Hi Imadud. There are various parameters that may yield important and different aspects of the effects of climate change on plants. The TIMESAT software program manual explains all parameters quite well. I suggest you read up on these and then decide. For instance, climate change could affect the timing of the growth season in which case you would probably look at start and end of season, while the effect on yield may be characterised by the large integral of your vegetation index among others. All depend on the question but as I said, the TIMESAT manual is quite helpful.

To quantify the impacts of climate variability on growth, you can measure root-shoot ratio (RSR), relative growth rate (RGR), absolute growth rate (AGR), net assimilation rate (NAR), specific leaf area (SLA), plant height, and number of tillers per plant.

For phenology suggested parameters are days to flowering, day to maturity, floret primodium initiation, anthesis date, duration from emergence to anthesis, duration from sowing to maturity, and growing degree days for yield you can measure number of spikes per plant, number of spikelets per spike, number of spikes per square metre, spike length (cm), number of grains per spike, grain weight per spike, 1000 grain weight, harvest index, grain yield per plant and seed index.

https://www.nature.com/articles/s41598-018-23101-2

Article Assessment of ozone toxicity among 14 Indian wheat cultivars...

Article Adapting wheat in Europe for climate change

You can find good ideas in relevant papers. For example:

Article Changes in yield variability of major crops for 1981-2010 ex...

Article Crop response to climatic variation

https://www.nature.com/articles/s41598-018-23101-2

https://academic.oup.com/jxb/article/60/9/2529/515341