Measurements of the quantum efficiencies of photosynthetic electron transport through photosystem II ( PSII ) and CO 2 assimilation ( CO 2 ) were made simultaneously on leaves of maize (Zea mays) crops in the United Kingdom during the early growing season, when chilling conditions were experienced. The activities of a range of enzymes involved with scavenging active O 2 species and the levels of key antioxidants were also measured. When leaves were exposed to low temperatures during development, the ratio of PSII / CO 2 was elevated, indicating the operation of an alternative sink to CO 2 for photosynthetic reducing equivalents. The activities of ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and superoxide dismutase and the levels of ascorbate and ␣-tocopherol were also elevated during chilling periods. This supports the hypothesis that the relative flux of photosynthetic reducing equivalents to O 2 via the Mehler reaction is higher when leaves develop under chilling conditions. Lipoxygenase activity and lipid peroxidation were also increased during low temperatures, suggesting that lipoxygenasemediated peroxidation of membrane lipids contributes to the oxidative damage occurring in chill-stressed leaves.Exposure of maize (Zea mays) crops to low temperatures during the early growing season in temperate regions results in depressions in photosynthetic productivity and canopy development (Miedema, 1982; Stirling et al., 1991; Baker and Nie, 1994). Chill-induced decreases in CO 2 assimilation in maize leaves are associated with inhibition of photosynthesis involving both increased dissipation of excitation energy in the PSII antennae and photodamage to PSII reaction centers (Ortiz-Lopez et al., 1990; Andrews et al., 1995; Fryer et al., 1995; Haldimann et al., 1996), decreases in the activities of Benson-Calvin cycle enzymes (Kingston-Smith et al., 1997), and poor development of the photosynthetic apparatus (Nie and Baker, 1991; Nie et al., 1992 Nie et al., , 1995.Under such environmental stress conditions, which reduce the capacity to assimilate C, it has been suggested that photosynthetic electron flux to O 2 will increase, resulting in the increased production of superoxide, H 2 O 2 , and hydroxyl radicals (Asada, 1996). These active O 2 species are extremely damaging to lipids, proteins, and pigments unless they are rapidly scavenged within the chloroplasts by a group of enzymes consisting of SOD, GTR, DHAR, MDHAR, and APX (Asada, 1996). There is some evidence, although not extensive, that increased levels of these scavenging enzymes may play a role in limiting the degree of photodamage experienced by maize at chilling temperatures (Jahnke et al., 1991; Massacci et al., 1995; Hodges et al., 1997).In maize leaves at normal growth temperatures, the relationship between photosynthetic electron transport and CO 2 assimilation is highly conserved over a wide range of light intensities and CO 2 concentrations and also during the induction of photosynthes...

The purposes of this study were to: (a) examine the differences within 11 specific kinematic variables and an outcome measure (ball velocity) associated with component developmental levels of humerus and forearm action (Roberton & Halverson, 1984), and (b) if the differences in kinematic variables were significantly associated with the differences in component levels, determine potential kinematic constraints associated with skilled throwing acquisition. Significant differences among component levels in five of six humerus kinematic variables (p <.01) and all five forearm kinematic variables (p < .01) were identified using multivariate analysis of variance. These kinematic variables represent potential control parameters and, therefore, constraints on overarm throwing acquisition.

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Wheat leaves were exposed to light treatments that excite preferentially Photosystem I (PS I) or Photosystem II (PS II) and induce State 1 or State 2, respectively. Simultaneous measurements of CO2 assimilation, chlorophyll fluorescence and absorbance at 820 nm were used to estimate the quantum efficiencies of CO2 assimilation and PS II and PS I photochemistry during State transitions. State transitions were found to be associated with changes in the efficiency with which an absorbed photon is transferred to an open PS II reaction centre, but did not correlate with changes in the quantum efficiencies of PS II photochemistry or CO2 assimilation. Studies of the phosphorylation status of the light harvesting chlorophyll protein complex associated with PS II (LHC II) in wheat leaves and using chlorina mutants of barley which are deficient in this complex demonstrate that the changes in the effective antennae size of Photosystem II occurring during State transitions require LHC II and correlate with the phosphorylation status of LHC II. However, such correlations were not found in maize leaves. It is concluded that State transitions in C3 leaves are associated with phosphorylation-induced modifications of the PS II antennae, but these changes do not serve to optimise the use of light absorbed by the leaf for CO2 assimilation.

The light-harvesting chlorophyll a/b proteins associated with PS II (LHC II) are often considered to have a regulatory role in photosynthesis. The photosynthetic responses of four chlorina mutants of barley, which are deficient in LHC II to varying degrees, are examined to evaluate whether LHC II plays a regulatory role in photosynthesis. The efficiencies of light use for PS I and PS II photochemistry and for CO2 assimilation in leaves of the mutants were monitored simultaneously over a wide range of photon flux densities of white light in the presence and absence of supplementary red light. It is demonstrated that the depletions of LHC II in these mutants results in a severe imbalance in the relative rates of excitation of PS I and PS II in favour of PS I, which cannot be alleviated by preferential excitation of PS II. Analyses of xanthophyll cycle pigments and fluorescence quenching in leaves of the mutants indicated that the major LHC II components are not required to facilitate the light-induced quenching associated with zeaxanthin formation. It is concluded that LHC II is important to balance the distribution of excitation energy between PS I and PS II populations over a wide range of photon flux densities. It appears that LHC II may also be important in determining the quantum efficiency of PS II photochemistry by reducing the rate of quenching of excitation energy in the PS II primary antennae.