Protandry is a problem in

Protandry is a problem in Walnut.
Protandry is the condition in which the male reproductive organs (stamens) of a flower mature before the female ones (carpels), thereby ensuring that self-fertilization does not occur. It is a state in hermaphroditic systems that are characterized by the development of male organs or maturation of their products before the appearance of the corresponding female product thus inhibiting self-fertilization and that is encountered commonly in mints, legumes, carrots, walnut, etc.
In walnut, pollen is shed before the ovules are mature.