Would have helped with a size-range and multi-focal pictures, but it could be Juglans (Walnut) or Amaranthaceae pollen (e.g. Chenopodium (Goosefoots) or Artiplex (Orach)) perhaps.

It seems to be a fresh (= with cytoplasm) grain of the group sometimes referred to as Chenopodiineae, (= Chenopodiaceae plus the genus Amaranthus), periporate. I agree with Carl Alexander Frisk that pictures of several focus levels would much facilitate the identification of the grain.

Thanks, Carl Alexander Frisk and Héctor L. D'Antoni, for your response.

Please see attached more images. The diameter is ca 24.53 micrometres.

I think it might be Cheno-Amaranth (Chenopodiineae).

It also could be Caryophyllaceae pollen, such as Arenaria or Colobanthus, with a similar size of pollen grain. But it could be also pollen of Chenopodiaceae or Amaranthaceae as mentioned above by other colleagues.