When a terrestňal plant is subjected to a mild drought stress, the leaf net CO2 uptake declines as a result of the stomatal closure. In contrast to previous beliefs the photosynthetic apparatus is vety resistant to dehydration. Particularly the relations between photochemistry, its regulation and leaf CO2 assimilation (f) are identical when F is changed either by increasing leaf water deficit or by decreasing the ambient CO2 concentration. When the stomata close in a leaf under water deficit the CO2 concentration within the leaf declines. As a result photorespiration is favoured, the photochemical yield of open photosystem 2 (PS 2) centres and the activity of some enzymes dechne (sucrose-phosphate synthase, nitráte reductase). The dechne of photochemical yield of PS 2 is consequential to an increase in thermal dissipation of the excitons trapped by PS 2 units. The dechne in CO2 concentration occurring in a desiccating leaf may trigger an integrated response of leaf metabohsm which still remains to be explored.
The main subject of our study are spherical (weakly spherical) graphs, i.e. connected graphs fulfilling the condition that in each interval to each vertex there is exactly one (at least one, respectively) antipodal vertex. Our analysis concerns properties of these graphs especially in connection with convexity and also with hypercube graphs. We deal e.g. with the problem under what conditions all intervals of a spherical graph induce hypercubes and find a new characterization of hypercubes: $G$ is a hypercube if and only if $G$ is spherical and bipartite.