Water deficit (WD) decreases photosynthetic rate (A) via decreased stomatal conductance to CO₂ (g_s) and photosynthetic metabolic potential (A_pot). The relative importance of g_s and A_pot, and how they are affected by WD, are reviewed with respect to light intensity and to experimental approaches. With progressive WD, A decreases as g_s falls. Under low light during growth and WD, A is stimulated by elevated CO₂, showing that metabolism (A_pot) is not impaired, but at high light A is not stimulated, showing inhibition. At a given intercellular CO₂ concentration (C_i) A decreases, showing impaired metabolism (A_pot). The C_i and probably chloroplast CO₂ concentration (C_c), decreases and then increases, together with the equilibrium CO₂ concentration, with greater WD. Estimation of C_c and internal (mesophyll) conductance (g_i) is considered uncertain. Photosystem activity is unaffected until very severe WD, maintaining electron (e⁻) transport (ET) and reductant content. Low A, together with photorespiration (PR), which is maintained or decreased, provides a smaller sink for e⁻_, causing over-energization of energy transduction. Despite increased non-photochemical quenching (NPQ), excess energy and e⁻ result in generation of reactive oxygen species (ROS). Evidence is considered that ROS damages ATP synthase so that ATP content decreases progressively with WD. Decreased ATP limits RuBP production by the Calvin cycle and thus A_pot. Rubisco activity is unlikely to determine A_pot. Sucrose synthesis is limited by lack of substrate and impaired enzyme regulation. With WD, PR decreases relative to light respiration (R_L), and mitochondria consume reductant and synthesise ATP. With progressing WD at low A, R_L increases C_i and C_c. This review emphasises the effects of light intensity, considers techniques, and develops a qualitative model of photosynthetic metabolism under WD that explains many observations: testable hypotheses are suggested.

Photosynthetic responses of sunflower plants grown for 52 d in ambient and elevated CO₂ (A=350 or E=700 µmol mol^–1, respectively) and subjected to no (control), mild or severe water deficits after 45 d were analysed to determine if E modifies responses to water deficiency. Relative water content, leaf water potential (Ψ_w) and osmotic potential decreased with water deficiency, but there were no effects of E. Growth in E decreased stomatal conductance (g_s) and thereby transpiration, but increased net CO₂ assimilation rate (P_n, short‐term measurements); therefore, water‐use efficiency increased by 230% (control plants) and 380% (severe stress). Growth in E did not affect the response of P_n to intercellular CO₂ concentration, despite a reduction of 25% in Rubisco content, because this was compensated by a 32% increase in Rubisco activity. Analysis of chlorophyll a fluorescence showed that changes in energy metabolism associated with E were small, despite the decreased Rubisco content. Water deficits decreased g_s and P_n: metabolic limitation was greater than stomatal at mild and severe deficit and was not overcome by elevated CO₂. The decrease in P_n with water deficiency was related to lower Rubisco activity rather than to ATP and RuBP contents. Thus, there were no important interactions between CO₂ during growth and water deficit with respect to photosynthetic metabolism. Elevated CO₂ will benefit sunflower growing under water deficit by marginally increasing P_n, and by slowing transpiration, which will decrease the rate and severity of water deficits, with limited effects on metabolism.

In many plant species that remain leafless part of the year, CO₂ fixation occurring in green stems represents an important carbon gain. Traditionally, a distinction has been made between stem photosynthesis and corticular photosynthesis. All stem photosynthesis is, sensu stricto, cortical, since it is carried out largely by the stem cortex. We proposed the following nomenclature: stem net photosynthesis (SNP), which includes net CO₂ fixation by stems with stomata in the epidermis and net corticular CO₂ fixation in suberized stems, and stem recycling photosynthesis (SRP), which defines CO₂ ling in suberized stems. The proposed terms should reflect differences in anatomical and physiological traits. SNP takes place in the chlorenchyma below the epidermis with stomata, where the net CO₂ uptake occurs, and it resembles leaf photosynthesis in many characteristics. SRP is found in species where the chlorenchyma is beneath a well-developed stomata-free periderm and where reassimilation of internally respired CO₂ occurs. SNP is common in plants from desert ecosystems, rates reaching up to 60% of the leaf photosynthetic rate. SRP has been demonstrated in trees from temperate forests and it offsets partially a carbon loss by respiration of stem nonphotosynthetic tissues. Reassimilation can vary between 7 and 123% of respired CO₂, the latter figure implying net CO₂ uptake from the atmosphere. Both types of stem photosynthesis contribute positively to the carbon economy of the species, in which they occur; they are advantageous to the plant because they allow the maintenance of physiological activity during stress, an increase of integrated water use efficiency, and they provide the carbon source used in the production of new organs.

Water relations and photosynthetic characteristics of plants of Lycium nodosum grown under increasing water deficit (WD), saline spray (SS) or saline irrigation (SI) were studied. Plants of this perennial, deciduous shrub growing in the coastal thorn scrubs of Venezuela show succulent leaves which persist for approx. 1 month after the beginning of the dry season; leaf succulence is higher in populations closer to the sea. These observations suggested that L. nodosum is tolerant both to WD and salinity. In the glasshouse, WD caused a marked decrease in the xylem water potential (ψ), leaf osmotic potential (ψ_s) and relative water content (RWC) after 21 d; additionally, photosynthetic rate (A), carboxylation efficiency (CE) and stomatal conductance (g_s) decreased by more than 90 %. In contrast, in plants treated for 21 d with a foliar spray with 35 ‰ NaCl or irrigation with a 10 % NaCl solution, ψ and RWC remained nearly constant, while ψ_s decreased by 30 %, and A, CE and g_s decreased by more than 80 %. An osmotic adjustment of 0·60 (SS) and 0·94 MPa (SI) was measured. Relative stomatal and mesophyll limitations to A increased with both WD and SS, but were not determined for SI‐treated plants. No evidence of chronic photoinhibition due to any treatment was observed, since maximum quantum yield of PSII, F_v/F_m, did not change with either drought in the field or water or salinity stress in the glasshouse. Nevertheless, WD and SI treatments caused a decrease in the photochemical (q_P) and an increase in the non‐photochemical (q_N) quenching coefficients relative to controls; q_N was unaffected by the SS treatment. The occurrence of co‐limitation of A by stomatal and non‐stomatal factors in plants of L. nodosum may be associated with the extended leaf duration under water or saline stress. Additionally, osmotic adjustment may partly explain the relative maintenance of A and g_s in the SS and SI treatments and the tolerance to salinity of plants of this species in coastal habitats.