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We show that the plane Cremona group over a perfect field  k  of characteristic  p  ≥ 0 contains an element of prime order ℓ ≥ 7 not equal to  p  if and only if there exists a two-dimensional algebraic torus  T  over  k  such that  T ( k ) contains an element of order ℓ. If  p  = 0 and  k  does not contain a primitive ℓth root of unity, we show that there are no elements of prime order ℓ &gt; 7 inand all elements of order 7 are conjugate.

On Elements of Prime Order in the Plane Cremona Group over a Perfect Field