The subject invention pertains to novel mutant polynucleolide molecules that encode enzymes that have increased heat stability. These polynucleotides, when expressed in plants, result in increased yield in plants grown under conditions of heat stress. In one embodiment, the polynucleotide molecules of the subject invention encode maize endosperm ADP glucose pyrophosphorylase (AGP) activity. Plants and plant tissue bred to contain, or transformed with, the mutant polynucleotides, and expressing the polypeptides encoded by the polynucleotides, are also contemplated by the present invention. The subject invention also concerns methods for isolating polynucleotides and polypeptides contemplated within the scope of the invention. Methods for increasing yield in plants grown under conditions or heat stress are also provided.