Few J papers that may be relevant  (leaf water potential and leaf photosynthesis)

Lampinen et al. (2004), reported that stress caused a clear reduction in stem water potential throughout the daylight period, as well as reductions in leaf conductance and photosynthesis, and all of these reductions were more severe with increasing water stress. Jarvis (1976) found that the stomatal conductance of illuminated leaves is a function of current levels of temperature, vapour pressure deficit, leaf water potential (really turgor pressure) and ambient CO$_2$ concentration. Paper by Koch et al. (1994) measured diurnal patterns of leaf conductance, net photosynthesis and water potential of five tree species at the top of the canopy in a tropical lowland rain forest in southwestern Cameroon. Photosynthesis was found negatively related to leaf temperature above midday air temperature maxima. These patterns were similar to those observed in other seasonally droughted evergreen communities, such as Mediterranean-climate shrubs, and indicate that environmental factors may cause stomatal closure and limit photosynthesis in tropical rain forests during the midday period. Ronquim et al. (2009) compared the growth and leaf nutrient content of young potted plants of Copaifera langsdorffii in sunny and shaded areas without water stress. Carbon assimilation and leaf water relations were evaluated by net photosynthesis, potential photochemical efficiency and leaf water potential during daily courses in dry and rainy periods under natural conditions in both contrasting irradiances.

References

B. D. LAMPINEN, K. A. SHACKEL, S. M. SOUTHWICK,W. H. OLSON and T. M. DEJONG. 2004. Leaf and canopy level photosynthetic responses of French prune (Prunus domestica L. ‘French’) to stem water potential based deficit.Irrigation. Journal of Horticultural Science & Biotechnology. 79 (4) 638–644.http://ucanr.org/sites/LampinenLab/files/80439.pdf

P.G. Jarvis. 1976.The Interpretation of the Variations in Leaf Water Potential and Stomatal Conductance Found in Canopies in the Field. Phil. Trans. R. Soc. Lond. B. 273 (927):  593-610.http://rstb.royalsocietypublishing.org/content/273/927/593

GEORGE W. KOCH,’ JEFFREY S. AMTHOR and MICHAEL L. GOULDEN. 1994. Diurnal patterns of leaf  photosynthesis, conductance and water potential at the top of a lowland rain forest canopy in Cameroon: measurements from the Radeau des Cimes. Tree Physiology. 14: 347-360. http://treephys.oxfordjournals.org/content/14/4/347.full.pdf

Carlos Cesar Ronquim; Carlos Henrique B. A. Prado; João Paulo de Souza. 2009. Growth, photosynthesis and leaf water potential in young plants of Copaifera langsdorffii Desf. (Caesalpiniaceae) under contrasting irradiances.Brazilian Journal of Plant Physiology. 21(3): 197-208.http://www.scielo.br/pdf/bjpp/v21n3/v21n3a04.pdf

Thanks Dr Kibria

Dear Adbul,

You can also find information in the following references, I hope this is useful:

Chaves MM. 2002. How Plants Cope with Water Stress in the Field? Photosynthesis and Growth. Annals of Botany 89, 907–916.

Chaves MM, Maroco JP, Pereira JS. 2003. Understanding plant responses to drought — from genes to the whole plant. Functional Plant Biology 30, 239.

Chaves MM. 1991. Effects of water deficits on carbon assimilation. Journal of Experimental Botany 42, 1-46.

Lawlor DW. 2002. Limitation to Photosynthesis in Water-stressed Leaves: Stomata vs. Metabolism and the Role of ATP. Annals of Botany 89, 871–885.

Lawlor DW, Cornic G. 2002. Photosynthetic carbon assimilation and associated metabolism in relation to water deficits in higher plants. Plant, Cell and Environment 25, 275–294.

The issue is more complicated. For unstressed plants lower water potentials mean higher stomatal conductances and therefore higher photosynthesis. (I do, however, not know a reference for it). However, all the references in Gemmas post are also true. 

This is true. My references are about water stressed conditions...I already assumed you were interested under this scenario

Best regards

Thanks Drs Molero and Berninger

Maybe this paper could provide some  informataion.

Niu S, Li Z, Xia J, Han Y, Wu M, Wan S. Climatic warming changes plant photosynthesis and its temperature dependence in a temperate steppe of northern China [J]. Environmental and Experimental Botany, 2008, 63(1-3): 91-101.

Thanks Dr Eidiani