The biochemical basis of heat/drought tolerance was investigated by comparing the response of antisense and sense transgenic soybean plants (containing the l-Δ¹-pyrroline-5-carboxylate reductase gene) with non-transgenic wild-type plants. The plants were subjected to a simultaneous drought and heat stress of 2 days, whereafter they were rewatered at 25°C. During this time the sense plants only showed mild symptoms of stress compared to the antisense plants which were severely stressed. Upon stress, nicotinamide adenine dinucleotide phosphate (NADP⁺) levels decreased in antisense while it increased in sense plants. Recovery with respect to NADP⁺ levels was best in sense plants. Sense plants had the highest ability to accumulate proline during stress and to metabolise proline after rewatering. Analyses of the fast phase chlorophyll-a fluorescence transients showed dissociation of the oxygen-evolving complex (OEC) upon stress in all plants tested. In the sense plants, which best resisted the stress, OEC dissociation was bypassed by proline feeding electrons into photosystem 2 (PSII), maintaining an acceptable nicotinamide adenine dinucleotide hydrogen phosphate (NADPH) level, preventing further damage. Upon recovery, NADPH is consumed during oxidation of accumulated proline providing high levels of NADP⁺ to act as electron acceptor to PSII, which indirectly may ameliorate the inhibition and/or the effect of uncoupling of the OEC.