Macrocystis pyrifera gametophytes growing under controlled conditions produce juveniles with different numbers of fronds, i.e. few (≤ 2) or multiple fronds (≥ 3). We describe these morphotypes and test whether the number of fronds may impact juvenile growth by performing laboratory and field experiments. Kelp reproductive material was collected in southern Chile and 133 female gametophytes were individually monitored for 90 days until they become fertilized and formed sporophytes. Samples of each individual frond were taken for genetic analysis to determine genetic homogeneity. A factorial experiment combining temperature (12 and 18 °C) and N-NO₃ (N-NO₃ limited and N + NO₃ non-limited concentrations) was conducted using juvenile sporophytes. Growth was assessed by measuring individual surface area and wet weight, and the physiological status by quantifying photosynthetic pigments, nitrate reductase activity and photosynthetic parameters. Field experiments with both morphotypes were carried out to measure growth, on the seabed within a natural kelp bed, and a farm using sporophytes. Following the development of female gametophytes with different numbers of oogonia, we found that those with a higher number of oogonia produced a sporophytic unit united by a single holdfast (i.e. multiple fronds morphotype). These sporophytic units were genetically diverse suggesting that oogonia were fertilized by antherozoids from different male gametophytes. The laboratory experiment indicated that these sporophytic units can withstand better nitrogen limiting conditions at 12 °C as shown by a significantly higher growth rate. This trend was also found for chlorophyll and partially supported by the photosynthetic parameters (higher light compensation point). Nitrate reductase activity revealed that under N limitation its activity increased only in the multiple frond morphotype, independent of temperature. M. pyrifera multiple frond morphotype out-planted to on-growing sites showed a significant higher biomass. These results indicate that morphotype is as important as genetic management when pursuing kelp growth.

在受控条件下生长的大孢子虫配子体产生的幼虫具有不同数量的叶状体，即很少（≤2）或多个叶状体（≥3）。我们描述了这些形态型，并通过进行实验室和现场实验来检验叶状体的数量是否会影响幼虫的生长。在智利南部收集了海带生殖材料，分别对133只雌配子体进行了90天监测，直到它们受精并形成了孢子体。采集每个个体叶片的样品进行遗传分析，以确定遗传同质性。结合温度（12和18°C）和N-NO ₃（限制N-NO ₃和N + NO ₃）的析因实验非限制性浓度）使用少年孢子体进行。通过测量个体表面积和湿重来评估生长，并通过定量光合色素，硝酸还原酶活性和光合参数来评估生理状态。在天然海带床上的海床上和使用孢子体的农场进行了两种形态型的田间试验，以测量生长情况。随着雌性配子体发展为不同数量的卵子，我们发现那些卵母细胞数量较多的雌配子体会产生孢子体，并由单个固着物（即多个叶状形态）联合在一起。这些孢子体在遗传上是多样的，这表明卵黄质被来自不同雄配子体的类Anzoozoids受精。实验室实验表明，这些孢子体单元可以在12°C的条件下承受更好的氮限制条件，这表明生长速率明显更高。叶绿素也发现了这种趋势并部分由光合作用参数支持（较高的光补偿点）。硝酸还原酶活性表明，在氮限制下，其活性仅在多叶形形态中增加，与温度无关。种植到生长点的火棘分枝杆菌多叶形态表现出明显更高的生物量。这些结果表明，追求海带生长时，形态型与基因管理同等重要。