Question

Effect of extreme cold and extreme hot weather on the plants.

Answer 1

Plants grow faster in moderate weather conditions. A rise in temperature increases the plant's growth rate but to an extent and if temperature goes on increasing growth rate starts falling and loss of water increases which will result into the death of plant. Rise in temperature also affects badly on seed germination.
Cold also affects plants badly if there is too much low temperature the water in plants start freezing results into the burst of plant cells. Bursting of plant cells cause plants in browning, wilt and eventually death.

Answer 2

Increased temperature will increase the rate of chemical reactions within the plant. This will mean the plant will need more oxygen, carbon dioxide and water supplied. If it is hot and subsequently dry the plant will reduce water loss by closing stomata slowing normal processes. This works to a limited extent whereby signs of water loss such as wilting occur.

Cold itself mostly just slows processes. Unless the plant is in its reproductive phase. Cold due to ice can depend on how the plant is adapted and how the ice settles. (A covering of ice over a bud can actually protect it from cold temperatures.) If it is sufficiently cold the water within the plant can freeze, causing the cells to expand and break. This could cause death.