Tens of millions of plants are killed by and for human beings every year. They are dug up, cut down, burned, poisoned, mutilated, starved and deprived of water and sunlight for the sole purpose of benefiting humankind. While this may be necessary for the continuance of the species, it is done without regard for the plants, without conscience and without consciousness.
To anyone who believes that life itself has some purpose, or is even its own reason for being, the wanton destruction of plants is inappropriate. The destruction of any life is not to be taken lightly, or presumed to be isolated in the scheme of things. It requires careful consideration of the responsibilities and alternatives. Some vegetarians therefore prefer to eat fruits and vegetables only at the peak of ripeness, when a cycle of the plant's life has been completed.
It is a fact of life that humanity's present state of evolution calls for the eating of plants, whether directly or indirectly, in order to ensure survival. Until nutrition can be converted directly from sunlight, it will have to be taken from the plant kingdom in order to do the least amount of harm to sentient beings.