Short answer: It’s a matter of stricter vows during Paryushan, not a universal “ban.” Many Jains give up lemon because it is khatta (sour) and used mainly to enhance taste, which goes against the spirit of austerity and sense‑control practiced in these days.

Main reasons people avoid lemon in Paryushan

• Ras-tyag (control of taste): Sour items like lemon increase taste-craving. During Paryushan we reduce such tastes to cultivate samyam (self‑restraint) and aparigraha (non‑attachment).
• Ayambil/Ekasana rules: If you are doing Ayambil or very simple meals, khatta (sour) items—including lemon—are generally not permitted.
• Fresh, raw, and seed-related caution: Many families avoid fresh fruits/vegetables and sometimes multi‑seeded items during these days to minimize possible himsa and to keep the diet extremely simple. Lemon typically falls out under these stricter family/community practices.
• Practical discipline: Those taking only boiled water in daylight cannot add lemon to it; so lemon naturally gets excluded for them.

Digambar and Shwetambar note

• Shwetambar: During Paryushan, many households drop khatta items (including lemon) along with green/root vegetables as part of stricter pachchakhān (vows).
• Digambar: During Dashlakshan (following Paryushan) and related observances, similar austerity is kept; sour/stimulating items are commonly avoided. The principle is the same—ahimsa and restraint—though exact home practices may vary.

In summary: We avoid lemon during Paryushan to keep the diet plain, reduce attachment to taste, and uphold ahimsa-focused austerity. If your family or local sangh observes stricter rules, follow those pachchakhān.

For simple guidance on what to eat/avoid in Paryushan:

• What to eat during Paryushan
• Rules during fast of Paryushan