Introduction Low temperature is one of the most important factors limiting the spread and production of plants worldwide. This is especially true for field crops of tropical or subtropical origin. In the case of chilling-sensitive maize plants, temperatures in the 10–15°C range decrease the capacity for biomass production, while the exposure of plants to still lower temperatures for a prolonged period may lead to irreversible damage and the death of the plants (; ). Efficient early germination and growth at cool temperatures in the spring is a critical part of resistance to low temperature stress in young maize plants.

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One of the main aims of breeders is to develop cold-tolerant genotypes, which can be sown either earlier in order to extend the vegetation period, or in cooler geographical zones. Xander for blackberry. Although chilling-sensitive plants are generally considered to lack the ability to develop chilling resistance when exposed to low but non-injurious temperatures, they can also be able to adapt to lower, usually non-freezing temperatures to some extent (;; ). The mechanisms of cold acclimation are mainly studied in overwintering cereals, where a certain period of low, non-freezing temperature is necessary to achieve the maximum level of freezing tolerance even in the case of frost-tolerant winter varieties.

This process is generally called frost hardening or cold hardening. Cold hardening is the result of various physical and biochemical processes, including the adjustment of membrane composition and the accumulation of certain protective substances, among them stabilising compounds such as polyamines, osmoprotectants or antioxidants (;,).

The synthesis of these compounds and the regulation of acclimation mechanisms are mediated by a complex signal transduction network (). However, it has long been known that light is necessary for the development of freezing tolerance as well as low temperature. Without enough light during the cold hardening period even winter cereals with a potentially high level of frost hardiness are incapable of achieving freezing tolerance (; ). Light has been shown to mediate the development of freezing tolerance via several biological processes. These include photosynthesis-related processes, the expression level of stress-related genes and the synthesis of various protective compounds (). A strong overlap has been shown between factors involved in sensing and transducing light and temperature in plants (, ). Acclimation to low temperatures also respond to light and temperature signals (; ).

In hardy plants light is a critical factor during the low temperature hardening period, but in chilling-sensitive plants light often shows another face during exposure to low temperature inducing photoinhibition, which contributes to the development of chilling symptoms in these plants (). Photoinhibition is a light-induced decline in photochemical activity and occurs when the light energy available exceeds the receptive capacity of the photosynthetic processes and the level that can be neutralised via different protective mechanisms (recently reviewed by ). The mechanisms underlying the development of freezing tolerance during the hardening period of model plants, such as Arabidopsis, or cold-tolerant cereal species, such as wheat or oat, have been widely studied, but these results cannot be generalised for the very chilling-sensitive C4 plants.

In recent years several cold-responsive genes have been identified in maize, but these studies were usually focused either on the comparison of genotypes with different levels of chilling tolerance (,; ) or on the different phases of cold stress responses (), and the role of light has not been discussed. In the present work we hypothesised that light, in spite of its photoinhibitory effects, may also have a similar role in the cold acclimation processes in chilling sensitive plants, similarly as it was found in cold tolerant winter cereals or in Arabidopsis. Experiments were designed in order to characterise the contribution of light during the cold acclimation period to the development of a certain level of cold tolerance in maize plants. A detailed microarray study was also carried out, and certain physiological, biochemical and genetic factors contributing to cold tolerance were tested.

Results may point out the possible involvement of low temperature-induced photoinhibition in the development of cold tolerance as a signal during the cold acclimation period in chilling sensitive plants. Materials and Methods Plant Material and Growth Conditions In the 1st set of experiments seeds of maize plants ( Zea mays L. Hybrid Norma) were germinated between wet filterpapers for 3 days then grown in a modified Hoagland solution () for a week at 22/20°C at a photosynthetic photon flux density (PPFD) of 180 μmol m −2 s −1 (growth light 1; GL1) with 16/8 h light/dark periodicity (control plants). The plants were then hardened at 15/13°C for 3 days under three different light conditions: GL1; low light intensity 1 (LL1): 46 μmol m −2 s −1; and low light intensity2 (LL2): 14 μmol m −2 s −1). After this the plants were transferred to 5°C at continuous GL1 for 3 days, and then back to 22/20°C for a 1-day recovery period. Free cracks and serial keys.