The cells of a pineapple plant have 50 chromosomes in their nuclei. How many chromosomes are there in the male nucleus in a pineapple pollen grain?

25 chromosomes

Explanation:

There are two principal types of cells in the organism: Somatic diploid cells (2n) that reproduce by the process of mitosis, and germ cells that are diploid reproductive cells in charge of gamete production. Germ cells divide by mitosis and meiosis. Through the process of mitosis, they originate more sexual cells, but through the process of meiosis, they give place to haploid gametes, called sperm and egg cells, through the gametogenesis process.  

Gametes´destiny is to merge during fecundation, and a new diploid cell called zygote emerges through fertilization. The zygote is a complete cell and suffers successive mitosis to form the new organism.  

Meiosis produces four daughter haploid cells (n) from a diploid germ cell (2n). Each daughter cell is haploid because they have half the number of chromosomes of the original one.

So, in the exposed example

"...cells of a pineapple plant have 50 chromosomes in their nuclei..."

These might be diploid somatic or germ cells. The number of chromosomes, 2n = 50.

Germ cells in plants suffer meiosis and produce gametes.

Gametes of plants are ovocell and pollen grains. Ovocell is the feminine gamete, while the pollen grain is the masculine gamete.

The pollen grain is haploid and its number of chromosomes n = 25.

enzyme speed up the reactions

srxdctfvygbuhinjmok,plkmojnhbgvfctdrft6gyfcgvhbuinjok