不同土壤水势条件下水曲柳幼苗的光合作用特征

SOME PHOTOSYNTHETIC CHARACTERISTICS OF FRAXINUS MANDSHURICA SEEDLINGS GROWN UNDER DIFFERENT SOIL WATER POTENTIALS

采用根区渗灌控水技术 ,将土壤水势长期控制在 0～ - 0 .0 2MPa(W1)、- 0 .0 2～ - 0 .0 4MPa(W2 )、- 0 .0 4～ - 0 .0 6MPa(W3 )、- 0 .0 6～ - 0 .0 8MPa(W4)、- 0 .0 8～ - 0 .16MPa(W5)范围内。系统研究了不同土壤水势条件下水曲柳 (Fraxinusmandshurica)幼苗叶片的光合速率、PSⅡ光化学效率和Rubisco羧化活性的日动态。结果表明 ,在所有土壤水势条件下 ,苗株皆在早晨达到净光合速率 (Pn)最高峰 ;不同处理间光合午休的程度随所处土壤水势递降而加剧。从W1至W5,叶片的日光合累积比例为 10 0∶96∶6 4∶6 0∶5 2。各处理晨后最初的Pn 降低主要是气孔导度下降引起的 ,W3 ～W5处理午间强烈的光合抑制则主要源于非气孔因素。各处理的PSⅡ光化学效率 (Fv/Fm)和Rubis co初始羧化活性也都表现为不同程度的午间降低 ,且所处的土壤水势越低 ,降幅就愈大 ,其中W3 、W4和W5处理的递降趋势尤为明显。苗木叶片光合作用的日均水分利用效率除W1显著较低外 ,其余处理间无显著差异。从充分供水条件下 (W1、W2 )Pn 仍有晨后降低分析 ,林外强烈的大气因子 (如高温、强光和低大气湿度 )已经构成苗木光合作用的胁迫因素 ,而土壤供水不足则大大加剧了胁迫的程度。

Manchurian ash, (Fraxinus mandshurica), typically distributed along lo wer streams and regenerated under the canopy of natural forests, is an important hardwood silvicultural species in the temperate forest regions of northeast Chi na. On most reforestation sites, however, Manchurian ash plantations exhibit ver y restricted growth. Therefore, the main objectives of this study were to gain a n insight into the physio-ecological mechanisms responsible for the poor growth and low productivity of Manchurian ash in plantations and to analyze the environ mental limitations of soil water on photosynthetic activity. Two-year old Manchurian ash seedlings were planted in rainproof soil pots (8 m length × 0.5 m width × 0.3 m depth). When seedlings had restored vigor, soil w ater potentials were maintained at five treatment levels in the different pots: 0 - -0.02 MPa (W 1), -0.02 - -0.04 MPa (W 2), -0.04 - -0.06 MPa (W 3), -0. 06 - -0.08 MPa (W 4) and -0.08 - -0.16 MPa (W 5). Soil water potential was controlled usi ng a specially designed root-sphere osmotic irrigation technique. After 3 months of soil water conditioning, the diurnal course of gas exchange, PSⅡ photochemi cal efficiency (F v/F m) and rubisco carboxylation activity of Manchurian ash seed lings grown under the different soil water conditions were investigated. All tre atments showed a maximum net photosynthetic rate (P n) in the early morning (appr ox. 6∶00) and a noontime depression of photosynthesis that was the greatest in t he low soil water treatment. The cumulative values of diurnal photosynthesis (mm ol CO 2·m -2 ·d -1 ) from W 1 to W 5 were in the proportion of 100 ∶96∶64∶60∶52. The in itial depression of P n in the early morning for all the treatments was cau sed pr imarily by lowered stomatal conductance, but the strong inhibition of photosynth esis that occurred from 10∶00 to 14∶00 for W 3, W 4, and W 5 treatments was due pre dominately to non-stomatal limitations. Simultaneous with the morning-to-noon or soil-water-ordered depression of P n, there was a corresponding decrease i n PSII photochemical efficiency (F v/F m) and in rubisco carboxylation activity, particu larly for the W 3, W 4 and W 5 treatments. There were no significant differen ces in diurnal average water use efficiency (WUE) among treatments with the except ion of a pronounced lower WUE in W 1. The morning to noon depression of P n observed in the well-watered treatmen ts (W 1 and W 2) indicated that climatic conditions (e.g. high light levels, h igh temper ature, and low humidity) of the experiment differed from the species natural env ironment and caused physiological stress on the wet-adapted and shade-tolerant M anchurian ash seedlings. This stress was intensified as soil water deficiency in creased from W 3 to W 5 treatments.