120 MassaGGiet al.

mays L) is one of the most important crops, noi only inthe tropics bui also in more temperate zones. Whencultivated in temperate regions, maize plants musi copewith low temperatures, particularly during the earlygrowth stages. For this reason, maize has been a favouritemodel system far scientists who have investigated planisusceptibility to low temperature.

The exposure of maize leaves to temperatures lowerthan 16°C has been seen to cause a wide range ofphysiological effects including a decrease of nei photosyn-thesis, stomatal conductance and mesophyll conductanceto CO2 (Long et al., 1983; Jahnke et al., 1991), bui alsoan increase of stomatal conductance and a consequentwater stress in low-temperature-sensitive varieties(Mustardy et al., 1982); the stimulation of ABA forma-tion which triggers stomatal closure and limits water lossin low-temperature-resistant varieties (Capell andDorffiing, 1993); and changes in the activity of enzymesof photosynthetic carbon metabolism (Stamp, 1987).

At a photochemical level, exposure of maize leaves tolow temperature has been reported to cause a reductionof the quantum yield of CO2 assimilation (Baker et al.,1983), a reduction of the fluorescence emitted from dark-adapted samples (Hetherington and Oquist, 1988), theonset of photoinhibition as indicated by changes in fluor-escence parameters (Greer and Hardacre, 1989) and bythe negative effect on the photochemical properties withthe simultaneous exposure to low temperature and highlight (Greer and Hardacre, 1989; Nie et al., 1992), areduction of some chloroplast proteins (Robertson et al.,1993), bui an increase of a 31 kDa polypeptide in maizechloroplast thylakoids (Covello et al., 1988), chlorophyllbleaching and leaf chlorosis (Taylor and Craig, 1971).

It has been suggested that, when carbon metabolism isreduced by low temperature, the energy absorbed by theleaf is in excess and cali drive the accumulation ofsuperoxide and hydroxyl radicals (Long, 1983). If theradicals afe noi effectively removed by the oxyradicalscavengers, they may damage the photosynthetic mem-branes and eventually cause photoinhibition (Satoh andFork, 1982;Krause and Cornic, 1987). Superoxide dismu-tase and ascorbate peroxidase catalyse the process ofdetoxification of oxygen radicals through the formationand removal of H2O2. The reactions driving towards theremoval of H202 have been described in detail (Bowleret al., 1992; Foyer et al., 1994). Briefly, H202 reacts withascorbate yielding monodehydroascorbate and H20.Monodehydroascorbate may either directly regenerateascorbate by taking electrons from the linear electrontransport or be oxidized to dehydroascorbate. Dehydro-ascorbate regenerates ascorbate by reacting with reducedglutathione. Carotenoids may also scavenge the excessradicals allowing the de-excitation of singlet oxygen ,andof chlorophyll molecules (Siefermann-Harms, 1987).Photoprotection of carotenoids may be associated with

mechanisms of chlorophyll de-excitation through thexanthophyll cycle (Demmig-Adams and Adams, 1992).

Plants growing at low temperature musi closelyco-ordinate metabolism, photochemical activity andenergy dissipation mechanisms. The activity of the oxy-radical scavenger enzymes and the concentration of carot-enoids frequently increase during acclimation to lowtemperature (HiiJlgren and Oquist, 1990; Badiani et al.,1993). It is likely that plants cali effectively cope withlow temperature only when these regulatory mechanismsafe in balance. Maize genotypes with resistance to lowtemperature may have developed effective regulatorymechanisms. Jahnke et al. (1991) have shown that theactivities of superoxide dismutase and ascorbate peroxid-ase sampled at 19 °C and 5 °C were higher in leaves ofZea diploperennis (a low-temperature-resistant species)than in leaves of Zea mays. However, this experimentwas carried out on plants grown at 25 cc. When plantswere exposed to low temperature far 5 d the activity ofascorbate peroxidase was similar to that observed at 25 °Cboth in Zea diploperennis and Zea mays.

To test if resistance to low temperature is associatedwith the development of regulatory mechanisms, twomaize genotypes selected far resistance and sensitivity tolow temperature (VA-36 and A-6l9, respectively) weregrown at low temperature and at a more optimal temper-ature. We present results indicating that the electrontransport rate is in excess of the photosynthetic require-meni of A-6l9 leaves grown at low temperature. We alsosuggest that the excess of reducing power damages thephotosynthetic apparatus of A-619 leaves grown at lowtemperature because the activity of the hydroxyl radicalscavenger ascorbate peroxidase and the amounts of caro t-enoids afe low.

Materials and methods

Plantmaterialand growth conditionsSeeds of two maize genotypes, A-619 and VA-36, were p1antedin 21 pots containing l/l/l, soi1/sand/peat. Plants were grownat CNR (Rome, Italy) in two greenhouses which weremaintained at different temperatures. Contro I plants were grownat a day/night temperature of 25/20 cC; plants exposed to lowtemperature were grown at 16/14 cc. With the exception oftemperature, alI of the other environmental parameters werethe same in the two greenhouses. Plants were watered threetimes a week to soil saturation and regularly fertilized. At leastten plants of each genotype were grown in each greenhouse andwere used during the experiments. Measurements were carriedout on the last fully expanded leaf 25 and 50 d after seeding.

Leaf absorbance measurements

Quantitative measurements of leaf reflectance and transmittancewere obtained using a LiCor 1800 portable spectroradiometerand a LiCor 1800-12 integrating sphere (LiCor, Lincoln,Nebraska, USA). The light absorbed by the leaf was calculatedmaking the difference between the total incident light and the