(1)① Generally, the anthers in the mononuclear stage are selected for culture, and the anthers before the mononuclear stage are inoculated, which are tender and easily broken; Anthers inoculated after mononuclear stage have some loose petals, which makes it difficult to disinfect materials.

② There are two ways of anther culture in vitro: one is dedifferentiation to form callus, and the other is dedifferentiation to form embryoid.

③ Promoting pollen cell division and growth depends on the ratio of cytokinin to auxin.

(2)① Pectinase catalyzes the decomposition of pectin in plant cell wall into galacturonic acid.

② According to the table in the stem, the juice yield is the highest at 40℃, indicating that the enzyme activity is the strongest at this temperature.

③ This experiment has two indicators to measure the results. One is to observe the volume of filtered apple juice at the same time. The large volume shows that pectinase activity is high, pectin is decomposed more, and juice flows out better. The other is to observe the clarity of juice.

So the answer is:

(1)① Single core?

② embryoid? callus

③ Cytokinin? plant hormone

(2)① galacturonic acid?

② Maximum at 40℃?

③ Pectinase decomposes pectin into small molecules, which can pass through filter paper, so the volume of apple juice reflects the ability of pectinase to catalyze pectin decomposition.