I would spend  some time on conceptual side. Either Yield potential or drought resistance and water use efficiency imply different responses that are coordinated by soil temperature and nutrient supply. Sensitivity analysis often varies among growth phases. http://plantstress.com/articles/up_drought_files/AR%20Blum.pdf

Dear Miranda Mortlock

You can also measure:

- leaf water potential

- membrane stability index

- florescence parameters 

- antioxidant enzymes activities

- proline and soluble sugar contents

- leaf chlorophyll content

Regards

Kazem Ghassemi-Golezani

Dear Miranda, I think you have to look at:

1. leaf water potential (potential of apoplastic water);

2. osmotic potential (potential of symplastic water):

3. From the difference between these you can & should determine turgor potential;

4. Relative water content (RWC) to be used as an approximation of cell volume which you need to determine osmoregulation.

Have a look at the papers of J.M. Morgan (in AJAR, AJPP) who did all this in the late 1970's, early 1980's with wheat. Neil C. Turner also wrote a useful review in 'Plant & Soil' 1981, Vol 58 on techniques of measuring plant water status.

Have fun!

  Mark

Yes- I am aware of  the measurements; I am interested in what is now the harder to measure? Especially when we may have larger field trials. Also the equipment is expensive and may be more automated- but how easy is it to calibrate and how does it drift in the field? 

Are there some measurements that we make- because they are easy- and if so how does that affect current crop physiology findings?

I'd say RWC is the easiest parameter to measure and perhaps one of the more reliable measurements provided you only sample fully expanded, non-growing tissue. Provided you place your samples in pre-weighed, air-tight tubes rapidly after excising and then put them in a cooled esky while carrying them around the field, you could takes lots of samples quickly. You'll have to take the fresh weight back in the lab, then float, then turgid weight & then into the oven for dry weight. You need a good 4 point balance at least but other than that the equipment is not expensive. You need to work consistently but that's always required. Determination of either water or osmotic potential is a slower more painstaking process and you have to ask yourself whether RWC will suffice or whether you need the extra info that the potentials provide. One needs generally to take measurements at the same time of the day (or pre-dawn) because the water status of plants varies with time of day so this will affect how many plants can be sampled.       

Perhaps a bit late, but may I ask what the aim is - to find correlations between various measurements eg RWC and yield? Or to simulate crop water relations and yields eg based on weather data and/or soil moisture?  I appreciate the point about costs and difficulty of measurements but these cannot be the main criteria - what is the scientific/practical  need and the value of the output, perhaps for scheduling irrigation or judging if a crop is appropriate for a particular area.  Once these aspects are decided - see @Bongi - then the details can be decided including their cost-effectiveness. The comments address the detail well.  I was involved many years ago with a project in barley crop   growth and yield and the effects of water deficits and feel that such work requires many types of measurement over time if processes are to be understood.  Look at Research Gate - Legg et al, Day et al, Lawlor et al for information. 

Thank you David for asking for extra clarification/rationale behind the posing of this question. 

The reason I ask: research staff costs are increasing, and our trials (plot numbers) are getting bigger. We are looking at screening traits in crops to find significant variation.  It may be tempting to measure only the faster and simpler water measurements? 

Dear Miranda, 

I think this is more crop specific. I am sure you have read papers from Ab Blum, RA Fischer, MM Ludlow, Pasiora R Richards Rana Mums, Rebetzkei so on. They reported various traits & biomass association in different crops. Our recent work we published in JEBot, 2015 suggested canopy conductance as a good trait for yield prediction in sugarcane, and Similarly RA Fischer found that Gs contributed a lot for yield improvements in CIMYYT breeding trials. Canopy conductance is not an easy traits to measure, but we have tested a surrogate trait Canopy temperature (Tc) for biomass prediction in sugarcane. This trait is very easy to measure with high throughput screening system in larger trials. I think Tc is an integrative traits for water relation mechanisms like transpiration efficiency, fluorescence yield and Ps, and so on.

I was interested in sorghum. It is relevant ofcourse as to which yield component you are interested in. Grain, grain per unit of water, etc. Answers  were helpful. Ofcourse one key aspect is that due to plasticity, adaption of plants to various types of seasons (early or late drought) our dry land crops are very complex  and crop physiology requires a  modelling approach AND field trials (see Prof G Hammer Seminar 14 March 2017 QAAFI)