Glasshouses are such an alternative and offer better control of
water supply and protection against too low temperatures.
Additional lighting in the glasshouse may ensure a minimal
daily irradiance and a fixed photoperiod while shade screens
can protect against high light intensities in summer (Max et al.
2012). Thus, greenhouses provide more buffered conditions for
growing plants; they often extend the period for experiments
(a strong economic argument) and to some extent, allow the
dissection of plant responses to different environmental
conditions. In practical terms, plants grown in glasshouses will
usually experience higher-than-outdoor air temperatures during
nights and winters and lower irradiance because of shading.
Most glasshouses have limited possibilities to reduce
temperatures during periods of strong solar irradiance in
summer. Air ventilation can control temperatures to some
degree and the same applies to the evaporative cooling effect
caused by the transpiration of a considerable amount of plants
present in the glasshouse. In many greenhouses there are
significant spatial heterogeneities in irradiance, due to shading
by the greenhouse structure itself and often also by neighbouring
buildings or other objects. Artificial lighting, although
compensating for temporal variation in external irradiance, can
also introduce new heterogeneities, including a marked vertical
gradient in irradiance intensity (see below). Overall, diurnal and
seasonal variation in growth conditions remain, but in a more
buffered way compared with the experimental garden