To change a plants Light settings go to the plants blueprint so for example the BP_TomatoPlant and select the BP_PlantComponent in the Components panel, then in the Details panel and inside the Plant Settings Category find the Light category like the image below.
Here you can change this plant’s Light settings, you can hover your mouse over each settings name to get a tooltip with more info about each setting.
You can enable or disable the Use Light setting to remove the plants need for Light.
Light update time controls how often the plant checks its current light value in seconds.
You can set the plant to take damage when it has no Light, this can be enabled or disable using the Use No Light Damage setting.
No Light Damage Time controls how often the plant will take damage in seconds and the No Light Damage is the amount of Damage the plant will take when it has no Light.
Light can effect how fast the plant grows and how fast the vegetables grow. This uses the Growth Multiplier setting, if set to 0 having Light wont effect the plants growth speed, if set to 0.5 the plant will grow 50% faster then its normal speed, this effect stacks with other stats grow multipliers.
For a plant to detect light from sources like the BP_BP_SmallGrowLamp the plants blueprint must have a BP_LightDetectorComponent, like shown in the image below. The ideal location for this component is just above the plants mesh like shown below.
If your plant doesn’t have this component use the add button in the components panel and search for BP_LightDetectorComponent then add the component, all the included plants already have this component setup so you don’t need to add it to them.