Drought tolerance can mean many things and many plant processes can be affected by drought, which can also mean many things. So it is not possible to use one measure to describe a complex situation. The question is too simplified. I have dealt with the problem in different publications listed in Research Gate and suggest  reading  them and the literature given there.  Chlorophyll fluorescence is a valuable technique but only under particular conditions, such as rapid and relatively severe dehydration but may not be useful under mild drought which affects growth processes and yield without altering photosynthetic metabolism greatly and so does not alter chlorophyll fluorescence. Chlorophyll content /concentrations in leaves are not a useful indicator of early drought effects unless the water deficit is rapid and large. If a specific list of references is required I can supply later. 

Hi Arun,

You forgot chlorophyll fluorescence. It is the most sensitive and fastest responding variable for drought monitoring. Fluorescence is directly related to photosynthetic electron transport. Hence, when water as an electron donor is in shortage, fluorescence will increase immediately.

Hav a look at:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1062669/

http://jxb.oxfordjournals.org/content/51/345/659.full

Success,

Frank

Hello. First of all, it depends whether the plant is homoiochlorophyllous or not: http://link.springer.com/article/10.1023%2FA%3A1005951908229.

Regards

Hi Renata,

If I am not wrong Arun has used the word drought and not dessication, didn't he? Makes a hell of a difference no?

Frank

Yes , it is the SPAD meter reading which is the relative chlorophyll content of the leaf which is related to drought tolerance, but may not necessarily related to yield under drought condition. The measurements are fastest under field in large population.

Hi,

I agree with Frank Veroustraete that the chlorophyll fluorescence is the most sensitive variable. On the other hand with SPAD meter can have fast mesurments of total chlorophyll content, which may be correlated to drought. Chlorophyll content (or ratio) changes are struclular thus they take more time to be induced. Changes in chloropyll fluoroscence are  functional that's why more sensitive.

The relationship between carbon assimilation measurements made by photosynthesis systems of the dark Calvin cycle, and measurements of variable fluorescence of photosystem II (PSII), made by chlorophyll fluorometers of the light reaction, are not always straightforward. For this reason, choosing the correct chlorophyll fluorescence protocol can also be different for C3 and C4 plants. It has been found, for example, that Y(II) and ETR are good tests for drought stress in C4 plants, but a special assay is required to measure drought stress in most C3 plants at usable levels In C3 plants, photorespiration, and the Mehler reaction, are thought to be a principal cause. (Flexas 2000). For more information, refer the Opti-Sciences plant stress guide.

There are volumes of research papers available for measuring most types of plant stress using chlorophyll fluorometers, and the various protocols available.[9]

When choosing the correct protocol, and measuring parameter, for a specific type of plant stress, it is important to understand the limitations of the instrument, and the protocol used. For example, it was found that when measuring Oak leaves, a photosynthesis system could detect heat stress at 30 °C and above, Y(II) could detect heat stress at 35 °C and above, NPQ could detect heat stress at 35 °C and above, and Fv/Fm could only detect heat stress at 45 °C and above. (Haldiman P, & Feller U. 2004) OJIP was found to detect heat stress at 44 °C and above on samples tested. (Strasser 2004).

This trait may be an important criteria for stay green which indirectly help in drought tolerance.

Chlorophyll fluorescence parameter, especially,  Phi PS II (light exposed quantum yield of PSII) is a  relevant parameter for the assessment of drought tolerant potential in many crops. Pulse amplitude modulated fluorescence (PAM) system is widely used for this measurement. Chlorophyll content/index  meter is a rapid system just to know the chlorophyll content or rate of photobleaching. Drought is little bit complex phenomena setting in field when cocomittant occurence of soil water deficit, high solar light, high temperature  and atmospheric VPD increase. Chlorophyll bleaching may  just occur due to high temperature or high light condition alone also, therefore, chlorophyll index  may not be considered for assessing the drought tolerence potential of any crop.

Annamalai Nathan

yes, i do agree with Dr Nathan

As it has been sugested, chlorophyll fluorescence is the fastest way to measure drought. I would say that chlorophyll content (or ratio) monitoring is usually made in drought stress assays, to confirm whether structural changes may occur as a drought stress response.

