What is it that switches a seed from dormancy to sprouting? It’s important to time it right, as once the seedling emerges from the ground, it is stuck with whatever environment it finds itself. It’s easy to see that seed will not want to germinate during a harsh winter or deep drought, but even when conditions are favourable, there are other problems. Plants rarely travel alone, and if a seed falls into soil somewhere, it’s likely to have neighbours all competing for the same resources.
It’s tempting to see germination among a group of seeds as a competition to be first. The earliest seedlings get the first light, with this extra energy they can shade out their neighbours and keep a greater share of sunlight to themselves. What has puzzled Lindsay Leverett and her colleagues is something that seems to get this all wrong. Some plants seem to delay germination when they have neighbours. Is this just because the other plants are dominating the soil and gobbling up more nutrients, preventing the late seeds from getting the signals they need to germinate? Or is it that something else is going on? In their paper The fitness benefits of germinating later than neighbors Leverett and colleagues list three ways in which late germination could be an advantage.
When two seedlings germinate around the same time, they’re in direct competition. They compete for the same soil with the same resources, and it’s quite a fight. What happens if one plant is much more massive? Surely the fight is one-sided? Leverett and her co-authors argue that the fight doesn’t happen. The bigger plant has deeper roots and has a different relationship to the soil than its smaller neighbour. Instead of fighting its neighbour, the smaller plant can work around it.
The mismatch in growth can also mean the later plant is competing at a different time to its neighbour. A plant can be growing while its neighbour is dying back. The time delay means that a plant can spend less time competing with its neighbours.
Finally, a larger neighbour can act as a windbreak, protecting smaller seedings from the cold. So while there are costs to later germination, there can also be benefits.
To examine how germination is timed, Leverett and colleagues looked at two questions. They measured how germination was affected by conditions in the seed environments. They also looked at whether the maternal environment made a difference. The authors then asked how late germination influenced fitness, but putting seedlings next to plants of various sizes to simulate early or late germination.
What they found was that seeds were more likely to germinate if the mother plant had more neighbours. They also found that germination would be delayed if there was a canopy over the seed site (simulating a neighbour) when the seed was watered.
The effects of neighbours were a bit more complicated.
If you want to survive as a young seedling, then the best you can hope for is to have a large neighbour. Growing alone was more likely to lead to an early death than growing with a large neighbour. However, get past that early stage, and you’re doing well as a sole plant. In contrast plants with neighbours were more likely to die as they aged. They also grew smaller and their ability to reproduce was diminished. However, the plants with larger neighbours did better than the plants with smaller neighbours. The results are consistent with there being benefits to late germination, and this might explain why seeds with neighbours could delay their germination.
The result that surprised me the most was the influence of the maternal environment on how seeds germinate – and it really shouldn’t have. Two of the authors of the AmJBot paper were also authors on an earlier Annals of Botany paper that showed the importance of the maternal environment.
The paper, Contrasting germination responses to vegetative canopies experienced in pre- vs. post-dispersal environments, looked at seeds maturing under vegetative canopies, simulated by adding a green filter to light.
The Annals paper shows that when the seed is maturing it’s being loaded with baggage from the maternal plant. In the case of the AmJBot paper, the seeds are growing on the mother plant which has the stresses of its neighbours. That’s how the mother plant influences the seeds germination when they’re independent in the soil themselves.
As each plant tackles a slightly different micro-environment, it helps show how variation in responses is being passed along to the next generation. But a seed isn’t merely preprogrammed to germinate. There’s a definite response to neighbours in an organism that appears to be dormant.
If you’d like to read more, the Annals paper is free access now. The AmJBot paper will be free access from January 2019, if you don’t have access to a subscription. Wiley has done an excellent job with the new AmJBot site in showing which issues of the journal are free to access.