Comstock J P, Mahall B E
Department of Biology, University of California, Santa Barbara, CA, USA.
Oecologia. 1985 Mar;65(4):531-535. doi: 10.1007/BF00379668.
Predawn xylem pressure potentials were measured on two California chaparral shrubs, Ceanothus megacarpus and Ceanothus crassifolius, throughout the winter and spring growing season and into the summer drought. On the days xylem pressure potentials were measured, leaf orientation measurements were made on a population of marked leaves from the same shrubs. Predawn xylem pressure potentials decreased from -0.1 MPa in both species to -7.8 and -6.6 MPa in C. megacarpus and C. crassifolius, respectively, between May and August, 1981. Leaf inclinations became more vertical during this period with the greatest change observed in C. crassifolius. This change in leaf inclination was reversible, and, in the late winter and early spring, one year old leaves became more horizontal. Leaf azimuths were random and did not change seasonally. Simulations of solar radiation interception indicated that the increase in leaf inclination associated with summer drought reduced the absorption of solar radiation in August by 6% for C. megacarpus and 20% for C. crassifolius. Standard leaf energy budget calculations suggest that steep leaf inclinations would result in slightly lower leaf temperatures and transpiration rates under summer conditions.
在整个冬季、春季生长季节直至夏季干旱期，对两种加利福尼亚丛林灌木——大果蓟（Ceanothus megacarpus）和厚叶蓟（Ceanothus crassifolius）的黎明前木质部压力势进行了测量。在测量木质部压力势的日子里，对来自同一灌木的一群标记叶片进行了叶片方向测量。1981年5月至8月期间，两种植物的黎明前木质部压力势均从-0.1兆帕分别降至大果蓟的-7.8兆帕和厚叶蓟的-6.6兆帕。在此期间，叶片倾斜度变得更加垂直，其中厚叶蓟的变化最为明显。叶片倾斜度的这种变化是可逆的，在冬末和早春，一年生叶片变得更加水平。叶片方位是随机的，没有季节性变化。太阳辐射截获模拟表明，与夏季干旱相关的叶片倾斜度增加，使得大果蓟在8月的太阳辐射吸收减少了6%，厚叶蓟减少了20%。标准的叶片能量收支计算表明，在夏季条件下，陡峭的叶片倾斜度会导致叶片温度和蒸腾速率略低。
