Cucumber seedlings were exposed to high and sub-high temperatures to see how it affected their growth and photosynthesis. The results showed that both high and sub-high temperatures slowed down the growth of the seedlings. The photosynthesis rate decreased, and the CO2 concentration inside the leaves increased under heat stress. The decrease in photosynthesis was mainly due to factors other than the stomata. The enzymes involved in photosynthesis were affected by the temperature and duration of the stress. High temperatures caused severe damage to the seedlings' photosynthesis system. Overall, short-term sub-high temperatures didn't harm the seedlings under optimal light conditions, but high temperatures did.