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Relative Water Content RWC = (FW-DW)/(TW-DW)

FW = fresh weight collect 8-10 1 cm² leaf pieces from 3-4 leaves wrap immediately in saranwrap to minimize evaporation, store in dark i) sample leaves from soybean and wheat ii) sample leaves from watered and dry treatments iii) sample leaf pieces from the leaf tip and leaf base (to see if there is a gradient in water potential down the leaf) iv) sample leaves from the top and bottom of the canopy (to see if there is a gradient of RWC down the plant) weigh the leaf pieces on a 4-digit balance (lab) immerse the leaf pieces in water overnight, place in fridge

TW = turgid weight Next day blot leaf pieces to dryness reweigh and place in a dryer at 60C (24hrs)

DW = dry weight reweigh and calculate RWC

Plant Water Potential

The most common method for measuring water in plants (e.g. dry matter % = DW/FW) has limitations of a) not allowing comparisons between species, b) not being directly related to measurementds of water in soil. Relative water content is better for comparing between species, but has no related value for soil Soil-Plant-Atmosphere Continuum (SPAC) The path of water flow from the soil to the atmosphere - through the plant. We need a unifying measurement that can define water status at any part of the continuum . Water potential Water physicists have developed the term water potential. Potential is defined as: the thermodynamic measure of energy of with water, and available to do work (Note - work is expressed as movement to an area of lower potential) - units are MPa (previously bars) Free water is defined to have a potential of zero. Water under pressure will have positive potential and move to areas of lower pressure. Water that is elevated will have positive potential and move to lower areas Water that has a disolved solute will have a negative potential and will attract free water (e.g. across a semi-permeable membrane). A) water potential of the atmosphere = (R.T)/MW x ln(e/e*) R=gas constant= 8.31 J /mol/K T=temperature (K) = C+273 MW= molecular weight of water = 18g/mol e/e* = relative humidity B) water potential in plants yt = yp + yp + yg Effect of changes in water potential on plant function (see handout) Diurnal patterns in water status (see handout)   Water potential in plants and soil can be measured by many methods, the 2 most common being thermocouple psychrometer pressure chamber

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