HYDRATURE AND PLANT PRODUCTION 281 



amounts of dissolved materials occurred (see Fig. 6, lettuce). In this case 

 the refractometer values are only relatively valid. In order to avoid an 

 incorrect interpretation we use principally the refractometer method as a 

 relative method, i.e. : with tliis apparatus we only try to indicate a quahta- 

 tive change of plant hydrature. Yet, for many investigations and practical 

 purposes this is sufficient, because as soon as the refractive index rises we 

 do know that plant hydrature changes to less than optimum conditions. 

 Plants should be irrigated in that case. 



The practical value of the refractometer method was tested and proved 

 with an alfalfa experiment in Abu Ghraib, Iraq in the summer of 1957, 

 after natural rainfall was over and when temperatures went up to about 

 50°C. Four treatments (2-4 replications) were watered after the refractive 

 index at the plots showed a value just higher than the optimum (= lowest 

 value possible), or higher than optimum +2%, optimum +4% and opti- 

 mum + 6% respectively. Because refractometer readings require a 

 minimum of time, it was possible to measure the hydrature of plants every 

 two days. The curves during that period (refractometer readings in 

 dissolved materials, per cent) can be seen in Fig. 7. All are clearly related to 

 the water supply, as also the curve of decreasing yield is to the average 

 refractometer readings for the various plots. 



Also here (Fig. 8) we find the above mentioned characteristic: the 

 decrease in the yield is more evident initially than at higher levels of 

 refractometer reading. 



Successful application of checking hydrature by the method of refracto- 

 meter determinations have been recently reported by Russian scientists 

 (Babuschkin, 1959; Belik, i960) for irrigation purposes. 



If factors outside the plant as suction tension or soil moisture stress are 

 employed for checking plant production instead of the osmotic value of 

 the cell sap, it can be found that even then the curve of decreasing yield 

 shows the above mentioned characteristic: it is hke an e-fimction in shape. 

 For instance, Wadleigh and Ayres (1945) got a similar result with bean 

 plants.Recent data reported by Sands andRutter (1959) show that height of 

 shoots, length of needles, and dry weight o£ young Pinus sylvestris plants 

 produce similar curves (Fig. 9) when plotted against increasing suction 

 tension in soil. In addition to this even the curve of decreasing net 

 assimilation rate, representing the real production process, follows the shape 

 of an e-fimction. 



Of course the decrease of yield with decreasing hydrature is appreciably 

 different in various crops. Those species which are relatively hygrophytic, 

 or even mesophytes, show a rapidly dechning yield curve. In plants which 



