In the last decade, our understanding of the processes underlying plant response to drought, at the molecular and whole-plant levels, has rapidly progressed. Here, we review that progress. We draw attention to the perception and signalling processes (chemical and hydraulic) of water deficits. Knowledge of these processes is essential for a holistic understanding of plant resistance to stress, which is needed to improve crop management and breeding techniques. Hundreds of genes that are induced under drought have been identified. A range of tools, from gene expression patterns to the use of transgenic plants, is being used to study the specific function of these genes and their role in plant acclimation or adaptation to water deficit. However, because plant responses to stress are complex, the functions of many of the genes are still unknown. Many of the traits that explain plant adaptation to drought - such as phenology, root size and depth, hydraulic conductivity and the storage of reserves - are those associated with plant development and structure, and are constitutive rather than stress induced. But a large part of plant resistance to drought is the ability to get rid of excess radiation, a concomitant stress under natural conditions. The nature of the mechanisms responsible for leaf photoprotection, especially those related to thermal dissipation, and oxidative stress are being actively researched. The new tools that operate at molecular, plant and ecosystem levels are revolutionising our understanding of plant response to drought, and our ability to monitor it. Techniques such as genome-wide tools, proteomics, stable isotopes and thermal or fluorescence imaging may allow the genotype-phenotype gap to be bridged, which is essential for faster progress in stress biology research.

Understanding plant responses to drought — from genes to the whole plant

Drought-induced responses of photosynthesis and antioxidant metabolism in higher plants

https://www.itqb.unl.pt/~pinheiro/CP_PDFs/2002_AnnBot.pdf

How Plants Cope with Water Stress in the Field? Photosynthesis and Growth

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

This review focuses on the effects of water deficits on photosynthesis and partitioning of assimilates at the leaf level. It is now established that the rate of C02 assimilation in the leaves is depressed at moderate water deficits, mostly as a consequence of stomatal closure. In fact, depending on the species and on the nature of dehydration, carbon assimilation may diminish to values close to zero without any significant decline in mesophyll photosynthetic capacity. This remarkable resistance of the photosynthetic apparatus to water deficits became apparent after the measurement of photosynthesis at saturating C02 concentrations was made possible. Whenever light or heat stress are superimposed a decline in mesophyll photosynthesis may occur as a result of a 'down-regulation' process, which seems to vary among genotypes. A major secondary effect of dehydration on photosynthetic carbon metabolism is the change in partitioning of recently fixed carbon towards sucrose, which occurs in a number of species in parallel to the increase in starch breakdown. This increase in compounds of low molecular weight may contribute to an osmotic adjustment. Controlling mechanisms involved in this process deserve further investigation.

Effects of Water Deficits on Carbon Assimilation

Differences in maximum leaf conductance in grapevine plants growing in soils with contrasting water availabilities during mid-summer in Portugal could be accounted for by differences in the concentration of ABA in xylem sap. This conclusion is reinforced by the observation that the relationship between leaf conductance and endogenous ABA concentration can be mimicked by the application of exogenous ABA to leaves detached from irrigated plants. During the day, leaf conductance decreased after a morning peak, even when the leaves remained in a constant environment at a moderate temperature and leaf-to-air vapour pressure difference. This decline in leaf conductance was not a consequence of an increase in the xylem ABA concentration or the rate of delivery of this compound by the transpiratory stream. The afternoon depression in leaf conductance was associated with an apparent limitation in stomatal opening potential, which persisted even when detached leaves were fed with water and rehydrated. The reason for this inhibition has still to be identified.

ABA xylem concentrations determine maximum daily leaf conductance of field‐grown Vitis vinifera L. plants

Recherche des causes de la diminution des echanges gazeux l'apres-midi chez la vigne. Apres avoir verifie que cette diminution de l'activite photosynthetique avait bien lieu dans les feuilles de plantes correctement arrosees et en absence de stress dus a la temperature ou a l'humidite de l'air; les caracteristiques photosynthetiques des feuilles exposees a des densites de flux de photons fortes et moderees ont ete comparees afin de comprendre le role d'une exposition prolongee a une forte luminosite

Afternoon Depression In Photosynthesis in Grapevine Leaves—Evidence for a High Light Stress Effect

