Question

stomata of desert plant remains closed during daytime. how do they take up carbon dioxide and perform photosynthesis

Answer 1

In desert plants the stomata open at night.
They take in carbon dioxide at night, and is stored in the form of an intermediate which is used during day for photosynthesis (When the stomata are closed).
Also, chlorophyll of the plant are located in the stem as levesare reduced to spines and stem takes the function of photosynthesis. 

Answer 2

In these plants stomata closes during day time to reduce the rate of transipration so during night time when stomata opens they absorb co2 and store in an intermediate form called malic acid during day time for performing photosynthesis malic acid splits and realise co2