You should look for all the factors that may have affected plant growth.

Maybe your "normal season" caused some other type of stress (other than heat).

Also, growth is positively affected by high temperatures just below the stress threshold, so high yields can be explained by optimal climate conditions that just preceded exposure to stress. You would need to analyze the climate conditions of the entire season in detail, in order to determine the degree of actual exposure to stress in each season. Also, do check for environmental optima (temperature, rainfall) for the plant species in question.

Martin Raspor clearly mentioned in his answer.

each stage of growth need proper condition. max photosynthesis rate and max respiration  rate are take place in different  temperatures. difference of these output and inputs determine yield 

The answer of Martin Raspor is a very good one. Just in case you eliminate what he suggests, I only have this comment: Another explanation would be that stress induces, not only increased antioxidative capacity, but epigenetic adaptation towards growth. Firstly check the sugars/amino acids ratio in the phloem and try to correlate it to antioxidants. I found (Alexou 2013) that under stress this ratio decreases, while ascorbate in the leaves increases. If your plants are under stress, they should show this too. Then you know that what you found is truly "stress-induced". You can also try the method I propose in my paper to measure ROS easily and make sure that the plants are truly under oxidative stress. Perhaps increased growth comes from the plant's need to overcome stress. Are you measuring growth only with GDD? Can it be that plants invest in root to acquire more water? Or they invest in seeds in a shorter time owing to stress? Anyways, I hope you find all this useful. Good luck!

Thanks to all. And I do agree with all.  In Chickpea, above 35° is considered as crucial and the flowering is mostly affected by the temperature.

We gave regular irrigation during its crop cycle to avoid the drought stress.

The temperature during vegetative phase is below 35°.

So is it possible that the plants accumulate sufficient biomass in stress in the vegetative stage (when the temperature was below 35°) due to high radiation (Summer sowing)? Therefore ultimately some lines which have better buffer capacity for heat produced more yield than its respective normal season yield.

And another way can we take the consideration of growing degree days for explain this incident?

Kindly discuss.

I have an experience of having some lines produced higher yield under drought stress than under the normal favorable conditions in faba bean. this could be explained by the fact that yield is a trait which depends on many other plant characteristics including physiological, biochemical and morphological, all are controlled by major genes as well as with other modifying genes which are highly affected by the prevailing environmental conditions in terms of gene expression. therefore yield is considered as quantitative character which is highly influenced by environment. a another  explanation is that heat or drought tolerance are traits their expression is much depending on  the interaction between other environmental factors, ie., temperature. light intensity, gases, salinity, relative humidity, etc. which can modify the severity of the stress under that specific stress conditions, at which the lines showed high unexpected yield. also the effect (severity) of  stress is highly associated with time of stress incidence ( ie., at which growth stage: vegetative or reproductive). accordingly genotypes show differential response to the stress as well as the variables gene expressions.

Hello Vaisali,

Crop production is a rather complex  function of plant morphological features - size and characteristics of all organs - which are determined by biochemical processes themselves determined by the genome, interacting with the environment, itself complex and varying in a non-uniform way. This we all know, but there is the idea that we can somehow explain yield by relatively few or even one factor (plant or environment).  Generally that is not the case, unless a major environmental factor affects the plant/crop. So it is not easy to answer your question without some data on the plant and environment. Martin in the first answer says this and I agree with him: others add supporting comments.  First try to consider what you mean by normal environment and then identify how the environments differ. Actually, environments are not `stressful' - they are neutral. It is only when the environment affects the crop in such a way that some feature of its growth or function is impaired that the environment may be said to be `stressful'.  I know that it is very common or normal to talk about stress as an environmental thing rather than the plant response. Perhaps the old or little used  term of` stressor' meaning the environmental factor and `strain' as the plant response might be better. 

Having said all that it will be necessary to get some data to really understand what is differing between the crops where you get better yields in a conditions which you expect to decrease them. By the way perhaps you  might like to look at some of my papers on Researchgate where these aspects have been considered. Best of luck!  David  

 @Awadalla Abdalla Abdelmula  Sir,

Thank you so much Sir for sharing your results with me  ncluding possible reasons. SIr, have you published any paper on that data so that I can refer to your paper. If yes please share with me. I will be highly thankful.

@David W. Lawlor, thanks Sir and I very much agree with you.

Here for chickpea the normal season is the Nov-Feb where the temperature remains optimum for the growth of chickpea and it is grown traditionally in this period only.

For Chickpea ,as I mentioned that above 35o C is the crucial for pod formation so If we sow the crop in First week of Feb then its flowering is exposed to heat above than 35oC and thus the yield is reduced. I referred this as stress for heat in Chickpea.

You told me to refer some of your papers so, could you please sugggest me specifically which paoers I should read.

Thank you so much for your response.

Regards,

Vaishali