Drought and salinity are two widespread environmental conditions leading to low water availability for plants. Low water availability is considered the main environmental factor limiting photosynthesis and, consequently, plant growth and yield worldwide. There has been a long‐standing controversy as to whether drought and salt stresses mainly limit photosynthesis through diffusive resistances or by metabolic impairment. Reviewing in vitro and in vivo measurements, it is concluded that salt and drought stress predominantly affect diffusion of CO₂ in the leaves through a decrease of stomatal and mesophyll conductances, but not the biochemical capacity to assimilate CO₂, at mild to rather severe stress levels. The general failure of metabolism observed at more severe stress suggests the occurrence of secondary oxidative stresses, particularly under high‐light conditions. Estimates of photosynthetic limitations based on the photosynthetic response to intercellular CO₂ may lead to artefactual conclusions, even if patchy stomatal closure and the relative increase of cuticular conductance are taken into account, as decreasing mesophyll conductance can cause the CO₂ concentration in chloroplasts of stressed leaves to be considerably lower than the intercellular CO₂ concentration. Measurements based on the photosynthetic response to chloroplast CO₂ often confirm that the photosynthetic capacity is preserved but photosynthesis is limited by diffusive resistances in drought and salt‐stressed leaves.

Photosynthetic responses to carbon dioxide concentration can provide data on a number of important parameters related to leaf physiology. Methods for fitting a model to such data are briefly described. The method will fit the following parameters: V_cmax, J, TPU, R_d and g_m[maximum carboxylation rate allowed by ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco), rate of photosynthetic electron transport (based on NADPH requirement), triose phosphate use, day respiration and mesophyll conductance, respectively]. The method requires at least five data pairs of net CO₂ assimilation (A) and [CO₂] in the intercellular airspaces of the leaf (C_i) and requires users to indicate the presumed limiting factor. The output is (1) calculated CO₂ partial pressure at the sites of carboxylation, C_c, (2) values for the five parameters at the measurement temperature and (3) values adjusted to 25 °C to facilitate comparisons. Fitting this model is a way of exploring leaf level photosynthesis. However, interpreting leaf level photosynthesis in terms of underlying biochemistry and biophysics is subject to assumptions that hold to a greater or lesser degree, a major assumption being that all parts of the leaf are behaving in the same way at each instant.

The conductance for CO(2) diffusion in the mesophyll of leaves can limit photosynthesis. We have studied two methods for determining the mesophyll conductance to CO(2) diffusion in leaves. We generated an ideal set of photosynthesis rates over a range of partial pressures of CO(2) in the stroma and studied the effect of altering the mesophyll diffusion conductance on the measured response of photosynthesis to intercellular CO(2) partial pressure. We used the ideal data set to test the sensitivity of the two methods to small errors in the parameters used to determine mesophyll conductance. The two methods were also used to determine mesophyll conductance of several leaves using measured rather than ideal data sets. It is concluded that both methods can be used to determine mesophyll conductance and each method has particular strengths. We believe both methods will prove useful in the future.

Some, but not all, plants emit isoprene. Emission of the related monoterpenes is more universal among plants, but the amount of isoprene emitted from plants dominates the biosphere–atmosphere hydrocarbon exchange. The emission of isoprene from plants affects atmospheric chemistry. Isoprene reacts very rapidly with hydroxyl radicals in the atmosphere making hydroperoxides that can enhance ozone formation. Aerosol formation in the atmosphere may also be influenced by biogenic isoprene. Plants that emit isoprene are better able to tolerate sunlight-induced rapid heating of leaves (heat flecks). They also tolerate ozone and other reactive oxygen species better than non-emitting plants. Expression of the isoprene synthase gene can account for control of isoprene emission capacity as leaves expand. The emission capacity of fully expanded leaves varies through the season but the biochemical control of capacity of mature leaves appears to be at several different points in isoprene metabolism. The capacity for isoprene emission evolved many times in plants, probably as a mechanism for coping with heat flecks. It also confers tolerance of reactive oxygen species. It is an example of isoprenoids enhancing membrane function, although the mechanism is likely to be different from that of sterols. Understanding the regulation of isoprene emission is advancing rapidly now that the pathway that provides the substrate is known.