The temperatures at which most physiological processes in plants go on normally is species depended. The temperature range in which plants can grow varies from approximately 0 °C to 45 °C. Some tropical species require a minimal temperature (Tb) higher than 10 °C. There are also plant species which require a low temperature before they are able to germinate or flower, this phenomenon is named vernalization (from Latin vernus, "of the spring"). Vernalisation temperatures are typically between 5 and 10 °C, but some species require a vernalisation temperature below 0 °C. Temperature has a strong influence on the rate of plant development. The plant response to temperature in terms of developmental rates from Tb onwards is almost linear until an optimum (To) and then de decreases until a maximum or ceiling temperature (Tc) (Figure 5.1). Each organ or growth stage of a given plant can have a separate response to temperature, i.e. Figure 5.1 can be drawn for germination, leaf initiation, flowering etc. However, within one cultivar these processes are quite similar. Aside from effects on development rate, sub- and supra optimal temperatures can also have directly damaging effects on the photosynthesis apparatus, assimilation speed, cells, proteins and DNA. Supra optimal temperature (range To to Tc) for example generate nutrient and hormone imbalances, protein misfolding and loss of radical scavenging activity. Not only the absolute temperature but also the fluctuation in temperature can have profound effects on plant growth and development, i.e. difference in day and night temperature can strongly influence plant length.
A complicating factor temperature management are differences in: air, substrate and plant organs, i.e. leaf, stem, flower, seed and roots and most importantly the meristem (growing tip of that organ) temperature. In advanced horticulture systems the root and shoot temperatures can be controlled separately. This is important for some crops to optimise the uptake rate of nutrients in relation to the growth speed of the shoots and to control the root to shoot ratio in terms of biomass. Controlling root temperature can also regulate the root pressure (upwards pressure of water through the vascular tissue), cooling can be important if high root temperature create high water pressure in the plant. Especially when leaf transpiration is hindered this high pressure can cause cell damage in plant tissue. Additionally, the night temperature in cultivation systems is often kept lower than the day temperature to reduce respiration rate at night when plant do not photosynthesize.
Figure 5.1: Schematic graph of the relation between temperature and plant developmental rate. For one plant each organ or growth stage can have a separate response to temperature, i.e. there is this graph for germination, leaf initiation, flowering etc. These processes are however within one plant variety quit similar.
Some examples of the impact of temperature on the nutritional value of crops
Supra optimal temperatures evoke stress in plants. Proteins function sub optimally and created reactive oxygen radicals (ROS) are not properly neutralised. In response plants manufacture more anti-oxidants. High temperatures (25/30°C) enhance antioxidant activity in strawberry, as well as anthocyanin and total phenolic content, compared with cool day and night temperatures (18/ 12°C) (Wang and Zheng 2001). Similarly, higher anthocyanin levels in grapes and apple skin are usually attributed to the combination of cool night temperatures and high day temperatures caused by high levels of irradiance during ripening. However, if temperatures exceed species and substance dependent thresholds biosynthesis of theses antioxidants will also decrease or stop. For example, in tomato fruit lycopene production is inhibited at temperatures below 12 °C and above 32 °C (Kläring et al. 2015) .
Not only absolute but also relative differences in night and day temperature are important for e.g. carotenoid synthesis and lycopene in particular (Dumas et al. 2003).
According to Cabañero et al. (2004) root zone temperatures of 35 °C resulted in higher calcium uptake by pepper plants and consequently in mitigation of negative salt effects and blossom end rot incidence in fruit. Yet, in tomato’s root temperatures higher than 20 °C have negative effects on fruit quality. In contrast for fast growing vegetables high temepratures can have beneficial effects(Adams 1999). In the same line, increasing nutrient solution temperature from 12 to 20 °C increased water flow through the stems of tomato plants by 250% which results in higher nutrient uptake and better fruit quality as shown by Kafkafi (2001) and Tindall et al. (1990) . Moreover, the combined effect of elevated air temperatures and low radiation are shown to increase glucosinolates, ascorbic acid and lutein content of greenhouse broccoli Schonhof et al. (2007).
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