WATER RELATIONS OF SPECIES: COMPARISONS 301 



10- 



9- 



SMT 

 (ATMS.) 



7- 



I- 

 Z 



U 6 — 



a 



5— 



UJ 



Q 



I 



< 



liJ 



3- 



MEAN TR. IMGMS,% PER MiN.) 

 2-15 p.m. to A-15p.nn. 



0-45 ; 



13 15 



TIME OF DAY 



I 

 17 



^ SMT 



\v (ATMS.) 



NAJl 5-0 to 7 

 \*-'(3contai 



O 

 iners) 



3-2 to 5-0 

 (1 container) 



BELOW 0-2 

 12 containers) 



I 

 19 



1 — r 



6 7 



Fig. 6. The effect of soil moisture tension (SMT) on leaf water deficit and transpiration 



rate. (24 August i960.) 



• Thelycrania sanguinea 

 X Primus padus 



The two species were planted alternately in the containers. Values are means for at 

 least 10 leaves. Three containers had SMTs within the highest range of SMT (4-0 to 

 6-7 atm at the beginning of the experiment) ; one at the middle range (2-3 to 4-0 atm) ; 

 two at the lowest range (below 0-2 atm throughout the experiment). Readings of 

 SMT were taken from four plaster-of-Paris blocks per container. 



water uptake, even for plants in soil at more than 5-0 atm SMT. Values at 

 7 a.m. were smaller on 25th than on the previous day, although SMT had 

 increased shghtly. The removal of two samples of leaves appreciably 

 reduced the leaf area of each plant. Hence if equilibrium between plant and 

 soil moisture was not achieved during the night, for plants with a normal 

 number of leaves, then a reduction in leaf area might result in smaller early 

 morning LWDs, which more closely approached the equihbrium value. 

 It is difficult to explain the generally small values of LWD which pre- 

 vailed, since environmental conditions were not very different from those 

 during the previous experiments, and the transpiration rates were not very 

 much lower than before. At 7 a.m., on both 24 and 25 August, there 

 were no significant differences between LWDs of the two species. At 7 

 p.m., in the intermediate SMT condition, the mean LWD for T. sanguinea 



