, transpiration stops and salts accumulate in the xylem of roots, drawing in water from the surrounding soil by osmosis. The excess water rises up the xylem tubes and is forced out at the leaf tips through openings called hydathodes. This exudation of plant sap is known rather inelegantly as guttation, and only happens at night. The water pressure is not strong enough to rise beyond 3 feet, so guttation is not seen on tree leaves. The thermal image (inset) taken by infrared photography shows the cooler temperature (blue) in the guttation droplets.
❦ When the drops dry, they sometimes leave behind a residue of salts and minerals. This is not a problem, unless the soil is over-fertilized resulting in fertilizer burn of leaf tips. In the same way, guttation droplets in corn seedlings were shown to have high levels of neonicotinoid compounds, used as pesticidal coatings on the seed. These concentrations could be a lethal dose for honey bees that sip on guttation drops as a water source. While shedding toxins through guttation drops protects the plant, it may have repercussions – both beneficial and harmful, on insects and other animals.
Inset of thermal image: http://thermal-imaging-blog.com/index.php/page/13/#.Vq1LIvkrLcs
REF on neonicotinoids in guttation droplets #openaccess: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4284396/