Scarcity of water is a severe environmental constraint to plant productivity. Drought-induced loss in crop yield probably exceeds losses from all other causes, since both the severity and duration of the stress are critical. Here, we have reviewed the effects of drought stress on the growth, phenology, water and nutrient relations, photosynthesis, assimilate partitioning, and respiration in plants. This article also describes the mechanism of drought resistance in plants on a morphological, physiological and molecular basis. Various management strategies have been proposed to cope with drought stress. Drought stress reduces leaf size, stem extension and root proliferation, disturbs plant water relations and reduces water-use efficiency. Plants display a variety of physiological and biochemical responses at cellular and whole-organism levels towards prevailing drought stress, thus making it a complex phenomenon. CO2 assimilation by leaves is reduced mainly by stomatal closure, membrane damage and disturbed activity of various enzymes, especially those of CO2 fixation and adenosine triphosphate synthesis. Enhanced metabolite flux through the photorespiratory pathway increases the oxidative load on the tissues as both processes generate reactive oxygen species. Injury caused by reactive oxygen species to biological macromolecules under drought stress is among the major deterrents to growth. Plants display a range of mechanisms to withstand drought stress. The major mechanisms include curtailed water loss by increased diffusive resistance, enhanced water uptake with prolific and deep root systems and its efficient use, and smaller and succulent leaves to reduce the transpirational loss. Among the nutrients, potassium ions help in osmotic adjustment; silicon increases root endodermal silicification and improves the cell water balance. Low-molecular-weight osmolytes, including glycinebetaine, proline and other amino acids, organic acids, and polyols, are crucial to sustain cellular functions under drought. Plant growth substances such as salicylic acid, auxins, gibberrellins, cytokinin and abscisic acid modulate the plant responses towards drought. Polyamines, citrulline and several enzymes act as antioxidants and reduce the adverse effects of water deficit. At molecular levels several drought-responsive genes and transcription factors have been identified, such as the dehydration-responsive element-binding gene, aquaporin, late embryogenesis abundant proteins and dehydrins. Plant drought tolerance can be managed by adopting strategies such as mass screening and breeding, marker-assisted selection and exogenous application of hormones and osmoprotectants to seed or growing plants, as well as engineering for drought resistance.

https://hal.archives-ouvertes.fr/hal-00886451/document

Plant drought stress: effects, mechanisms and management

http://nhjy.hzau.edu.cn/kech/ssyy/pdf/drought/4.PDF

Photosynthesis under drought and salt stress: regulation mechanisms from whole plant to cell

Cold, salinity and drought stresses: an overview.

Stomata, the small pores on the surfaces of leaves and stalks, regulate the flow of gases in and out of leaves and thus plants as a whole. They adapt to local and global changes on all timescales from minutes to millennia. Recent data from diverse fields are establishing their central importance to plant physiology, evolution and global ecology. Stomatal morphology, distribution and behaviour respond to a spectrum of signals, from intracellular signalling to global climatic change. Such concerted adaptation results from a web of control systems, reminiscent of a 'scale-free' network, whose untangling requires integrated approaches beyond those currently used.

http://www.bio.bristol.ac.uk/research/HetheringtonLab/pdfs/nature01843.pdf

The role of stomata in sensing and driving environmental change.

Stressful environments such as salinity, drought, and high temperature (heat) cause alterations in a wide range of physiological, biochemical, and molecular processes in plants. Photosynthesis, the most fundamental and intricate physiological process in all green plants, is also severely affected in all its phases by such stresses. Since the mechanism of photosynthesis involves various components, including photosynthetic pigments and photosystems, the electron transport system, and CO2 reduction pathways, any damage at any level caused by a stress may reduce the overall photosynthetic capacity of a green plant. Details of the stress-induced damage and adverse effects on different types of pigments, photosystems, components of electron transport system, alterations in the activities of enzymes involved in the mechanism of photosynthesis, and changes in various gas exchange characteristics, particularly of agricultural plants, are considered in this review. In addition, we discussed also progress made during the last two decades in producing transgenic lines of different C3 crops with enhanced photosynthetic performance, which was reached by either the overexpression of C3 enzymes or transcription factors or the incorporation of genes encoding C4 enzymes into C3 plants. We also discussed critically a current, worldwide effort to identify signaling components, such as transcription factors and protein kinases, particularly mitogen-activated protein kinases (MAPKs) involved in stress adaptation in agricultural plants.

Photosynthesis under stressful environments: An overview

Regulation of photosynthesis of C3 plants in response to progressive drought: stomatal conductance as a reference parameter.

Drought and salinity (i.e. soil water stress) are the main environmental factors limiting photosynthesis and respiration and, consequently, plant growth. This review summarizes the current status of knowledge on photosynthesis and respiration under water stress. It is shown that diffusion limitations to photosynthesis under most water stress conditions are predominant, involving decreased mesophyll conductance to CO 2 , an important but often neglected process. A general failure of photochemistry and biochemistry, by contrast, can occur only when daily maximum stomatal conductance (g s ) drops below 0.05-0.10 mol H 2 O m -2 s -1 . Because these changes are preceded by increased leaf antioxidant activities (g s below 0.15-0.20 mol H 2 O m -2 s -1 ), it is suggested that metabolic responses to severe drought occur indirectly as a consequence of oxidative stress, rather than as a direct response to water shortage. As for respiration, it is remarkable that the electron partitioning towards the alternative respiration pathway sharply increases at the same g s threshold, although total respiration rates are less affected. Despite the considerable improvement in the understanding of plant responses to drought, several gaps of knowledge are highlighted which should become research priorities for the near future. These include how respiration and photosynthesis interact at severe stress, what are the boundaries and mechanisms of photosynthetic acclimation to water stress and what are the factors leading to different rates of recovery after a stress period.

