Singular fibers in Lefschetz fibrations on manifolds with b2+=1

We determine the minimal number of singular fibers of a nontrivial genus-g Lefschetz fibration X→S2 (with g⩾14 or g⩾6 and even) satisfying b2+(X)=1. Partial results for g odd with 9⩽g⩽13 are also discussed. Furthermore we prove a (weaker) lower bound for the number of singular fibers in a genus-g Lefschetz fibration in general.