The whole issue boils down resistance and tolerance of crops to the weather. Whereas some crops are resistant or tolerant to cold and hot seasons, some other crops are resistant or tolerant to either cold or hot seasons which makes them seasonal, 

Dear Caser,

This question has to do with cellular mechanism of crop. Most of the perennials can withstand subzero temperature. Plants do not produce metabolic heat to keep them warm, but there are several mechanisms (species specific) that plants adopt to tolerate low temperature by avoiding formation of ice crystals in the cells:

1. Accumulation of solutes (sucrose, mainly, but also other organic compounds such as proline) by the cells to depress the freezing point of water (think salting ice on the sidewalk during winter) and to stabilize membranes. 

2. So-called “antifreeze” proteins help prevent ice crystals from forming in the extracellular spaces (outside cell); plant cells that make these proteins typically secrete them into cell wall region (intercellular spaces).

3. The plant cells may synthesize proteins called “dehydrins”, which are inside the cell (cytoplasm), may bind water molecules and alter the collective structure of water in the cell to stabilize membranes.

4. Plant cells can alter lipid composition of cellular membranes in order to adjust the fluidity (functionality) to colder temperatures.

I hope this helps.

Thanks!

Dear Dr Abdel

As i understand,  when a cool season crop in cultivated in a contrasting season for eg. hot season, its fullest yield potential can't be achieved. However with the advent breeding new varieties and introduction of varieties from foreign countries, it is possible to cultivate these crops in both the seasons with minor adjustments in productivity. For eg In india, in certain states the Maize crop hitherto kharif (Jun-Aug) crop is cultivated nowadays in rabi season (Sep-Dec) also. This may also be due to change in weather conditions  dictated by the ostensible climate change too.  

Dear  Collegues: 

The optimal functioning position of enzymes satuated or related to cellular membrane are when cellualar membrane in physical state of 50% Sol-----Gel. The orientation of reactive group is perfect. Since, membrane flexibility and solublity depend upon Unsaturated/saturated faty acids that assymbled the cellular membrane, Cool season crops the ratio is for the favour of unsaturated where, %0% sol-----Gel be achieved at low temperature degrees. Osmosity and other accumulated chemicals to avoid low temperatures come through enzyme activities. Thanks indeed 

Dear sir, 

As i understood, these seasonal crops are susceptible to weather. Means summer crops are succeptible to cold n cool season crops cant grow in hot weather. Now a days due to advancement of breeding many more cultivars of different crops are developed which hv the capacity to regist or tolerate the adverse climate. So by adopting such varieties we can do the crops in off seasons. Rather than that in green house with controlled atmosphere we can produce the off season vegetables anytime n these ll be of good qualities too. Yes if u wanna do summer crop in winter in such green houses the cost ll be less coz u can adopt low cost technologies to mng these bt if u wanna do winter crops in summer then it ll be costlier coz dropping the temp. Inside green house by completely controlled atmodphere

For growers the classification of vegetable crops by similarities in their temperature requirements (Thermo-classification) or by their ability to withstand frost (Hardy ness classification) assists growers in planning their planting schedules. However hardy only refers to frost and some hardy vegetables will not withstand summer drought, while some tender crops do not thrive in cool weather even when there is no frost.  On the one side vegetable crops are defined on the basis of hardiness and on another side not unlike hardyness thermo-classification is also based on temperature. The difference is that crops are classified according to their optimum temperature requirements and are divided into two groups either as cool temperature or warm temperature crops. Cool temperature crops have an optimum temperature of 12-20oC and are able grow as low as 1oC, no mention of their ability to withstand frost are given in this classification. Warm temperature crops have an optimum of 18-20 oC Cool temperature crops are as follows: Green peas, lettuce, cabbage, cauliflower, broccoli, onions and spinach are all ‘cold season ’ crops. ‘Warm season’ crops are; Beans, cucumber, eggplant, okra, pepper, summer squash, watermelon, and tomatoes. This system of classification gives the grower an indication of the crops ability to withstand high temperatures but nothing in regards to frost. In order to have a clear idea on which crop to use when planning a planting schedule it is advised that the grower use the ‘hardyness classification’ in conjunction with the thermo-classification.

Hanks for all