Study On The Relationship Between Maize Photosynthesis And The Zinc Content And The Carbonic Anhydrase Activity Of Its Leaves In Karst Areas

Maize is a gramineae, annual plant which belongs to Zea mays L. It is one of the mostimportant crops cultivated in the world, with its total yield only lower than wheat. As the maingrain crop, the feed crop or the industry processing raw material crop, maize has importantfunction, which can not be substituted in the global economic development. The sown area ofmaizeinGuangxi is 400000hm2 each year,onlylowerthan that of sugar cane. Therefore,maizeplaysakeyroleinprovidingthepopulationwithfood.Raisingthequalityandperunitarea yieldassoonaspossibleisanurgenttaskinreleasingpovertyinkarstmountainousareas.The photosynthetic efficiency is used as the probe in revealing the running mechanisms ofphotosynthetic organs. It is also a basic determiningfactor of the plant productive forces and thecrops output. To improve photosynthetic efficiency is a core topic of new"green revolution",which can make contribution to solving the problems of grain, resources and environment thehumanfacesfor.The carbonic anhydrase (CA, the EC 4.2.1.1) containing the ion of zinc, a kind ofmetalloenzyme, exists widely in plant, animal and microorganism. CA can catalyze theconversion reaction between CO2 and the HCO3-. CA showed tremendous influence on thedissolution and precipitation of carbonate rocks, the element migration and vegetation's growth.Liu et al (1997) found that after adding CAinto the experimental solutions, the dissolution rateof limestone was enhanced by a factor of about 10 under the condition of higher CO2 partialpressure, while it is a factor of about 3 for the dolomite dissolution, but under the lower partialpressure of CO2. CA is also an important photosynthetic enzyme, which can reduce diffusionalresistance of CO2 in the mesophyll cell. It provides also the substrate for the carboxylation andadjuststhephotosynthesis.This study aimed at exploring the function of zinc and the carbonic anhydrase in maizegrowth processes in the karst soil. Through the change in zinc content and the carbonicanhydrase activity of maize, the changes in photosynthetic efficiency of the maize leaf, theoutput and the overground biomass of the maize were investigated. An instrument Li-6400 was used to determine the photosynthetic characteristic. Li-6400 may also give the light quantumflux density(PAR,μmol/m2?s), atmospheric CO2 density(Ci,μL/L) and photosynthetic fluxofleaf(Pn,μmol/m2?s).Theresultsshowedthat:1. The stomatal conductance, transpiration, and the effective radiation all influenced thephotosynthetic flux. On sunny days, from the early morning at 7:00 the photosynthetic fluxincreased with the temperature gradually. At about both 11:00 and 14:00 the photosynthetic fluxreachedpeakvalues. Betweenthe2peaks was alowvalue,whichwas so-called"thenoonbreakof plant"in literature. The maize stomatal conductance (Cond) and transpiration rate (Tr) are allhigher under conditions with muck or zinc fertilizer spreading, showing that the photosynthesisandgrowthofmaizewereraisedbyzinc.2. At the first flush stage of maize growth, the zinc content of maize leaf was onlybetween15mg/kg and 45mg/kg, while it was between 40mg/kg and 50mg/kg at the full flush stage ofmaize growth. All showed that the zinc content of maize leaf on the soil with zinc fertilizerspreading was higher than that on the soil with muck fertilizer spreading. However, at the laststage of maize growth, the zinc content of maize leaf was generally below 10mg/kg. The zinccontent ofhealthyleafshouldbebetween15mg/kgand100mg/kg.Therefore,there appearedthephenomenon of zinc deficit at the last stage of maize growth. Related to the content of zinccontent, CAactivitywas between0.3U/mgand 2U/mgat the first flush stage of maize growth; atthe full flush stage, CA activity was between 2 U/mg and 7U/mg; and at the last growth stage,CAactivitywasthelowest,beingabout0.2U/mg.3. At the full flush stage of maize growth, the CAactivitywas the highest of all, and so didthephotosyntheticflux,beingbetween22.11and31.06μmolCO2m-2s-1.Atthefirstflushstageofmaize growth, the photosynthetic flux was between 16.79 and 19.98μmolCO2m-2s-1,corresponding to higher CA activity. At the last stage of maize growth, the photosynthetic fluxwas the lowest, being between 10.69 and 13.21μmolCO2m-2s-1, corresponding to the lowest CAactivity.4. On the cultivated land of karst area with zinc fertilizer spreading, the yield and theoverground biomass of maize increased by 11.5% and 15.0% respectively, as compared to 5.3%and6.6%respectivelyfornon-karstsoil.Onthefieldofkarstareawithmuckfertilizerspreading,the yield and the overground biomass of maize increased by 15.2% and 17.8% respectively, ascompared to 7.9% and 8.2.% respectively for non-karst soil. These results showed that thepotential to increase production by improving karst soil is larger than that by improvingnon-karstsoilwithafactorofabout2.In a word, the study provided sound scientific basis for improving karst soils, and thus for raisingtheyieldandqualityofmaize.