The Development of Chloroplast Ultrastructure and Hill Reaction Activity During Leaf Ontogeny in Different Maize (Zea Mays L.) Genotypes

Changes in Hill reaction activity (HRA) and ultrastructure of mesophyll cell (MC) chloroplasts were studied during the ontogeny of third leaf of maize plants using polarographic oxygen evolution measurement, transmission electron microscopy, and stereology. The chloroplast ultrastructure was compared in young (actively growing), mature, and senescing leaves of two different inbreds and their reciprocal F1 hybrids. Statistically significant differences in both HRA and MC chloroplast ultrastructure were observed between different stages of leaf ontogeny. Growth of plastoglobuli was the most striking characteristic of chloroplast maturation and senescence. The chloroplasts in mature and senescing leaves had a more developed system of thylakoids compared to the young leaves. Higher HRA was usually connected with higher thylakoid volume density of MC chloroplasts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Ultrastructure and Dimensions of Chloroplasts in Leaves of Three Maize (Zea mays L.) Inbred Lines and their F1 Hybrids Grown Under Moderate Chilling Stress

Influence of moderate chilling stress on vascular bundle sheath cell (BSC) and especially mesophyll cell (MC) chloroplasts of mature maize leaves was studied by electron microscopy and stereology. Plants of two inbred lines of maize, differing in their photosynthetic activity, and their F₁ hybrids were cultivated during autumn in heated or unheated glasshouse. Generally, chilling temperatures resulted mainly in the decrease in stereological volume density (VD) of both granal and intergranal thylakoids of MC chloroplasts, while the ratio of granal to all thylakoids (granality) was less affected. The VD of peripheral reticulum and plastoglobuli usually increased after cold treatment of plants. The volume of MC chloroplasts usually increased under chilling stress, the shape of the chloroplasts changed only slightly. The ultra-structure of chloroplasts differed between individual genotypes; chilling-stressed hybrid plants showed positive heterosis particularly in the granal thylakoids' VD of MC chloroplasts.     

Photosynthetic Parameters of Maize (Zea mays L.) Inbred Lines and F1 Hybrids: Their Different Response to,and Recovery from Rapid or Gradual Onset of Low-temperature Stress

The activity of photosystems (PS) 1 and 2, together with the content and ratio of photosynthetic pigments, were measured in three inbred lines and two F₁ hybrids of maize (Zea mays L.), grown in either optimum or low temperature (LT) conditions. The ability of chilling-stressed plants to deal with the negative effects of long-term exposure to LT and to recover the efficiency of photosynthetic apparatus after their return to optimum temperatures was examined during spring and autumn seasons. The aim was to analyse the possible differences between the rapid and gradual onset of LT on the response of young maize plants to chilling stress. The distinctive superiority of hybrids over their parental lines, found during the exposure of maize plants to LT, was not always retained after the return of chilling-stressed plants to optimum growth conditions. The response of individual genotypes to chilling stress, as well as their ability to recover the photosynthetic efficiency from the cold-induced damage, strongly depended also on the duration and the rapidity of the onset of LT.     

Genetically based differences in photochemical activities of isolated maize (Zea mays L.) mesophyll chloroplasts

Photochemical activity of isolated mesophyll chloroplasts was measured as Hill reaction activity (HRA) and photosystem 1 (PS1) activity in three diallel crosses of maize (Zea mays L.) inbred lines and F₁ hybrids. Statistically significant differences between genotypes together with positive heterotic effect in F₁ generation were found for both traits studied. These differences were more pronounced when HRA or PS1 activity was expressed per leaf area unit or dry matter unit compared to the expression per chlorophyll content unit. Analysis of variance showed that both the genetic and non-genetic components of variation in the photochemical activity of isolated mesophyll chloroplasts are present in all three diallel crosses examined. The positive heterosis in F₁ hybrids probably arises from non-additive genetic effects of a positive dominance type. Additive genetic effects were also statistically highly significant. We found no differences between reciprocal crosses. This revised version was published online in September 2006 with corrections to the Cover Date.     

The Ultrastructure of Chloroplasts,Content of Photosynthetic Pigments,and Photochemical Activity of Maize (Zea mays L.) as Influenced by Different Concentrations of the Herbicide Amitrole

The effect of three different concentrations of amitrole (AM), a bleaching herbicide affecting carotenogenesis, on chloroplast ultrastructure, photosynthetic pigment contents, and photochemical activity was studied in two maize genotypes differing in photosynthetic characteristics. The content of photosynthetic pigments in leaves of plants treated with low (20 M) AM concentration was similar to control plants and no damaging effect of the herbicide on the ultrastructure of either mesophyll (MC) or bundle-sheath (BSC) cell chloroplasts was observed. Higher (60 and 120 M) concentrations of AM caused a significant decrease in the content of carotenoids (especially xanthophylls), which was followed by photooxidative destruction of chlorophylls and some alterations of chloroplast ultrastructure. MC chloroplasts appeared more sensitive to the damaging effect of AM compared to BSC chloroplasts. A significant decrease in the amount of both granal and intergranal thylakoids in MC chloroplasts was observed with the increasing concentration of AM. As regards BSC chloroplasts, rapid decrease in the volume density of starch inclusions was found in plants treated with higher concentrations of AM. When 120 M AM was used, both MC and BSC chloroplasts contained just a few thylakoid membranes that were strongly altered. The changes in the ultrastructure of MC chloroplasts were accompanied by the changes in their photochemical activity. The formation of chloroplast protrusions after treatment of plants with AM as well as in control plants was also observed.