The Physiology of Flowering Plants - KHAM PHA MOI

106 MINERAL NUTRITIONIt also is needed for normal membrane function. In B-deficient roots,ion uptake capacity deteriorates but when such roots are suppliedwith B, recovery is considerable by 20 minutes and complete withinan hour. Such fast action suggests a primary action at the membranelevel, B either affecting membrane permeability or acting on membrane-boundenzymes. There is also some evidence for B affectingenzymes of auxin and ascorbate metabolism. The greatest demandfor B is during the reproductive phase; in the absence of B, pollengrain formation fails, and pollen tubes germinating in the absence ofB swell and burst.Regarding the beneficial elements, individual species differ intheir requirements. Sodium benefits many species, being able tosubstitute to some extent for K. Rubidium and strontium alsoprobably owe their beneficial effect to an ability to replace some ofa plant’s requirements for K and Ca respectively. Rubidium enhancesgrowth most markedly in K-deficient media. Aluminium is morelimited in its beneficial effects; it is beneficial to tea (Camellia sinensis),Fallopia sachalinensis, and a number of grasses. Selenium is accumulatedin large amounts by some ‘accumulator species’ e.g. certainspecies of Astragalus growing in Se-rich soils, also by Lupinus albus andPhleum pratense; it affords some protection against insect attack andprotects against toxicity from excess Pi. The beneficial effects ofsilicon have already been discussed in connection with its essentialityfor some species.Complex interactions take place between mineral elements andmetabolism over and above the primary roles of the minerals. Severalexamples of interactions between mineral supply and growth hormonemetabolism have been reported. In sunflower plants, deficiencyof N, P or K in the rooting medium has been found to decrease the flowof cytokinin hormones from the roots to the shoots. Macronutrientdeficiency can thus act on plant development not only through directshortage of elements, but via the hormone supply. The biosynthesis ofthe gaseous hormone ethylene is promoted by Ca 2+ ions. Ethylene actsantagonistically to the hormone auxin in a number of effects. ThusCa 2+ antagonizes auxin by promoting ethylene biosynthesis. Cobaltinhibits the biosynthesis of ethylene and hence Co salts are used toprolong the life of cut flowers, ethylene being normally produced bythe flowers and promoting senescence. More such interactions havebeen reported and undoubtedly more still remain to be noted.4.3 Ion uptake and transport in the plant4.3.1 Ions in the soilWith the exception of C, H and O, which are derived from water andCO 2 and are incorporated by photosynthesis as described in Chapter 2,plants acquire all other elements as inorganic ions. Even C can beobtained as the carbonate or bicarbonate ion. Organic nitrogenouscompounds can act as the N source, but normally are available to