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

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

Photosynthesis is one of the key processes to be affected by water deficits, via decreased CO2 diffusion to the chloroplast and metabolic constraints. The relative impact of those limitations varies with the intensity of the stress, the occurrence (or not) of superimposed stresses, and the species we are dealing with. Total plant carbon uptake is further reduced due to the concomitant or even earlier inhibition of growth. Leaf carbohydrate status, altered directly by water deficits or indirectly (via decreased growth), acts as a metabolic signal although its role is not totally clear. Other relevant signals acting under water deficits comprise: abscisic acid (ABA), with an impact on stomatal aperture and the regulation at the transcription level of a large number of genes related to plant stress response; other hormones that act either concurrently (brassinosteroids, jasmonates, and salycilic acid) or antagonistically (auxin, cytokinin, or ethylene) with ABA; and redox control of the energy balance of photosynthetic cells deprived of CO2 by stomatal closure. In an attempt to systematize current knowledge on the complex network of interactions and regulation of photosynthesis in plants subjected to water deficits, a meta-analysis has been performed covering >450 papers published in the last 15 years. This analysis shows the interplay of sugars, reactive oxygen species (ROS), and hormones with photosynthetic responses to drought, involving many metabolic events. However, more significantly it highlights (i) how fragmented and often non-comparable the results are and (ii) how hard it is to relate molecular events to plant physiological status, namely photosynthetic activity, and to stress intensity. Indeed, the same data set usually does not integrate these different levels of analysis. Considering these limitations, it was hard to find a general trend, particularly concerning molecular responses to drought, with the exception of the genes ABI1 and ABI3. These genes, irrespective of the stress type (acute versus chronic) and intensity, show a similar response to water shortage in the two plant systems analysed (Arabidopsis and barley). Both are associated with ABA-mediated metabolic responses to stress and the regulation of stomatal aperture. Under drought, ABI1 transcription is up-regulated while ABI3 is usually down-regulated. Recently ABI3 has been hypothesized to be essential for successful drought recovery.

/pdf/photosynthesis-and-drought-can-we-make-metabolic-connections-50uqjztojl.pdf

Photosynthesis and drought: can we make metabolic connections from available data?

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

* Whether photosynthesis is limited during water stress and recovery because of diffusive or biochemical factors is still open to debate, and apparent contradictions appear when various studies on species with different growth forms are compared. * Ten Mediterranean species, representing different growth forms, were subjected to different levels of water stress, the most severe followed by rewatering. A quantitative limitation analysis was applied to estimate the effects of water stress on stomatal (S(L)), mesophyll conductance (MC(L)) and biochemical limitations (B(L)). * Results confirmed a general pattern of photosynthetic response to water stress among C(3) plants when stomatal conductance (g(s)) is used as a reference parameter. As g(s) values decreased from a maximum to approx. 0.05 mol H(2)O m(-2) s(-1), the total photosynthetic limitation rose from 0 to approx. 70%, and this was caused by a progressive increase of both S(L) and MC(L) limitations, while B(L) remained negligible. When lower values of g(s) were achieved (total photosynthetic limitation increased from 70 to 100%), the contribution of S(L) declined, while MC(L) still increased and B(L) contributed significantly (20-50%) to the total limitation. * Photosynthetic recovery of severely stressed plants after rewatering showed a dominant role of MC(L), irrespective of the degree of photosynthesis recovery.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2007.02087.x

Photosynthetic limitations in response to water stress and recovery in Mediterranean plants with different growth forms

Drought is one of the major constraints limiting crop production worldwide. Crop growth models predict that this issue will be more severe in future. Drought impairs normal growth, disturbs water relations, and reduces water use efficiency in plants. Plants, however, have a variety of physiological and biochemical responses at cellular and whole organism levels, making it a more complex phenomenon. The rate of photosynthesis is reduced mainly by stomatal closure, membrane damage, and disturbed activity of various enzymes, especially those involved in ATP synthesis. Plants display a range of mechanisms to withstand drought, such as reduced water loss by increased diffusive resistance, increased water uptake with prolific and deep root systems, and smaller and succulent leaves to reduce transpirational loss. Low-molecular-weight osmolytes, including glycinebetaine, proline and other amino acids, organic acids, and polyols also play vital roles in sustaining cellular functions under drought. Plant growth substances such as salicylic acid, auxins, gibberellins, cytokinins, and abscisic acid modulate plant responses toward drought. Polyamines, citrulline, and several enzymes act as antioxidants and reduce adverse effects of water deficit. Plant drought stress can be managed by adopting strategies such as mass screening and breeding, marker-assisted selection, and exogenous application of hormones and osmoprotectants to seeds or growing plants, as well as engineering for drought resistance. Here, we provide an overview of plant drought stress, its effects on plants’ resistance mechanisms and management strategies to cope with drought stress.

