Response of anatomical structure and photosynthetic characteristics to low light stress in leaves of different maize genotypes

Prolonged rainy weather during the maize growing season can lead to insufficient light,and thus dramatically lower yields.Maize,Zea mays L.,is one of the most important food crops in China,and around the world,and a high and stable yield is necessary to guarantee food safety.Previous studies into the effects of low light stress on maize have predominantly focused on aboveground biomass,leaf area,chlorophyll content,net photosynthetic rate,root activity,soluble protein content,grain yield,nutrient levels,and the activity of some enzymes,including superoxide dismutase(SOD),peroxidase(POD),catalase(CAT),sucrose-phosphate synthase(SPS),sucrose synthase(SS),soluble starch synthase(SSS),granule-bound-starch synthase(GBSS),adenosine diphosphate glucose pyrophorylase(ADPGPPase),and uridine diphosphate glucose pyrophorylase(UDPGPPase).However,the potentially damaging effects of low light stress on the anatomical structure of maize photosynthetic systems have seldom been mentioned in the literature,and the relationships between anatomical structure and photosynthetic characteristics remain unclear.Our objective was to determine the effects of low light stress on the anatomical structure of maize photosynthetic systems,and the relationship between photosynthetic characteristics and anatomical structure,in different maize cultivars at the seedling stage. We experimented on the seedling leaves of two maize cultivars: the shade-tolerant cultivar Zhengdan 958(ZD958) and the shade-sensitive cultivar Yuyu 22(YY22).These were grown in experimental pots in the field.50% shade treatment was used,with non-shaded seedlings as controls.Under low light stress,plant growth was inhibited;leaves became thinner,numbers of chloroplasts per unit of leaf area decreased,number of grana increased,grana thickness increased,and the thickness of the grana lamellae increased.In the shade-sensitive cultivar YY22,most of the chloroplasts became dysplastic,the stroma lamellae swelled and the grana lamellae became loose.In contrast,in the shade-tolerant cultivar ZD958 the chloroplasts were almost normal with some healthy parts.The net photosynthetic rate(Pn),photochemical efficiency(Fv/Fm),and actual photochemical efficiency(ΦPSⅡ) all decreased under shade.Chlorophyll content,intercellular CO2 concentration(Ci) and non-photochemical quenching(NPQ) increased;however these were all reversible.The values of all variables investigated were significantly higher or lower in the shade-sensitive cultivar YY22 than in the corresponding control.Shading had no significant influence on the shade-tolerant cultivar ZD958 except for Pn and Ci.We speculate,therefore,that low light stress has different significant effects on the anatomy and photosynthesis of maize with different genotypes.The shade-tolerant cultivar ZD958 maintains more normal anatomical structure and higher photosynthetic function than the shade-sensitive YY22 under low light stress;these are the major anatomical and physiological traits necessary for shade tolerance.