不同水分胁迫方式对沙地樟子松幼苗光合特性的影响

沙地樟子松引种栽培的成功已使该树种成为中国北方沙区人工造林的首选树种, 但由于早期引种的沙地樟子松人工林出现了衰退现象, 使得人们对在干旱、半干旱沙地进行大面积樟子松造林产生疑问. 为进一步研究沙地樟子松人工林衰退问题, 该文以2年生沙地樟子松幼苗为材料, 采用盆栽控水和聚乙二醇(PEG)处理法对苗木进行水分胁迫试验,比较两种胁迫处理苗木的光合特性. 结果表明, 土壤水分胁迫与PEG模拟水分胁迫(处理1h)对2年生樟子松幼苗光合生理特征及其水分利用效率影响基本一致; 当土壤含水量为40％田间持水量时, 沙地樟子松已表现出干旱胁迫, 土壤含水量为20%田间持水量时胁迫达到最大. 10%PEG处理对2年生樟子松幼苗光合生理指标影响与对照具有相同趋势, 表明该处理未对苗木造成严重干旱胁迫. 20%与30%PEG处理对樟子松幼苗光合指标影响的趋势相同, 胁迫超过2 h后樟子松幼苗光合速率、气孔导度、蒸腾速率都降到较低值且相对稳定. 土壤含水量为20%田间持水量的胁迫对樟子松幼苗的水分利用效率几乎没有影响, 轻度(40%田间持水量)胁迫甚至增高了水分利用效率; PEG胁迫的前期(4 h之前), 苗木的水分利用效率低于对照; 在胁迫处理4 h后, 20％与30％PEG处理的樟子松的水分利用效率均超过了对照. 这表明樟子松在较低的土壤含水量下, 具有忍耐、适应干旱胁迫的能力. 另外, 不同形式的强度胁迫处理(30%PEG和20%田间持水量)的各光合特征指标相对值之间没有差异, 表明樟子松苗木在强度胁迫条件下各指标相对值已降至相当低的程度. 3种PEG浓度(10%、20%、30%)干旱胁迫处理在2 h以内的各指标的相对值与3种土壤水分胁迫处理(40%、30%、20%田间持水量)基本一致, 因此, 可以认为2 h的PEG胁迫处理与土壤水分胁迫处理(7～10 d)具有相同的效果. 

Comparison of different types of drought stresses affecting photosynthesis of Mongolian pine seedlings on sandy soils

Mongolian pine (Pinus sylvestris L. var. mongolica Litv.) is one of the most drought resistant tree species. It has been widely introduced on sandy soils in the northern region of China (north, northwest and northeast China) but a decline in forest growth and timber quality has occurred in the earliest Mongolian pine plantations on Keerqin sandy soils located at Zhanggutai, Liaoning Province. In order to study this decline problem, we carried out an experiment to examine the effects of the following two drought stresses: soil water (SW) stress and polyethylene glycol (PEG) stress, on photosynthesis of 2_year old Mongolian pine seedlings on sandy soils. As well, we tested the response of photosynthesis on different types of drought stresses.The results indicated no significant differences between the effects of SW and PEG stress, processed in 1 hr, on indices of instantaneous photosynthetic rates, stomatal conductance, transpiration rates and CO2 concentration inside cells. When the soil water content was 40% of field water capacity, the seedlings started to become stressed and became heavily stressed at 20%. The cause of the stress was the change in stomatal conductance. The effects of PEG stress on photosynthetic indices showed the following characteristics: 1) a 10% PEG stress had the same effect on treated seedlings as on seedlings of the control group and there was less stress on the 2_year old seedlings; 2) 20% and 30% PEG stresses had the same effect and the values of instantaneous photosynthetic rates, stomatal conductance, transpirati on rates and CO2 concentrations inside cells were very low and stable after a 2 hr stress period. At 20% field water capacity there was a slight SW stress effect on water use efficiency (WUE) of the seedlings; at 40% capacity, SW stress improved WUE. Under conditions of PEG stress, the WUE of stressed seedlings was lower than that of control seedlings within 4 hrs since the start of the stress. For the 20% and 30% PEG stresses, WUE was higher for treated seedlings than for the control after a 4 hr stress. These results show that Mongolian pine seedlings can endure and adapt to drought stresses under conditions of lower soil water content. Comparisons between PEG stresses at 10%, 20% and 30% within 2 hrs and SW stresses at 40%, 30% and 20% of field water capacity showed that the relative values of photosynthetic indices were very similar. Therefore, PEG stresses within 2 hrs had the same effect as SW stress within 7～8 days.