Irrigation is used to alleviate soil drought in order to improve the productivity of poplar plantations. However, little is known about the responses of plants with adequate water supply to low air humidity which prevails in spring and early summer in Northern China. To study this aspect, rooted cuttings of Populus 'I-214' (I), P. ' Robusta'(R), P. 'Popularis 35-44' (P) and P. berolinensis (B) were grown in pots with soil moisture maintained at 90% of full field capacity. Using an Li-6200 Photosynthesis System, the net photosynthetic rate (PN) and other related parameters were determined for 3 clear days with different air humidities : 4345%, 2631% and 2022%. The effect of air humidity on PN was considerable and the difference in response to air drought among the 4 poplar clones was significant. Compared with PN at RH 4345%, PN under RH 2631% in I, R, P decreased by about 66%, but only by 41% in B. At RH 2022%, PN of I, R, P decreased by 7076%, but by 51% for B. A similar change was found in stomatal resistance (SR) of the 4 clones.
To study the response of stomatal apparatus to air drought, the changing course of SR was recorded using an Li-1600 Porometer at 9:00 am on a clear day. RH for measuring SR was maintained at 17.6%. Air and leaf temperatures were recorded simultaneously. The changing courses of SR for all clones were sigmoid in shape, but significant differences in the speed of response were found between clones. The first turning point in the sigmoid curve was considered as the beginning of stomatal closing, and the second turning point as the time of full closure. Experiments demonstrated that the stomata of R responded promptly (in 9 min.) to dry air (RH 17.6%), and took 11.6 min. to complete full closure. Stomata in I began closing in 13 min., but took only 4.4 min. to complete full closure. Therefore, both I and R were very sensitive to air drought. P was less sensitive: stomatal closure commenced at 18.4 min. and achieved full closure in 12 min. Resistance of stomata of B to air drought was surprisingly high; it began closing by 49.2 min. and was fully closed after 14.8 min. Knowledge about the difference in resistance to air drought is important for choosing poplar clones in semi-arid area with irrigation.
Key words: poplar, resistance to air drought, photosynthesis, stomatal resistance.
Correspondence: Wang Shasheng, Beijing Forestry University, Beijing 100083, China
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