This is based on my personal experience of having grown many different annual and perennial plants over the years.
The first growth (that is when the seed sprouts and produces the cotyledons, two flat oval leaves) is proportional to the size of the seed and the intensity of the light available. The roots starts establishing and then the plant produces a true leave or a pair of true leaves (which would in your case have the shape of an apple leave).
The length, the diameter and the colour of the shoot tell you right away if there is enough light.
If it is white, light is missing. If it's green, your doing all right.
Outdoors, the cotyledons of a cucumber, for instance, will be almost flat to the ground, the stem will be stout and very green.
Under sufficient but artificial light, the cotyledons of the same can be as much as an inch from the ground sometimes, but still needs to be green.
Whitish shoots gets leggy and fall over.
The light available will determine the strength of the stem and overall success, and I would think you need14 hours of direct light, natural or artificial, or a combination of both for ideal growth.
Otherwise you get a long shoot with not strenght.
Québec, the province, north of Ottawa
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