摘要
在根区渗灌条件下，将士壤水势长期控制在5组变动范围内。研究了5种土壤水分状态下水曲柳(Fraxinus mandshuric)幼苗的水分生态、光合作用和生长反应，所得结果如下：
在高于田间持水量的充分供水条件下，苗木日蒸腾作用强烈，表现为高水平的单峰形日变化；土壤供水不足，苗木蒸腾显著受抑制，表现为低水平的双峰形日动态过程。在充分供水条件下，苗木细根的日均吸水速率达2.67 mmol／m~2．s，；而当土壤水分状态在毛管水断裂含水量以下时，细根日均吸水速率为1.35mmol／m~2．s。土壤充分供水使叶水势在-0.76～-1.46Mpa范围内变动，而供水不足则使水势的变动范围降至-1.25～-1.94Mpa。土壤水势递降显著增大了SPAC体系的水流阻力，其中土壤—细根阻力随土水势降低而升高的相对速率远高于其他两项阻力。
充分供水使光合目动态曲线呈左偏旗形，无明显的午休谷；供水不足则出现典型的午休谷，甚Pn为负值。从气孔导度，胞间CO_2浓度和气孔抑制值的变化及叶水势、叶温等方面综合考虑，推断田间持水量以下土壤供水不足引起的光合作用减弱主要是非气孔抑制的结果，即PSII光化学反应中心钝化所致。
水分胁迫显著降低了苗木生长和干物质积累。对于水曲柳这种喜湿树种来说，土壤干旱并未促使苗木将相对更多的生物量投向吸收器官或根系。
从不同土壤水势条件下水曲柳幼苗的水分生理特点、光合作用及生长反应等方面综合考虑，适宜的土壤水分状态应该为亚饱和状态或田间持水量状态，其土水势范围大约为0～-40Kpa。对于喜湿的水曲柳幼苗来说，若在全光下生长，土壤湿度低于田间持水量即可导致苗株水分胁迫。
Under different osmotic irrigation conditions ,the soil water potential was controlled within the range of 0~20Kpa(W1), -20~-40Kpa(W2), -40~-60Kpa(W3), -60~-80Kpa(W4) and -80~-160Kpa (W5). Ash (Fraxinus mandshurica) seedlings were planted on water-controlled soils, and the water ecology, photosynthesis and growth properties of seedlings were studied. It resulted as the following:
Transpiration of seedlings decreased in the sequence of soil water potential ,of which max transpiration rate was 5~6 mmol/ m2.s (W1),and min transpiration rate was 1~2 mmol/m2.s (W3~W5).It indicated that drought stress inhibited transpiration significantly .The water suction rate of fine roots of W1 reached 2.67 mmol/ m2 in daily average and 4.44 mmol/ m2at midday,but it was only 1.35 mmol/ m2 in the case of W5 treatment .Soil water stress brought about declination in water saction rate of fine roots . Low soil water potential led to lower fluctuation levels of the leaf water potential.The leaf water potential of W1 treatment fluctuated in the range of -0.76~-1.46Mpa ,root-water potential fluctuated in the range of -0.08~-0.19Mpa ;while the leaf water potential of W5 treatment only ranged from -1.25~-1.94Mpa. Soil water stress significantly increased the water resistance of SPAC system of which soil-fine-root resistance increased more rapidly than the other two resistances.
W3~W5 treatment declined more rapidly than W1 and W2,forming deep valley of photosynthesis at midday. From a comprehensive consideration of stomatal conductance (Gs), intercellular Co2 concentration (Ci),leaf temperature and leaf water potential ,the mechanism of photosynthetic restrain of W3~W5 treatments should be that of non-stomatal.
In the soil water sequence of W1~W5, the leaf area of seedlings decreased at the ratio of 100: 65: 37:33:30,fine root area decreased at the ratio of 100:54:30:25:20,hight of new shoot decreased at the ratio of 100:70:52:45:44 ,diameter of new shoot decreased at the ratio of 100:88:68:64:57,whole -plant biomass decreased at the ratio of 100:71:43:38:35.The root /canopy ratio of seedlings were 1.38,1.37,1.17,1.18 and 1.04 from W1 to W5 treatment, thus ,soil water stress did not impeled the plant to distribute relatively more biomass to root system .
From a comprehensive consideration of plant water status photosynthesis and seedling growth ,the appropriate soil water conditions should be in the range of 0~-40Kpa ,that's from water saturation to filed capacity.
引文
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