FRESH WATER IS A LIMITED NATURAL RESOURCE REQUIRED FOR PLANT GROWTH AND DEVELOPMENT. PLANTS NEED TO BALANCE THE TRADEOFF BETWEEN CO2 UPTAKE FOR PHOTOSYNTHESIS AND TRANSPIRATIONAL WATER LOSS. BY INCREASING TRANSPIRATION EFFICIENCY, VALUABLE SOIL MOISTURE CAN BE CONSERVED AND USED LATER TO DELAY DROUGHT STRESS WHEN CONDITIONS BECOME DRY. RECENT WORK INDICATES THAT CORN IS NOT OPTIMIZED FOR TRANSPIRATION EFFICIENCY, AND SUPPORTS THE FEASIBILITY OF INCREASING TRANSPIRATION EFFICIENCY WITHOUT A REDUCTION IN YIELD. FURTHERMORE, UNPRODUCTIVE TRANSPIRATION AT NIGHT MAY ALSO PROVE TO BE A TARGET FOR INCREASING PLANT WATER-USE EFFICIENCY. THE IDENTIFICATION OF LINES WITH NATURAL VARIATION IN TRANSPIRATION WOULD GIVE NEW INSIGHTS INTO WAYS OF DEVELOPING WATER-USE EFFICIENT CROPS. OUR PRELIMINARY DATA SHOWS A LINK BETWEEN THE RATIO OF NATURALLY OCCURRING STABLE CARBON ISOTOPES (13C AND 12C) IN CORN LEAF TISSUE AND WATER-USE EFFICIENCY. THUS, THE STABLE CARBON ISOTOPE RATIO OF THE LEAF HAS THE POTENTIAL TO BE USED AS A PROXY TRAIT FOR WATER-USE EFFICIENCY. THE OBJECTIVE OF THIS PROPOSAL IS TO UNDERSTAND THE MECHANISMS AND THE GENETIC UNDERPINNINGS THAT DRIVE THE RELATIONSHIP BETWEEN LEAF STABLE CARBON ISOTOPE RATIO AND WHOLE PLANT WATER-USE EFFICIENCY IN CORN. THE COMPLETION OF THE PROPOSED AIMS WILL ADVANCE OUR BASIC KNOWLEDGE OF PHOTOSYNTHESIS AND ALSO PRODUCE TRANSLATIONAL RESULTS FOR THE BREEDING AND ENGINEERING OF CROPS WITH INCREASED PHOTOSYNTHETIC AND TRANSPIRATION EFFICIENCY. THIS PROJECT ALIGNS WITH THE PROGRAM AREA PRIORITY PHYSIOLOGY OF AGRICULTURAL PLANTS IN THE AREA OF MECHANISMS OF PLANT RESPONSE TO ABIOTIC STRESSES, INCLUDING WATER USE EFFICIENCY.