In most of the cases it was observed that protein content decreased with increased intensity of abiotic stress (salt/drought). but a few papers also showed increased protein content. you can several papers regarding this.

decrease in protein content is due to oxidative damage caused by ROS generated under abiotic stress condition.

the answer for this question can't be generalised. It depends on the situation how a particular protein behaves. the total protein content increases or decreases is dependent on the plant's response to the stress condition.

It's depends. The plant protein content might increase when under stress, probably due to the production of stress-related proteins in response to the environment. Please also note that the reduction of protein content might due to the losses of some proteins during extraction.

Many researches have reported that the level of total soluble protein in crop plants decreases under abiotic stress. Stress injury causes damage to protein synthesizing mechanism. Proteins undergo accelerated hydrolysis due to increased activity of protease. Under stress condition, reductions in the activity of many enzymes such as RuBisCO depresses the synthesis of protein and accelerates their degradation and disturb the soluble amino acid and protein ratio in plants. However this response of crop plants depends upon intensity and duration of stress, plant species, the stage of development and also the growing season.

Some researchers indicated that the total soluble protein content increase under abiotic stress. On the other hand, some of them reported conflicting results. It depends on the type of stress, intensity, etc. May be you can find good information in this paper:

Hendawey MH (2009) Effect of salinity on proteins in some wheat cultivars. Australian Journal of Basic and Applied Sciences 3: 80-88.

Of course, many proteins are up regulated after any stress as a response. Those proteins are usually related to the defensive mechanism of the plant. Meanwhile, there are other proteins that are down regulated in response for the same stress. Every stress has it's own way of cell response. Take a look at the attached two papers to learn more about the stress response

Article Heat Shock Activation of Phospholipase C Signaling Pathway i...

Many protein were implicated in responses of stress. Under cold stress the protein contents were increased in tolerant plants and decreased in sensibles.  see this attached papers

I think it's the opposite, during stress increases the rate of protein

hello you can find an answer to your question and may be others in this book

It will differ between species - type of stress - whether plant is resistant - tolerant or susceptible to the particular stress condition.

The net increase in free amino acids under various abiotic stresses must necessarily cause a decline in total soluble proteins in spite of the synthesis and/ or activation of certain stress related proteins such as HSPs and antioxidant enzymes.

Under abiotic stress, the levels of soluble proteins may vary depending on plant species, sensitivity of plant, stress type, and severity of stress.....

If plant is tolerant to particular stress then there is more possiblity of increase in stres related proteins (SRPs). For example, if plant is drought tolerant then a group of proteins called dehydrins found to be increased. If you want to know soluble proteins content under abiotic stress then proteins which are associated with antioxidative enzymes possibly shows increase. Most importantly, it depends on plant whether it tolerant or susceptible.

Crop plants when experiences abiotic stress the tolerant genotypes/ varieties shows increase in an soluble protein level.

The increase in the proteins such as HSPs in heat stress, and dehydrins in drought stress can be observed.