Field investigations in Welwitschia mirabilis during a severe drought .2. Influence of leaf age, leaf temperature and irradiance on photosynthesis and photoinhibition

During drought Welwitschia mirabilis, a desert plant indigenous to the west coast deserts of southern Africa exhibited a two-peaked pattern of diurnal CO2 exchange, with carbon losses for most time of the light period over all age classes of the approximately 5 years old leaf Leaf conductances for water vapour and water potentials were high only during the cooler, humid morning and declined thereafter with increasing temperatures, water vapour deficits of the air and irradiation. Although there was no gradient in water deficit over the whole leaf, leaf conductances were lower and carbon losses higher in 4 to 5 years old leaf sections. The latter also showed substantially reduced CO2 uptake capacity and quantum yield. In those leaf parts photoinhibition, estimated from the diurnal reduction in the ratio of dark adapted variable (F-V) to maximum fluorescence (F-M), was about 55% larger than in the young (approximate to 1 year old) tissue. The mature mid-leaf tissue behaved intermediate. Inhibition of F-V/F-M was due to the reduction of F-M, the dark adapted initial fluorescence (F-0) increased only slightly. q(1), the slowly relaxing component of non-photochemical quenching, was only 26% higher in the older leaf parts. F-0-quenching (q(0)) was low and constant in the young tissue but increased when q(1) was very high in the old one. The sum of q(1) and q(0) was linearily correlated with the diurnal inhibition of F-V/F-M. Except in the morning hours carbon exchange followed any changes in leaf temperature, while variations in F-V/F-M and q(1) were due to the synergism of temperature and radiation.