Transpiration: Venue for nutrients delivery

The concept of soil-plant-atmosphere continuum visualizes plant as the conduit for transportation of water, solutes and photoassimilates between soil and the atmosphere. Soil as the source of water and plant nutrients is studied extensively, whereas the climatic conditions as the driving force for water flow is mostly glossed over. More importantly, the control of stomates for water efflux is seen as too dynamic such that the demand side of the plant is hardly dealt with. Closing down of stomates leads to slower rate of flow in the xylem, lower supply of CO2 for photosynthesis and limited rate of translocation in the phloem. Without measurement of the plant under field condition, the disruption of the water flow inside the plant will go unnoticed. In the case studies of mangosteen, salak and oil palm presented, leaf gas exchange was measured concurrently with the leaf water potential gradients, the soil water status, the climatic conditions and the sap flow rate in the branch (in mangosteen). The results showed that extreme levels of radiation and vapor pressure deficit (vpd) exerted control on the rates of transpiration and sap flow. Irrigation can become wasteful during dry season of high vpd when transpiration is limited by the low stomatal conductance. Measurement of soil matric potential revealed several incidents of the root zone being water saturated for a long period during rainy season, with the crown exposed to low air vpd. Restricted transpiration limits the delivery rate of plant nutrients to the leaf and the fruit, which leads to several physiological disorders in fruit.