The role of water relations and abscisic acid (ABA) in the responses to drought were studied in a mediterranean forage crop, Trifolium subterraneum L. under field conditions. Soil and plant water status, leaf gas exchange parameters, and xylem sap ABA content were determined at different times during a long-term soil drying episode in irrigated and droughted plants. The diurnal time-courses of these parameters were also measured at the end of a drought period. In response to soil drying stomatal conductance (g) was reduced early to 50% that of irrigated plants before any substantial change in water potential was detected. A close logarithmic regression between photosynthesis rate (4) and g was present. For the first weeks of drought the decline in A was less pronounced than in g, thus increasing water use efficiency. Stomatal conductance during diurnal time-courses showed no consistent relationships with respect to either ABA or leaf water potential. Throughout the experimental period dependence of g on leaf water status was evident from the tight correlation (^ = 0.88, P<0.01) achieved between stomatal conductance and midday water potential, but the correlation was also high when comparing g with respect to ABA content in xylem sap (^ = 0.83, P< 0.001). However, the stomata from drought acclimated plants were apparently more sensitive to xylem ABA content. For similar xylem ABA concentrations stomatal conductance was significantly higher in irrigated than in waterstressed plants.

/pdf/the-role-of-abscisic-acid-and-water-relations-in-drought-3s0fctexky.pdf

The role of abscisic acid and water relations in drought responses of subterranean clover

We studied the effects of drought on leaf conductance (g), leaf water relations and on the concentration of abscisic acid (ABA) in the xylem sap of lupinus albus L. plants. Drought was imposed by withholding watering until predawn leaf water potential (ψ pd ) decreased below − 0.7 MPa. The abaxial stomata of waterstressed plants closed to a greater extent than the adaxial stomata and the same trend was evident after feeding exogenous ABA. Under moderate water deficits (ψ pd = − 0.32 ± 0.05 MPa) xylem ABA concentration was significantly higher in water-stressed plants (0.1 mmol m −3 ) than in the controls, and the observed depression in g could be mimicked when excised leaves of wellwatered plants were fed with exogenous ABA at the same concentration found in the xylem sap of moderately stressed plants. As soil drying progressed xylem ABA concentration increased further, reaching 0.8-0.9 mmol m −3 after 17 d without watering. However, feeding a similar concentration of exogenous ABA to leaves of well-watered plants failed to reproduce the depression in g observed in severely stressed plants. After rewatering, full recovery of g was not achieved immediately, in spite of xylem ABA concentration and ψ pd returning to pre-stress levels. Our results indicate that the increase in xylem ABA concentration is quantitatively sufficient to account for the depression in leaf conductance observed in white lupin plants subjected to moderate water deficits, but the role of xylem ABA in controlling g apparently decreases when soil drying and plant water stress becomes severe

The control of leaf conductance of white lupin by xylem ABA concentration decreases with the severity of water deficits

Changes in the photosynthetic rate (A), stomatal conductance (g), water relations, photosynthetic pigments, Rubisco and soluble sugars accumulation were studied in different aged leaves of white lupin during soil drying and following rehydration. In water-stressed plants, A and g sharply declined and recovered only partially after rewatering. The way Ci and A/gchanged with drought was strongly dependent on leaf age; only in the young leaves did A/g increase and Ci decrease. Drought induced accumulation of soluble sugars was also age dependent, decreasing as leaves aged. In response to soil drying, the contents of photosynthetic pigments, total soluble protein and Rubisco protein increased in the young leaves and were either not affected or slightly decreased in the older ones. Rehydration accentuated the losses in pigments and Rubisco in the old leaves of water-stressed plants. These results suggest that the contribution of mesophyll limitations to explain drought inhibition of photosynthesis increases with leaf age, decreasing the ability to recover after rewatering. In young leaves the tolerance of the photosynthetic apparatus to dehydration and rehydration episodes is high and it is associated with high contents of Rubisco and in soluble sugars, particularly hexoses.

Maria João Correia

Papers

ABA xylem concentrations determine maximum daily leaf conductance of field‐grown Vitis vinifera L. plants

Afternoon Depression In Photosynthesis in Grapevine Leaves—Evidence for a High Light Stress Effect

The role of abscisic acid and water relations in drought responses of subterranean clover

The control of leaf conductance of white lupin by xylem ABA concentration decreases with the severity of water deficits

Leaf age effects on photosynthetic activity and sugar accumulation in droughted and rewatered Lupinus albus plants