Air temperature is routinely measured in many different scenarios such as in wheatear stations, greenhouses or growth chambers and temperature sensors are not expensive and rather easy to implement. However, temperature sensors may often be directly affected often by the incoming short and long wave radiation which may substantially heat or cool the sensor. Thus, to ensure that the temperature of the surrounding air is the same as the temperature measured by the sensor, the sensor has to be shaded from sunlight and it should be exposed to adequate ventilation. In meteorology, ventilated radiation shields are standardized to enable a high level of comparability. In plant sciences, temperature measurement is not well defined and it is becoming increasingly important to standardize this very basic environmental factor to enable comparability between experiments at different locations in plant experiments.
Plant surface (meristem)
Measurement of the plant (leaf) surface is frequently monitored to assess the temperature dependent processes. Leaf temperature directly affects water fluxes by influencing the vapor pressure deficit between the leaf and surrounding air and thus driving the evaporative demand (>> section VPD). Additionally all biochemical processes are temperature dependent and since the plant, leaf or meristem temperature may be different then the air temperature direct measurement of plat surface temperature should be monitor. In practical terms, the time needed for organ development or between developmental stages is dependent on temperature and can be related to cumulative heat (>>thermal time). Thermal time used in this way has an advantage over the use of time after seeding or germination of a plant. It is important to adequately measure the organ temperature and to take into account that the surface temperature may be influenced by other environmental factors, such as light. Thermal time is a sum of cumulative differences between mean temperature between different time intervals and a speciﬁed base temperature described with units of degree-days (°C days). This relation is approximately linear within an optimal developmental temperature and it is lost when the plant response to temperature is beyond the optimum (very high or low temperatures) which results in a curvilinear model. (For more details see Sadok et al 2007, Trardieu 2013).
How to measure the meristem temperature in maize:
Sadok, W., et al. (2007) Leaf growth rate per unit thermal time follows QTL-dependent daily patterns in hundreds of maize lines under naturally fluctuating conditions. Plant, Cell & Environment 30: 135-146.
Tardieu, F. (2013). Plant response to environmental conditions: assessing potential production, water demand, and negative effects of water deficit. Frontiers in Physiology 4: 17.