It seems root exudation is considered to happen at night when transpiration
ceases ...
From another website :
Root Exudation and Rhizosphere Biology -- Walker et al. 132 (1): 44 -- PLANT PHYSIOLOGY
Modern cryo-scanning microscopy has helped researchers determine that the rhizosheath of a plant is more hydrated in the early morning hours compared with the midday samplings (McCully and Boyer, 1997). This implies that the exudates released from the roots at night allow the expansion of the roots into the surrounding soil. When transpiration resumes, the exudates begin to dry and adhere to the adjacent soil particles. Thus, the rhizosheath is a dynamic region, with cyclic fluctuations in hydration content controlled to some extent by roots.