Drought stress in plants: an overview

Recovery responses of photosynthesis, transpiration, and stomatal conductance in kidney bean following drought stress

Water retention, contact angle of water droplets, and amount of epicuticular wax on leaf surfaces were examined in two lines of near-isogenic barley with normal and glossy leaves. In addition, the effect of rainfall on the wettability of leaf surfaces was examined. Mean water retention of leaves from the 1st to 5th node was 5.6mg/cm2 in normal leaf lines and 12.8mg/cm2 in glossy leaf lines. The mean contact angle of water drops was 125° in normal leaf and 93° in glossy leaf. The mean epicuticular wax content was 17.3μg/cm2 in normal leaf and 9.3μg/cm2 in glossy leaf. In glossy leaves, contact angles decreased with increasing duration of rainfall treatment. After treatment for 5 days, the angle was about 70% of that without rainfall treatment. Reduction of epicuticular wax amount by rainfall exposure occurred in all leaf positions of both barley lines. The reduction rates in the normal leaf line were 10-20%. In the glossy leaf line, the reduction rates were 15% in the 1st leaves and about 50% in the 2nd-3rd leaves.

Effect of Rainfall Exposure on Leaf Wettability in Near-Isogenic Barley Lines with Different Leaf Wax Content

The present investigation was designed to examine the differences between the effects of rainfall and those of submergence. The growth response and injury of kidney bean plants were examined after the artificial rainfall (mist), root submerging and whole plant submerging treatments. Plants were exposed to mist (Precipitation: 5mm/h, Diameter of water drop: 0.24mm) or submerged in the growth chamber (20°C, 12hr light+12hr dark) for 5 days.(1) Growth response and injury immediately after the treatments:Considerable differences in growth response and injury were observed with the three treatments. In general, the inhibitory effects on growth and injury immediately after the treatments were marked on the shoot by mist, on the root by root submergence and on both the shoot and root by whole plant submergence.Mist treatment decreased the area, dry weight and chlorophyll content of the leaves, but it increased the fresh weight of the stem and elongation of the root (longest root). The greatest wilting response was observed one hour after the end of mist exposure.Root submergence decreased elongation, fresh weight and dry weight of root, but increased the dry weight of the stem. Wilting was not observed.Whole plant submergence decreased markedly elongation, fresh weight of the leaf and root, dry weight of the shoot and root, and chlorophyll content of the leaf. Wilting was observed, but its degree was weaker than that of mist.(2) Growth response and injury 12 days after the treatments:Mist treatment caused growth inhibition, but the degree of inhibition was weaker than that immediately after the treatments. The aftereffect of mist remained on fresh weight of leaf and dry weight of shoot and root to some extent. Foliage leaves that opened after mist treatment were injured strikingly.Root submergence decreased root length, leaf area, fresh weight (leaf, stem) and dry weight (shoot, root) to some extent, but foliage leaves were not injured.Whole plant submergence decreased elongation, fresh weight and dry weight of shoot and root to a great extent. Foliage leaves were injured, but less severely than by mist.These results suggest that the exposure to rainfall not only causes weak effects of submergence, but also has a different action on the plant.

https://www.jstage.jst.go.jp/article/agrmet1943/38/4/38_4_371/_pdf

Studies on Plant Response to Rainfall.

植物体内成分に対する雨濡れの影響を明らかにするため, 降雨日を想定した弱い日射条件 (61W/m2) で, 5日間の降雨処理によるインゲンマメ植物の生体重, 乾物重および可溶性糖, デンプン, 窒素含量の変化について検討した。1) 降雨処理により植物体の乾物重, 可溶性糖, デンプン, 窒素含量は減少した。降雨処理を受けた植物の乾物重は無処理植物の78%, 可溶性糖含量は72%, デンプン含量は57%, 窒素含量は83%の値に減少した。乾物重, 可溶性糖, デンプン含量の低下は地上部 (葉, 茎) において根よりも顕著であった。窒素含量の低下の程度は少ないが, 根が葉, 茎よりも減少した。2) 5日間の降雨処理期間中の可溶性糖, デンプン, 窒素含量の増加率は無処理植物よりも少なくなったが, 特に茎のデンプン含量は減少が著しく, 降雨処理開始時の約1/2の値になった。3) 本実験の結果から, 植物体の雨濡れは糖類の含量に著しく影響を与え, 特に葉, 茎のデンプン含量の低下をもたらすことが明らかになった。本研究の経費の一部は文部省科学研究費補助金 (一般研究C 課題番号63560249) によった。

Kazuyoshi Kimura

Papers

Recovery responses of photosynthesis, transpiration, and stomatal conductance in kidney bean following drought stress

Effect of Rainfall Exposure on Leaf Wettability in Near-Isogenic Barley Lines with Different Leaf Wax Content

Studies on Plant Response to Rainfall.

Changes of Sugar and Nitrogen Contents in the Bean Plants by Rainfall Exposure

Studies on plant response to rainfall. (IV). Effect of rainfall on leaf expansion