光强对木麻黄幼苗根系形态、解剖结构及其碳氮含量的影响

对光环境的灵敏响应使得森林中常见的光照异质性成为影响植物自我更新的关键因素,然而植物地下根系结构对光照的响应较为难测而缺乏深入研究。为探究不同光强下木麻黄根系响应策略,以一年生木麻黄(Casuarina equisetifolia)幼苗为试验材料,模拟森林幼苗生长的林外(CK)、林缘(L1)、林窗(L2)和林下光环境(L3)设置4种光照强度,测定及分析木麻黄幼苗的生长、根系形态、细根解剖结构及碳氮含量等指标。结果表明:(1) L1下,幼苗采取维持高度,降低横向生长的方式,保证正常累积生物量,随光照强度的下降,株高、地径、叶片生物量及地上部分生物量逐渐下降。(2)在根系表型上,幼苗随光限制的加重呈现抑制纵伸但促进根系的横向生长,其中总根长、根平均直径及根体积达到显著差异。在径级结构上,细根发育程度随光照减弱而下降;而适当的遮光(L1)促进粗根生长但L3时除根尖数较CK上升外,根长度、根表面积、根体积均显著下降。(3)1-3级细根解剖变化较大,相较CK,1级细根皮层细胞面积显著增加,但根半径、维管柱结构、表皮厚度等指标则显著下降,2级细根根半径、皮层细胞面积、表皮厚度明显减少,但维管柱结构仅在L2、L3时显著下降;3级细根根半径、皮层细胞面积和维管柱面积均较CK显著增大,L1时维管柱结构下降,但随光照减弱加重,维管柱面积和中柱占比均明显增加。(4)在碳氮含量上,CK与L1无显著差异,TC在L2时显著下降,TN则在L2时显著上升,TC、TN均在L3达到最大,而C∶N随光强降低逐渐下降。综上所述,光限制时,木麻黄生物量及碳分配稳定根茎部分生长,采取“弱化吸收,强化储存”收缩型生长策略;当限制加重时,光合和呼吸作用失衡导致植物对细根投入养分的浪费,并最终造成林木死亡。研究结果为林下植被的更新提供理论参考。

Effects of light intensity on root morphology, anatomical structure and carbon and nitrogen content of Casuarina equisetifolia seedlings

The sensitive response to light environment makes the common light heterogeneity in forests to be a key factor affecting plant self-renewal. However, the underground root structure of plants lacks in-depth research due to its difficulty in measuring light. In order to further understand the root response strategy of Casuarina equisetifolia under different light intensities, one-year-old C. equisetifolia seedlings were used as experimental materials, and four light treatments were set up to simulate the light environment outside the forest (CK), in the forest edge (L1), in the forest gap (L2), and under the forest (L3). The growth, root morphology, anatomical structure of roots and carbon and nitrogen contents of the seedlings were measured and analyzed. The results showed that: (1) Under L1, the seedlings took to maintain height and reduce lateral growth to ensure normal accumulation of biomass. With decreasing of light intensity, the plant height, ground diameter, leaf biomass and aboveground part of the biomass gradually decreased. (2) In terms of root phenotype, the seedlings showed a tendency to inhibit longitudinal elongation and promote lateral root growth with the increasing light limitation, in which total root length, mean root diameter and root volume reached significant differences. In terms of radial structure, the development of fine roots decreased with the decreasing light. The coarse root growth was promoted in appropriate shading(L1), and the root length, root surface area, and root volume decreased significantly in excessive shading(L3), except for a rise in the number of root tips compared with CK. (3) The anatomical changes of grade 1-3 fine roots were large. Compared with CK, the cortical cell area of fine roots in grade 1 increased significantly, while the root radius, vascular column structure and epidermal thickness decreased apparently. The root radius, cortical cell area and epidermal thickness of grade 2 fine roots decreased markedly, while the vascular column structure decreased obviously only at L2 and L3. The root radius, cortical cell area and vascular column area of grade 3 fine roots increased significantly. Although the vascular column structure decreased at L1, both the area of the vascular column and the percentage of the middle column increased significantly with the continuous decrease of light. (4) There was no significant differene between CK and L1 in terms of carbon and nitrogen content. At L2, TC decreased significantly and TN increased significantly, both of which reached the maximum at L3. C:N decreased gradually with decrease of light intensity. In summary, when light is restricted, the biomass and carbon allocation of C. equisetifolia stabilized growth in the rhizome portion of the plant, adopting a contraction growth strategy of "weak uptake and strong storage". However, when the light limitation intensified, the imbalance of photosynthesis and respiration led to the imbalance of plant nutrient inputs to fine roots, and eventually caused the death of trees. The results provide a theoretical reference for the regeneration of understory vegetation.