There are three components of nutrient and photosynthate transfer in plants that might respond to temperature in different ways: (1) transfer of metabolites across cell membranes, including the exchange between apoplasm (space external to cell membranes) and the symplasm (space contained by cell membranes); (2) cell to cell transfer (within the symplasm) via plasmodesmal connections, and (3) movement of metabolites over long distances through phloem sieve elements.
Selective transfer of metabolites across a membrane against a concentration gradient is an energy-requiring step involving respiratory activity and membrane-bound ATPase (see Figure 1 in Case study 2.1). This metabolically active process is responsive to temperature. Membrane transfer is associated with many physiological functions including movement of metabolites and photoassimilate loading within source leaves, exchange with storage tissues along the path of transport and photoassimilate loading into sinks. The response of long-distance photoassimilate translocation to low temperature varies widely between species. In many temperate Gramineae (including wheat) a drop in temperature along the path of transport to 1°C has no measurable effect on the translocation of photosynthate, but at this temperature in chilling-sensitive species such as bean and sorghum there is a marked reduction in the translocation of photosynthate. In some chilling-tolerant species such as sugar beet, there is an initial rapid decline in translocation following application of low temperature along the path of transport, but this inhibition is transient in nature, with translocation returning to normal in a number of hours. In species that are more sensitive to chilling such as squash there is some adjustment to low temperature, resulting in partial recovery. Such adjust-ments vary between ecotypes within the one species, for example in Canada thistle where behaviour is related to altitude and thus temperature in natural surroundings (Figure 14.14). Lowering the temperature of the transport pathway also reduces the lateral transfer of carbon into adjacent tissues, or alternatively the remobilisation of stored carbohydrate back into the transport system. By contrast, translocation can be relatively insensitive to temperatures up to 40°C, but will be inhibited by prolonged periods of temperature >40°C. Prolonging a high-temperature treatment, unlike the low end of the scale, does not result in recovery; rather, blockage intensifies.
The exact nature of the inhibition of long-distance transport at temperature extremes is still uncertain. Inhibition appears not to be energy related in a metabolic sense, but low-temperature effects may be due to a displacement of proteinacious material. This would lead to transient responses that reverse in time. More sustained responses under either low- or high-temperature treatments are likely to be due to a build up of callose (a β-1,3 glucan) in sieve-plate pores.
Whether reduced translocation is the cause of poor growth at extreme temperatures is often difficult to assess. This is partly because of the transient nature of the temperature response in many species and partly because the response of the transport system to temperature has been examined in isolation from other processes. In sorghum, where temperatures below 20°C effectively reduce translocation, this reduction is minor in comparison with the direct effect of the same temperature on growth. Current findings suggest that long-distance transport in the phloem does not have a direct role in regulating whole-plant responses to temperature.