Light is the energy resource of plants captured through the process of photosynthesis, but it also affects plants in myriad other ways. The light affecting photosynthesis is expressed as photosynthetic photon flux density (PPFD) as mmol photons m~2s~\ PPFD at sea level on a bright summer day is approximately 2000 mmolm ~2s~\ Alternatively, beneath a forest canopy the PPFD may be 5-50 mmolm ~2s~\ Most plants demonstrate maximum photosynthetic rates with PPFD between 150 and 500 mmolm ~2s~\ At lower irradiances, light is limiting to photosynthesis and plant growth. Plant species with high irradiance requirements are called sun plants, while those with lower light requirements are shade plants. Sun plants can photosynthesize at greater rates, but are often unable to maintain themselves under low-light environments. Because light is largely directional, competition for light is caused by differences in vertical plant size and horizontal distribution of leaves.
Plants also differ with respect to their tolerance to the periodicity of light. In virtually any canopy environment, some light passes through the canopy relatively unaffected by leaves and branches. This creates a light fleck (or sunfleck) on the surface below it. These light flecks differ considerably in their duration and energy content. In a forest understory the proportion of light received in these light flecks approaches 50% of the total daily irra-diance received, but each light fleck lasts only a few seconds. The photosynthetic apparatus of plants requires the prior presence of light to react quickly to a light fleck, a phenomenon called photosynthetic induction. Understory plants have a low irradiance requirement to remain induced, while sun plants require higher light to maintain induction. These differences contribute to the ability of plants to survive in the understory.
Another response to periodicity reflects changes in day length accompanying seasonal changes in temperate regions. For many plants the length of the photoperiod controls flowering. Short-day plants will initiate flowering when day length gets shorter than some critical period. Alternatively, long-day plants will flower when day length exceeds a critical period. This ability to time flowering enables plants to complete the reproductive cycle before some adverse environmental change would limit their activity. In the prairies of North America grasses must initiate flowering early enough to complete their reproductive cycle before the first killing frost. For the same species in the north, flower initiation will occur in June, while in the south flowers are initiated in October. The two populations are ecotypes, and their differences in flowering time are maintained when they are grown together in the same environment. Light period provides a cue to how long the remaining growing season will be.
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