Think of a plant's relationship with light as being a pair of bookends, with one bookend being the plant's LCP (light compensation point) and the other being light saturation point. The LCP varies by species and occurs when there is enough light to synthesize (via photosynthesis) exactly the same amount of food (sugar/ glucose) as the plant 'burns' during the respiration phase. Plants need to produce more energy than they use to keep their systems and processes balanced and orderly in order to grow and produce fruit. Just as a flashlight "dies" when the energy stored in its batteries is depleted, plants unable to create at least as much energy as they use during respiration will die unless the trend can be reversed by amending cultural conditions to enable creation of more food/energy.
True growth is measured by the plant's increase in dry mass. When the amount of food/energy created during photosynthesis is the same as that burned during respiration are the same, the formation and consumption of organic matter are equal, and dry matter cannot be accumulated. The plant might show conspicuous branch/stem extension and perhaps some new leaves/branches) but if plants are shedding as much dry mass as they are creating, they are not "growing" in the true sense of the word.
The other half of the 'bookend' pair, as mentioned, is the light saturation point. For any given plant and light wavelength, the rate of photosynthesis increases until reaching the light saturation point, at which the photosynthetic rate reaches its maximum (flatlines), and where additional photo photo load produces no increase in or a negative effect on photosynthesis.
Also in play is how the plant is genetically programmed to supply its various energy sinks with food/energy. An energy sink is simply a part of the plant that requires energy input, and all energy sinks are not equal, they have a predictable pecking order. When plant parts call for food/energy, the primary allocation goes to respiratory function, i.e. to maintenance of living tissues, then, to production of fine roots, followed by flower and seed/fruit production, then primary growth (extension of both roots and shoots), then secondary growth (thickening), and finally, the synthesis of defensive chemicals. From this, we can see that the plant must have created enough energy to satisfy the energy needs of its living tissues and growth of new roots or replacement of roots damaged or killed by poor conditions in the rhizosphere before it can produce reproductive parts (blooms, fruit/ seeds).
Al
