Annals of Botany

In the heat of the night

Unravelling the complex network of factors that contribute to yield in crop plants has provided an ongoing challenge for physiologists and breeders. This is nicely illustrated by the work of Morita et al. (Chikugo, Morioko and Kukuyama, Japan, pp. 695–701). These authors have studied the effects of temperature on grain size in rice. Temperature regimes, applied from the heading phase onwards, were control (22 °C during day and night), high day temperature (HDT, 34 °C/22 °C) and high night temperature (HNT, 22 °C/34 °C). Grain growth rates were initially highest in the HDT treatment, intermediate in the HNT treatment and lowest in the controls, but growth stopped 5 days earlier in the experimental treatments than in controls. So an increased mean daily temperature leads to increased grain growth rate, more so when the higher temperature was applied in the day, but decreased growth duration. Further, final grain dry weight was approx. 10 % lower in the HNT treatments than in the HDT and control treatments. The earlier cessation of growth has been ascribed to earlier senescence of the sink organs, but we also need to ask about what is happening at the cellular level in the endosperm, which provides the bulk of grain weight. Measurements of endosperm cell numbers and size again indicated the complexity of the situation. Cell numbers in both HDT and HNT treatments were greater than in controls but similar to each other. The equality of grain size between the control and HDT treatments was achieved by a greater cell area in the controls, while the lower grain weight in the HNT treatment was a result of the cells themselves being smaller. Thus, the relationship between cell division and cell size was different in the three treatments. Further, the effects of temperature vary according to the timing of the high temperature exposure in relation to photosynthetic function.