assimilation is determined by measuring the rate at which the leaf assimilates . The change in is calculated as flowing into leaf chamber, in μmol mol−1 , minus flowing out from leaf chamber, in μmol mol−1. The photosynthetic rate (Rate of exchange in the leaf chamber) is the difference in concentration through chamber, adjusted for the molar flow of air per m2 of leaf area, mol m−2 s−1. The change in H2O vapour pressure is water vapour pressure out of leaf chamber, in mbar, minus the water vapour pressure into leaf chamber, in mbar. Transpiration rate is differential water vapour concentration, mbar, multiplied by the flow of air into leaf chamber per square meter of leaf area, mol s−1 m−2, divided by atmospheric pressure, in mBar. Calculations used in 'closed system' systems; A leaf is placed in the leaf-chamber, with a known area of leaf enclosed. Once the chamber is closed, carbon dioxide concentration gradually declines. When the concentration decreases past a certain point a timer is started, and is stopped as the concentration passes at a second point. The difference between these concentrations gives the change in carbon dioxide in ppm. Net photosynthetic rate in micro grams carbon dioxide s−1 is given by; (V • p • 0.5 • FSD • 99.7) / t where V = the chamber volume in liters, p = the density of carbon dioxide in mg cm−3, FSD = the carbon dioxide concentration in ppm corresponding to the change in carbon dioxide in the chamber, t = the time in seconds for the concentration to decrease by the set amount. Net photosynthesis per unit leaf area is derived by dividing net photosynthetic rate by the leaf area enclosed by the chamber. Applications Since photosynthesis, transpiration and stomatal conductance are an integral part of basic plant physiology, estimates of these parameters can be used to investigate numerous aspects of plant biology. The plant-scientific community has generally accepted photosynthetic systems as reliable and accurate tools to assist research. There are numerous peer-reviewed articles in scientific journals which have used a photosynthetic system. To illustrate the utility and diversity of applications of photosynthetic systems, below you will find brief descriptions of research using photosynthetic systems; Researchers from the Technion - Israel Institute of Technology and a number of US institutions studied the combined effects of drought and heat stress on Arabidopsis thaliana. Their research suggests that the