Lintunen Anna, Lindfors Lauri, Nikinmaa Eero, Hölttä Teemu
Department of Forest Sciences, University of Helsinki, P.O. BOX 27, FI-00014 Helsinki, Finland.
Department of Physics, University of Helsinki, P.O. BOX 64, FI-00014 Helsinki, Finland.
Tree Physiol. 2017 Apr 1;37(4):491-500. doi: 10.1093/treephys/tpw114.
Trees experience low apoplastic water potential frequently in most environments. Low apoplastic water potential increases the risk of embolism formation in xylem conduits and creates dehydration stress for the living cells. We studied the magnitude and rate of xylem diameter change in response to decreasing apoplastic water potential and the role of living parenchyma cells in it to better understand xylem diameter changes in different environmental conditions. We compared responses of control and heat-injured xylem of Pinus sylvestris (L.) and Populus tremula (L.) branches to decreasing apoplastic water potential created by osmotic stress, desiccation and freezing. It was shown that xylem in control branches shrank more in response to decreasing apoplastic water potential in comparison with the samples that were preheated to damage living xylem parenchyma. By manipulating the osmotic pressure of the xylem sap, we observed xylem shrinkage due to decreasing apoplastic water potential even in the absence of water tension within the conduits. These results indicate that decreasing apoplastic water potential led to withdrawal of intracellular water from the xylem parenchyma, causing tissue shrinkage. The amount of xylem shrinkage per decrease in apoplastic water potential was higher during osmotic stress or desiccation compared with freezing. During desiccation, xylem diameter shrinkage involved both dehydration-related shrinkage of xylem parenchyma and water tension-induced shrinkage of conduits, whereas dehydration-related shrinkage of xylem parenchyma was accompanied by swelling of apoplastic ice during freezing. It was also shown that the exchange of water between symplast and apoplast within xylem is clearly faster than previously reported between the phloem and the xylem. Time constant of xylem shrinkage was 40 and 2 times higher during osmotic stress than during freezing stress in P. sylvestris and P. tremula, respectively. Finally, it was concluded that the amount of water stored in the xylem parenchyma is an important reservoir for trees to buffer daily fluctuations in water relations.
在大多数环境中，树木经常经历低质外体水势。低质外体水势会增加木质部导管中形成栓塞的风险，并给活细胞造成脱水胁迫。为了更好地理解不同环境条件下木质部直径的变化，我们研究了木质部直径响应质外体水势降低时的变化幅度和速率，以及活薄壁细胞在其中所起的作用。我们比较了欧洲赤松（Pinus sylvestris (L.)）和欧洲山杨（Populus tremula (L.)）枝条的对照木质部和热损伤木质部对渗透胁迫、干燥和冷冻所产生的质外体水势降低的响应。结果表明，与预先加热以损伤活木质部薄壁组织的样本相比，对照枝条中的木质部在响应质外体水势降低时收缩得更多。通过操纵木质部汁液的渗透压，我们观察到即使在导管内不存在水张力的情况下，质外体水势降低也会导致木质部收缩。这些结果表明，质外体水势降低导致细胞内水分从木质部薄壁组织中撤出，从而引起组织收缩。与冷冻相比，在渗透胁迫或干燥过程中，质外体水势每降低单位值时木质部收缩的量更高。在干燥过程中，木质部直径收缩既涉及木质部薄壁组织与脱水相关的收缩，也涉及导管因水张力引起的收缩，而在冷冻过程中，木质部薄壁组织与脱水相关的收缩伴随着质外体冰的膨胀。研究还表明，木质部中质体和质外体之间的水分交换明显比之前报道的韧皮部和木质部之间的水分交换要快。在欧洲赤松和欧洲山杨中，木质部收缩的时间常数在渗透胁迫期间分别比冷冻胁迫期间高40倍和2倍。最后得出结论，木质部薄壁组织中储存的水量是树木缓冲水分关系日常波动的重要储备。
