Physiology of seed development | Agriculture Optional Notes for UPSC PDF Download

Seed — a living plant in a quiescent state

In the ovary, within the ovule, after egg and sperm unite to form the zygote, the zygote cell repeatedly divides and develops into an embryo. The embryo differentiates into different structures — plumule, radicle, cotyledon(s), and the endosperm, seed coat, or pericarp develop. Once this development has occurred, the embryo’s metabolism slows down to nearly zero and the maternal plant stops pumping energy into the seed. The nascent plant contained within the seed — really an embryo surrounded by nutritive tissue and a protective covering — is now independent of the mother plant. The seed, which is the next generation of plant, enters a quiescent phase.
Recall that the seed contains:

• An embryo, which is the new plant,
• A nutrient source (typically endosperm and/or cotyledon), and
• A protective covering (typically a seed coat and/or pericarp)

Why do plants produce seeds? Because seeds allow plants to:

• Propagate the next generation
• Generate genetically variable offspring that can be sorted through natural selection
• Survive harsh conditions
• Disperse into new environments

The first of these reasons is obvious, to propagate. One parent plant generates many seeds, and through these seeds potentially bears many offspring. A kidney bean plant, for instance, might average 4 seeds per pod and have 20 pods hanging on the plant, so one plant yields 80 seeds. A nice ear of field corn will have 16 rows of kernels with 40 kernels per row, for a yield of 640 seeds. The tiny hot pepper in your garden has over 50 seeds, and 20 peppers on a plant would yield 1,000 seeds.

The second reason, genetically variable offspring, results from cross-pollination (mentioned above under Seed Morphology), which is particularly common in wild, undomesticated plants. When plants cross-pollinate, an egg is formed within the maternal plant. The genetic constitution of the developing embryo within the seed is 50% from the paternal plant, 50% from the maternal plant. The particular combination of genes in the developing seed is different from that in either parent plant, and from the other seeds on the same maternal parent. The seeds share some of the same genes, but the specific combination of genes is different. That difference results in genetic variability, which can be expressed as differences in plant height, flower color, leaf shape, fruit size, or other morphological or physiological characteristics. In nature, this variability is the raw material on which natural selection operates. In plant breeding, it is the resource that sustains our efforts to select improved plant varieties.

Due to their protective coating and quiescent metabolism, seeds can survive harsh conditions that will kill the parent plant such, as freezing cold, protracted drought, and even fires. Once conditions are again favorable for plant growth, the seeds can then germinate.

Seed Dispersal

Seeds disperse from the maternal parent plant “in space” through many wonderful and creative mechanisms for hitching rides on the wind, on animals, and sometimes in animals as they are eaten, pass through the gut, and are excreted.

You have probably seen fluffy cottonwood (poplar) seeds floating on the summer breeze, pulled burdock seeds off your sweater after bushwhacking through the woods in the fall, or washed bird guano full of mulberry seeds off your car’s windshield. These seeds all used strategies for dispersal in space. Vanderbilt University’s Bioimages (optional) includes photos of mechanisms that help spatially disperse seeds.

Seeds also disperse from the maternal parent plant “in time.” Some have dormancy mechanisms that delay germination until the next favorable growing season, which might be a year from now or even several years from now. Seeds are considered dormant if they are alive and don’t germinate even if provided with favorable conditions for germination. There are many dormancy mechanisms; we’ll address some of them later.

If you’ve ever dug up new sections of your yard for a garden, did you notice that, a few weeks after planting veggies or flowers in the spring, you saw a big flush of weeds? And if you pulled up all of those weeds and somehow prevented any new seeds from landing in the garden, you still got a big flush of weeds the next year? This is called the soil’s seed bank, and it’s due to dormant seeds that are resting in the soil. Every year a percentage of those seeds lose their dormancy and germinate, leaving you wondering in frustration whether the weeding will ever end.