As elevation increases the atmosphere, radiation and water availability can all be affected. What is also important is the soil condition which will reflect in grain quality. The ability to have the best temperature of a crop can be found at the high elevation because temperature will rise as one goes down the mountain and decrease as one climbs. Dry sides of mountains will have greater ability to have unlimited light for photosythesis and not be so limited by humidity which will stimulate disease. The dry side of mountains can optimize the water relationship by supplemental irrigation. With high irradiation and supplemented water at the right temperature grain quality should be optimized. In many grain crops the too high temperature especially at night causes plants to waste energy in photorespiration. On the island of Maui the draining of cool night air down the mountain on a southern slope produces a low temperature that minimizes photorespiration and in the day the temperature and sunlight is unimpeded. As such this environment especially in the long day situation and the adequate supplementation of water and nutirents represents an environment for top quality and production yield. In lowland tropical environments the lack of depressed nightime temperature is highly determinant to poor physiological quality. In lowland alluvial situations the erosion of materials can provide a mineral rich environment and shallow top soils can be major limitation to crops and their production.

I think the main concept which you have to know is your crop itself, such as adaptation type, physiological proccess including photosynthesis pathway, morphological character, and etc. Each crops has there own behavior, thats way it is important to identify your crop first. every physical and non-physical plant parts will respon toward environmental changing after that you can make your assumption and choose several main variabels from another correlated journal research because there are numerous variabel to predict grain and also to represent environmental effect. Finally you can perform statistical analysis such as path analysis or advance regression and correlation to explain the interaction of variables with your goals.

Up to 1200 m a.s.l.

I think the info you have given to us is very insufficient. But here are some points that you may consider. You need some more information about your crop. you should know whether you crop is C3 or C4 or cam plant. Generally, C3 and C4 plants require different agro-climatic conditions for thriving well. Long day or short day plants, temperature requirements of your crop. As high altitudes may have huge difference in day and night temperatures. Altitude is also not clear, it could be 1000 feet or 5,000 feet or more above the sea level. Nutrients requirements. You need to know the rainfall data of that area. Moreover, quality of grains itself is so wide term. 

I cannot comment on either impact of altitude on grain quality as my expertise is with non-grain crops and comes from New Guinea and nearby islands to the east and west.

As you know, as you go up in altitude, the temperature drops in a linear manner. There are some other factors which impact on the temperature, such as aspect, but altitude is by far the most important. The lapse rate in New Guinea is 5.2 degrees per 1000 m and this is linear between ca 500 m asl and 4000 m. (Near the ocean the maximum is lower than predicted and the minimum is higher because of the moderating influence of the ocean on temperatures).

Altitude (= temperature) influences rate of crop development. Where I work in New Guinea, crops take longer to grow with increasing altitude, but yield per unit area is generally higher. So yield expressed as weight/unit area/unit time is more-or-less similar. The advantage of a higher yield per unit area is that yield per unit labour (or other) input is greater. As the labour inputs are more or less constant, but one obtains a higher yield, albeit with more time needed. So the staple food in NG, sweet potato takes about 4-5 months to maturity at sea level with yields typically 15 t/ha; it takes ca 6-7 months at 1600 m, with yields typically 25 t/ha; and it takes 9-10 months at over 2200 m altitude.

There are impacts on quality for some crops, especially fruit, with the slower maturity time. Typically these differences are small until the last few hundred metres of the crop's altitudinal range, then they become large. So a papaya, for example, does not taste too different at 100 m or at 1400 m asl, but the quality drops off rapidly near the crops upper limit (1700 m).

I have published a paper on the usual and extreme altitudinal range of 230 economic crops in PNG if anyone is interested.

Sorry I cannot help you with grain quality. Where I work, grains are not grown.

Michael Bourke, can you plz send your paper ?

The citation and URL for the paper is given below. The paper is on the web.

Best wishes

Mike

R.M. Bourke

_________________________

Bourke, R.M. (2010). Altitudinal limits of 230 economic crop species in Papua New Guinea. In S.G. Haberle, J. Stevenson and M. Prebble (eds). Altered Ecologies: Fire, Climate and Human Influence on Terrestrial Landscapes. Terra Australia 32. ANU E-Press, The Australian National University, Canberra. pp473-512.

            http://epress.anu.edu.au/wp-content/uploads/2011/02/ch271.pdf

There is a section on the influence of temperature (= altitude), rainfall, latitude on crop yield, production time etc in Section 1.13 of our book 'Food and Agriculture in Papua New Guinea'. The citation and URL for the link on the web are:

Bourke, R.M. and Harwood, T. (eds) (2009). Food and Agriculture in Papua New Guinea. ANU E Press, The Australian National University, Canberra.

http://epress.anu.edu.au/food_agriculture_citation.html