Keeping a positive carbon balance under adverse conditions: responses of photosynthesis and respiration to water stress

The effect of diffusional and photochemical limitations to photosynthesis was assessed in field-grown water-stressed grapevines (Vitis vinifera L.) by combined measurements of gas exchange and chlorophyll fluorescence. Drought was slowly induced, and the progressive decline of photosynthesis was examined in different grapevine cultivars along a continuous gradient of maximum mid-morning values of stomatal conductance (g), which were used as an integrative indicator of the water-stress conditions endured by the leaves. Initial decreases of g were accompanied by decreases of substomatal CO2 concentration (Ci), the estimated chloroplastic CO2 concentration (Cc) and net photosynthesis (AN), while electron transport rate (ETR) remained unaffected. With increasing drought, g, AN, Ci and Cc further decreased, accompanied by slight decreases of ETR and of the estimated mesophyll conductance (gmes). Severe drought led to strong reductions of both g and gmes, as well as of ETR. The apparent carboxylation efficiency and the compensation point for CO2 remained unchanged under severe drought when analysed on a Cc, rather than a Ci, basis, suggesting that previously reported metabolic impairment was probably due to decreased gmes.

Effects of drought on photosynthesis in grapevines under field conditions: an evaluation of stomatal and mesophyll limitations.

Summary • Whether decreases in Rubisco activity and the availability of ribulose-1,5bisphosphate (RuBP) regeneration are responsible for drought-induced depression of photosynthesis is under debate. • Here, leaf water potential and relative water content, gas exchange, chlorophyll fluorescence, initial and total Rubisco activity and RuBP content were determined during the time course of drought development in five C 3 species: Rhamnus alaternus , Rhamnus ludovici-salvatoris , Nicotiana sylvestris , Phaseolus vulgaris and Vitis vinifera . Water was withheld until photosynthesis approached zero (between 6 and 12 d depending on the species). • Relative water content and water potential progressively dropped with drought in Rhamnus and Vitis , but not in the other two species. While RuBP content and Rubisco activity remained constant, declining eventually only in the more severe drought situations, light-saturated stomatal conductance ( g s ) and photosynthesis ( A N ) decreased progressively during drought in all species. This strongly suggests a dominant role of decreased g s in photosynthesis downregulation during drought in these species, which is supported by increased electron transport to A N ratio. • It is concluded that impairment of Rubisco activity and RuBP content do not limit photosynthesis until drought is very severe. Moreover, the relative water content at which these mechanisms are impaired is strongly species-dependent.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2004.01056.x

Is photosynthesis limited by decreased Rubisco activity and RuBP content under progressive water stress

*† These authors contributed equally to this work. Summary Leaf mesophyll conductance to CO 2 (gm) has been recognized to be finite and variable, rapidly adapting to environmental conditions. The physiological basis for fast changes in g m is poorly understood, but current reports suggest the involvement of protein-facilitated CO 2 diffusion across cell membranes. A good candidate for this could be the Nicotiana tabacum L. aquaporin NtAQP1, which was shown to increase membrane permeability to CO 2 in Xenopus oocytes. The objective of the present work was to evaluate its effect on the in vivo mesophyll conductance to CO 2, using plants either deficient in or overexpressing NtAQP1. Antisense plants deficient in NtAQP1 (AS) and NtAQP1 overexpressing tobacco plants (O) were compared with their respective wild-type (WT) genotypes (CAS and CO). Plants grown under optimum conditions showed different photosynthetic rates at saturating light, with a decrease of 13% in AS and an increase of 20% in O, compared with their respective controls. CO 2 response curves of photosynthesis also showed significant differences among genotypes. However, in vitro analysis demonstrated that these differences could not be attributed to alterations in Rubisco activity or ribulose-1,5-bisphosphate content. Analyses of chlorophyll fluorescence and on-line 13 C discrimination indicated that the observed differences in net photosynthesis (AN) among genotypes were due to different leaf mesophyll conductances to CO2, which was estimated to be 30% lower in AS and 20% higher in O compared with their respective WT. These results provide evidence for the in vivo involvement of aquaporin NtAQP1 in mesophyll conductance to CO 2.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-313X.2006.02879.x

Josefina Bota

Papers

Regulation of photosynthesis of C3 plants in response to progressive drought: stomatal conductance as a reference parameter.

Keeping a positive carbon balance under adverse conditions: responses of photosynthesis and respiration to water stress

Effects of drought on photosynthesis in grapevines under field conditions: an evaluation of stomatal and mesophyll limitations.

Is photosynthesis limited by decreased Rubisco activity and RuBP content under progressive water stress

Tobacco aquaporin NtAQP1 is involved in mesophyll conductance to CO2 in vivo