As suggested by the previous researcher, chlorophyll fluorescence is the best method to study dought tolerance of a plant

If you want measuring chlorophyll a, b, total chlorophyll and a/b ratio, it is so easy. You can use Aceton 80% for extract chlorophyll from leaf and then using spectrophotometer according to Arnon (1949). But if you have lots of genotypes in the field and you want a rapid instrument, I suggest you using SPAD minolta- 502. It will give you the greenness of leaf and it is correlated with total chlorophyll.

Yes. I Agree with Dr SANAZ with respect to field studies dealing large populations

Drought tolerance can mean many things and many plant processes can be affected by drought, which can also mean many things. So it is not possible to use one measure to describe a complex situation. The question is too simplified. I have dealt with the problem in different publications listed in Research Gate and suggest  reading  them and the literature given there.  Chlorophyll fluorescence is a valuable technique but only under particular conditions, such as rapid and relatively severe dehydration but may not be useful under mild drought which affects growth processes and yield without altering photosynthetic metabolism greatly and so does not alter chlorophyll fluorescence. Chlorophyll content /concentrations in leaves are not a useful indicator of early drought effects unless the water deficit is rapid and large. If a specific list of references is required I can supply later. 

When I used chlorophyll fluorescence in a drought experiment to determine differences between treatments, to our surprise we did not find significant differences, although measurements with a Minolta SPAD meter showed clear differences.

I concluded that the drought treatment was not severe enough (i.e. the plants did not develop significant water deficit stress or adapted rapidly to the treatment).

So chlorophyll fluorescence is not always a reliable measurement.

Abraham Blum has a website with methodologies that can be used for drought and other abiotic stresses:

http://www.plantstress.com/methods/index.asp

Might be worth a look!

Dr. Wouter L. Ballizany

Yes. Based on field studies with large number of sorghum genotypes, we found SPAD meter reading is the best for assessing phenotypic difference governing drought tolerance in dryaland conditions of SAT climate 

Hi there Arun Prabhu,

Fluorescence has been mentioned by my predecessors. And I agree, it is a variable which is very sensitive toi drought in fact to any environmental impact hitting on the electron transport of the photosynthetic light reactions.

But lets not  forget the thermal band information content. With thermal information one can estimate evapotranspiration at the same time scale as fluorescence and at field or plot scale. At that level of airborne measurements fluorescence is being developed as we speak. The methodology for evapotranspiration estimation is more common practice right now. This may change pretty fast by the new algorithms developed to estimate fluorescence from airborne measurements. Evapotranspiration is a variable directly linked to plant water status. Electron transport however is slightly less directly linked to the water household of plants, especially when you want to stratify species resistances to drought. In any case drought resistance according to the type of photosynthetic metabolism is decreasing from:

Crassulacean metabolism > C4 metabolism > C3 metabolism.

There you have a first stratification of drought resistance at the metabolic level.

At the species level I wish you good luck. Maybe the combination of fluorescence and evpotranspiration is the best approach.

Cheers and success,

Frank

Hello David W. Lawlor ,

Thanks for your answer. I would be happy to read more about it. Could you please give some related references?

The answer to this question is, in my view, not a simple one. The reason is that there is complex interaction between the rate, intensity and duration of the stress under which the plant develops and grows. In rapidly developing short-term stress (see Lawlor and Tezara 2009) chlorophyll fluorescence (NPQ) is increased with small decrease in relative water content (RWC) as stomata close and CO2 supply falls. Also, leaf temperature increases but this is not considered in this publication. Chlorophyll content may not change much in younger leaves - but old leaves may start to senesce. In long term stress development (see Lawlor et al, 1981, Legg et al 1978) in the field for example, the plant adjusts developmentally - growing fewer tillers and fewer, smaller leaves, in the case of barley in the experiments but this is a standard response of all plants. The smaller plants adjust to the water supply so that they are not stressed with small RWC. And metabolic changes of the type discussed in the Lawlor and Tezara paper do not become so important. Fluorescence may be relatively little affected, leaf temperature may be higher, but adjusted plants have pretty open stomata so temperature may not be affected much. But older leaves die more quickly, so they lose chlorophyll. Very few studies have examined these aspects - because we insist on believing that a single factor in metabolism (such as a gene change) will be dominant and determine the response to drought.

