Guttation: Mechanism, Momentum and Modulation

Guttation is one of several visible physiological events of plant kingdom. The mechanism of this phenomenon is however, obscure and embedded deep into mysteries despite its vital significance for the plants and people. In this paper attempts have been made, in the light of recent discoveries, and new and novel findings, to review the past and present works reflecting on an integrated view of the mechanism of this phenomenon and its regulation. In this context, the gene transfer technology and other modern developments in instrumentation and experimentation have enabled to a large extent the understanding of the mechanism of plant secretion of liquid having solutes, both organic and inorganic, dissolved in it. In all probability, this process seems to originate primarily on account of root pressure that involves osmotic withdrawal and transport of water from growth medium and subsequently activities of living cells of xylem parenchyma at the expense of energy fuelled by ATP leading to energetically uphill co-transport of water and solutes in xylem vessels building hydrostatic pressure in the roots pushing the saps upward (Wegner in Journal of Experimental Botany 65:381–393, 2014; Singh 2016). This process though genetically governed, appears to be triggered and regulated by the interaction of several internal, external and edaphic factors finely-tuned by chemico-mechanosensors such as pH gradient, hormones, mechanical loading, turgor potential, membrane potential, temperature, light, touch, sound etc. influencing cellular contractile proteins and gating of aquaporins. The positive hydrostatic pressure so developed is, in turn, transmitted via stem to the leaves as release of water droplets of guttation through permanently open hydathodes located at the tips, margins, adaxial and abaxial surfaces of uninjured leaves.