An optimal strategy of stomatal control during a drought period, in pl
ants adapted to a humid climate, is derived by maximizing the photosyn
thetic production luring the expected duration of drought. The expecte
d duration of drought is calculated from the probability that rain occ
urs during a certain period, which is assumed constant. The underlying
plant model describes photosynthetic production and the consumption o
f water from the soil, with a given initial soil water content. Water
is consumed through transpiration at a rate dependent on water vapour
deficit, temperature and stomatal conductance and carbon is assimilate
d at a rare dependent on light intensity and stomatal conductance. The
optimization problem is solved with driving variables and the probabi
lity of rain corresponding to a Fenno-Scandian climate. The resulting
optimal stomatal control consists of two processes with different time
constants: (1) daily variation depending on the driving variables, an
d (2) a declining trend as a function of the initial soil water conten
t and the probability of rain. The result allows for a physical interp
retation of the so-called 'cost of water' used in similar optimization
studies. An approximate model is derived from the optimal solution, s
uch that the 'cost of water' is a function of the soil water content.
(C) 1996 Annals of Botany Company