ऊर्ध्वे शुषिपूरोः

Operational rule vidhi

The affix णमुल् comes after the roots शुष् 'to dry' and पूर् 'to fill' when the word ऊर्ध्व denoting an agent is in composition with them.

Word by word (padaccheda)

Right context ऊर्ध्वे saptamī · ekavacanaTarget शुषि- पूरोः ṣaṣṭhī · dvivacana

Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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Full reading

ऊर्ध्वे शुषि-पूरोः

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

The affix णमुल् comes after the roots शुष् 'to dry' and पूर् 'to fill' when the word ऊर्ध्व denoting an agent is in composition with them.
Thus ऊर्ध्वशोषं शुष्यति वृक्षः = ऊर्ध्वे शुष्यति 'the tree is dried up while it is still standing'. Compare यद्वोर्ध्वशोषतृणवद्विशुष्कः 'or wither like the up-heaved grass drying up' (Bhatti. III.14); ऊर्ध्वपूरं पूर्यते 'is filled full to the brim'. Rule 3.4.46 applies here also.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.