I suggest references in which I have been involved. . Other researches have different views (see J. Flexas and views on the role of mesophyll conductance, which is questioned in Lawlor and Tezara 2009).

Best wishes and stay well

David

References available on Researchgate (any problems please let me know):

Causes of decreased photosynthetic rate and metabolic capacity in water-deficient leaf cells: a critical evaluation of mechanisms and integration of processes

DW Lawlor, W Tezara - Annals of botany, 2009 - academic.oup.com

Background Water deficit (WD) decreases photosynthetic rate (A) via decreased stomatal conductance to CO2 (gs) and photosynthetic metabolic potential (A pot). The relative importance of gs and A pot, and how they are affected by WD, are reviewed with respect to light intensity and to experimental approaches. Scope and Conclusions With progressive WD, A decreases as gs 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 …

Cited by 590

Growth of spring barley under drought: crop development, photosynthesis, dry-matter accumulation and nutrient content

DW Lawlor, W Day, AE Johnston, BJ Legg… - The Journal of …, 1981 - cambridge.org

The effects of water deficit on growth of spring barley were analysed under five irrigation treatments. One crop was irrigated at weekly intervals from emergence throughout the growing season, and one was not irrigated at all after emergence. Soil water deficits in the …

Cited by 100 Related articles All 6 versions

The effects of drought on barley growth: models and measurements showing the relative importance of leaf area and photosynthetic rate

BJ Legg, W Day, DW Lawlor… - The Journal of Agricultural …, 1979 - cambridge.org

In a field experiment on the effects of drought on spring barley the crop was protected from rain by automatic rain shelters. Various plots received irrigation at different times to give a range of drought treatments from full irrigation to no irrigation between emergence and …

Cited by 207

Easy question, very difficult answer! As already explained in the previous answers, there is enormous complexity to have a precise determination for each environmental condition (stress level - drought and heat), plant species (size, cycle, ages, metabolic type, etc.), adaptive capacity of the responses (genetic, physiological, anatomical and metabolic), limitations of the instruments (technologies and operation) and methodologies used (sensitivity and repeatability) and evaluation of the results obtained (correct statistical procedures).

To all of these responses, I would add anatomical leaf variations (thickening) and changes in mesophilic tissues as a result of increased stress level. The use of SPAD, for example, may have greater readings caused by greater leaf thickening, without necessarily increasing chlorophyll synthesis. The pigments determinations by spectrophotometry also have their variations in relation to the time and type of extraction. This depends on the leaf tissues properties and their degree of resistance to extraction, not to mention the care regarding their degradation during handling. As for fluorometry, it is a punctual and dynamic data that, for a correct interpretation, depends on other information linked to the water status and on the plant's own morphological protection capacity (waxes, hair, cuticle).

In addition, other variables covered by the nutritional aspects and models used for data analysis enter. Many of the works require a more complete integrated and multivariate analysis, as this would facilitate the visualization of plant responses to environmental stresses.

According to Fritschi, Felix & Ray, J. (2007). Soybean leaf nitrogen, chlorophyll content, and chlorophyll a/b ratio. Photosynthetica. 45. 92-98. 10.1007/s11099-007-0014-4... Chl contents and composition were examined among 833 soybean (Glycine max L. Merr.) accessions and related to SPAD meter readings and leaf N content.….two distinct groups according to leaf Chl a/b ratios, with the majority of a mean ratio of 3.79. However, approximately 7 % (60) of the genotypes could be readily assigned to a group with a mean Chl a/b ratio of 2.67.

Our preliminary results evidenced the isohidric status for first group (stomata сloused quickly upon drought, but may suffer hungry) and anisohidric status for second group (stomata opened during prolonged period, inclusive air dryness and heat, but more need for water).