<P> Ratio fluorescence emission can be used to measure chlorophyll content . By exciting chlorophyll "a" fluorescence at a lower wavelength, the ratio of chlorophyll fluorescence emission at 705 nm + / - 10 nm and 735 nm + / - 10 nm can provide a linear relationship of chlorophyll content when compared to chemical testing . The ratio F735 / F700 provided a correlation value of r 0.96 compared to chemical testing in the range from 41 mg m up to 675 mg m . Gitelson also developed a formula for direct readout of chlorophyll content in mg m . The formula provided a reliable method of measuring chlorophyll content from 41 mg m up to 675 mg m with a correlation r value of 0.95 . </P> <P> In plants, chlorophyll may be synthesized from succinyl - CoA and glycine, although the immediate precursor to chlorophyll a and b is protochlorophyllide . In Angiosperm plants, the last step, the conversion of protochlorophyllide to chlorophyll, is light - dependent and such plants are pale (etiolated) if grown in darkness . Non-vascular plants and green algae have an additional light - independent enzyme and grow green even in darkness . </P> <P> Chlorophyll itself is bound to proteins and can transfer the absorbed energy in the required direction . Protochlorophyllide occurs mostly in the free form and, under light conditions, acts as a photosensitizer, forming highly toxic free radicals . Hence, plants need an efficient mechanism of regulating the amount of chlorophyll precursor . In angiosperms, this is done at the step of aminolevulinic acid (ALA), one of the intermediate compounds in the biosynthesis pathway . Plants that are fed by ALA accumulate high and toxic levels of protochlorophyllide; so do the mutants with the damaged regulatory system . </P> <P> Chlorosis is a condition in which leaves produce insufficient chlorophyll, turning them yellow . Chlorosis can be caused by a nutrient deficiency of iron--called iron chlorosis--or by a shortage of magnesium or nitrogen . Soil pH sometimes plays a role in nutrient - caused chlorosis; many plants are adapted to grow in soils with specific pH levels and their ability to absorb nutrients from the soil can be dependent on this . Chlorosis can also be caused by pathogens including viruses, bacteria and fungal infections, or sap - sucking insects . </P>

In plants primary growth occurs at a growing center called a/an