In vitro potato plants grow well at 25C/18C day,night temperature with a 16h/8h -photoperiod. Light intensity should be around 100 micromolar m-2,s-1. The type of MS media used is important and should be optimized for potato. Make sure the pH is 5.8 to 6.

You should consider the possibility of lack of calcium due to poor availability or poor uptake due to growth conditions. Would the latter be the case then adding calcium to the medium would probably not solve the problem, spraying would.

High temperature and high humid is one reason to induce tip blackening.

Quite an interesting topic.thanks fellow researchers

I agree with Xianping Li, in our tests, high temperature and keeping the invitro plants very close to lightings in growth chambers cause to top necrosis of young plants. Check the temp to be 20 to 22 not more and alco check the real temp inside of your growth chamber by mannual thermometers not complex sensors

Its very common problem in sprouts of potatoes due to lack of humidity and high temperature. It could be corrected by spraying either Calcium chloride or carbonate. Whereas in plantlets its due the high temperature and less humidity. Check your container what you are using to grow it. If cap or cover is not properly fitted, it would cause such symptoms.

You might consider a leaf tissue analysis. MS media (as Nathan mentioned) has its issues. By having tissue ashed and analyzed in the same manner crop materials are examined for macro and micro elements, you can determine a proper salt medium nutrient balance. Of course, you will have to compare the ash analysis with a standard derived from that of comparable young healthy tissue growth in situ.