The affix u comes in the chandas (Vedas) after the roots that have taken the Denominative affix kya in the sense of 'agent having such a habit etc.'.,
The affix उ comes in the Chhandas, after the roots that have taken the Denominative affix क्य in the sense of 'the agent having such a habit &c'.
The term क्य includes the three affixes क्यच्, क्यङ् and क्यप् (see sutras (III. I. 8), (II. 13), &c). As मित्रयुः, संस्वेदयुः, सुम्नयुः (Rig. Ved. I. 3. 4).
In the modern Sanskrit तृन् will be employed. As मित्रीयिता. By sutra 7.4.33, the long ई replaces the final अ, when क्यच् follows; as पुत्र + क्यच् = पुत्री + य = पुत्रीय. Why then in the case of मित्रयुः there is not the substitution of long ई for the अ of मित्र ? Because the rule 7.4.33 is set aside by 7.4.35 which declares that in the Chhandas the long ई is not substituted for the अ ।,