Setor de Fisiologia Vegetal, Centro de Biotecnologia e Genética, Universidade Estadual de Santa Cruz, CEP, Ilhéus, Bahia, Braz il.
Setor de Fisiologia Vegetal, LMGV, Centro de Ciências e Tecnologias Agropecuárias, Universidade Estadual do Norte Fluminense, Campos dos Goytacazes, Rio de Janeiro, Brazil.
Ann Bot. 2019 Nov 27;124(6):979-991. doi: 10.1093/aob/mcz040.
Although hypernodulating phenotype mutants of legumes, such as soybean, possess a high leaf N content, the large number of root nodules decreases carbohydrate availability for plant growth and seed yield. In addition, under conditions of high air vapour pressure deficit (VPD), hypernodulating plants show a limited capacity to replace water losses through transpiration, resulting in stomatal closure, and therefore decreased net photosynthetic rates. Here, we used hypernodulating (nod4) (282.33 ± 28.56 nodules per plant) and non-nodulating (nod139) (0 nodules per plant) soybean mutant lines to determine explicitly whether a large number of nodules reduces root hydraulic capacity, resulting in decreased stomatal conductance and net photosynthetic rates under high air VPD conditions.
Plants were either inoculated or not inoculated with Bradyrhizobium diazoefficiens (strain BR 85, SEMIA 5080) to induce nitrogen-fixing root nodules (where possible). Absolute root conductance and root conductivity, plant growth, leaf water potential, gas exchange, chlorophyll a fluorescence, leaf 'greenness' [Soil Plant Analysis Development (SPAD) reading] and nitrogen content were measured 37 days after sowing.
Besides the reduced growth of hypernodulating soybean mutant nod4, such plants showed decreased root capacity to supply leaf water demand as a consequence of their reduced root dry mass and root volume, which resulted in limited absolute root conductance and root conductivity normalized by leaf area. Thereby, reduced leaf water potential at 1300 h was observed, which contributed to depression of photosynthesis at midday associated with both stomatal and non-stomatal limitations.
Hypernodulated plants were more vulnerable to VPD increases due to their limited root-to-shoot water transport capacity. However, greater CO2 uptake caused by the high N content can be partly compensated by the stomatal limitation imposed by increased VPD conditions.
尽管豆类（如大豆）的高结瘤表型突变体具有较高的叶片氮含量，但大量的根瘤会降低植物生长和种子产量所需的碳水化合物供应。此外，在高空气蒸气压亏缺（VPD）条件下，高结瘤植物表现出有限的能力通过蒸腾来替代水分损失，导致气孔关闭，从而降低净光合速率。在这里，我们使用高结瘤（nod4）（每株植物 282.33 ± 28.56 个根瘤）和非结瘤（nod139）（每株植物 0 个根瘤）大豆突变体系来明确确定大量根瘤是否会降低根水力传导能力，从而在高空气 VPD 条件下降低气孔导度和净光合速率。
将大豆接种或不接种根瘤菌（菌株 BR 85，SEMA 5080）以诱导固氮根瘤（在可能的情况下）。播种后 37 天测量绝对根导率和根电导率、植物生长、叶片水势、气体交换、叶绿素 a 荧光、叶片“绿色度”[土壤植物分析开发（SPAD）读数]和氮含量。
除了高结瘤大豆突变体 nod4 的生长受到抑制外，由于其根干质量和根体积减少，这些植物的根供应叶片水分需求的能力下降，导致绝对根导率和根电导率按叶面积归一化后降低。因此，观察到 1300 h 时叶片水势降低，这导致光合作用在中午受到抑制，这与气孔和非气孔限制有关。
由于高结瘤植物的根到叶的水分输送能力有限，它们更容易受到 VPD 增加的影响。然而，由于 VPD 条件增加引起的气孔限制，高氮含量导致的 CO2 吸收增加可以部分得到补偿。
