PHOTOSYNTHESIS AND GROWTH OF POPLAR TREES 263 



assimilation activity of the trees, that it can become the decisive factor of 

 production in dry periods, it must also be shown how the hmiting effect 

 was again suspended, when the soil was naturally or artificially resaturated. 

 Eight days after the drought period the soil of the southern border was 

 water-saturated to a depth of at least 60 cm but the total assimilation activity 

 was not yet restored (Fig. 5). It can be seen that the tops of the northern 

 border still show the highest assimilation intensity. But they are closely 

 followed by the southern border base, then comes the northern border base 

 and finally the tops of the southern border. It is quite evident that when the 

 investigations were carried out the leaves of the tree on the southern border 

 had reached full turgidity only in the base fohage but not yet in the top 

 foHage. 



In the course of the drought period, we interfered with the soil drying 

 process of some trees of the southern border by artificial irrigation. The 

 result of this measure is to be seen in Fig. 6. This time we measured the 

 assimilation intensity of the mean crown region. From June 29 to July 3 the 

 progressive soil drought affected photosynthesis in such a manner-upper 

 figure -that the incipient ascent in the morning -as a consequence of a 

 modest resaturation of the fohage during the night-was soon reversed. 

 Before the watering on June 29 the treated tree at the southern border 

 showed the same course of the assimilation curve, with a morning rise 

 after 6 o'clock, a steep fall after 9 o'clock and a continuous depression until 

 sunset. An intense soaking with 100 1. per square metre had already led to a 

 restitution of half of the assimilation activity by the next day, and to a total 

 one on the following day: the curve of July 7 holds the level reached at 

 7 o'clock in the morning-notwithstanding a httle decrease -until about 

 3 o'clock in the afternoon, whereupon stomatal closing and a fall in 

 assimilation occurred as a consequence of the great air dryness (maximum 

 saturation deficit 'S'). It is quite evident that in COo-absorption the hght 

 factor docs not play an essential role here either. 



As is known, too great or too small a quantity of hght may hmit the 

 activity of photosynthesis, when there is a sufficient water supply. Measure- 

 ments with a poplar of our stand in the course of a cloudy day with hght 

 fluctuations between 2000 and 12,000 lux have resulted in an almost 

 synchronous course of hght intensity and assimilation (Fig. 7). The results 

 may be re-arranged to reveal an almost hnear relation between assimilation 

 and hght intensity, as shown in the lower half of the figure. The hmiting 

 effect of high radiation was revealed by measurement in the Hungarian 

 Alcah Steppe with single trees of the common oak and the so-called 

 'Zerreiche' [Quercus cerris). 1 may insert here two figures and two illustra- 



18 



