Changing Responses of Stomata to Abscisic Acid and CO2as Leaves and Plants Age

Abstract

Willmer, C. M., Wilson, A. B. and Jones, H. G. 1988. Changing responses of stomata to abscisic acid and C02 as leaves and plants age.—J. exp. Bot. 39: 401-410. Stomatal conductances were measured in ageing leaves of Commelina communis L. as plants developed; stomatal responses to C02 and abscisic acid (ABA) in epidermal strips of C. communis taken from ageing leaves of developing plants and in epidermal strips from the same-aged leaves (the first fully-expanded leaf) of developing plants were also monitored. Stomatal behaviour was correlated with parallel measurements of photosynthesis and leaf ABA concentrations. Stomatal conductance in intact leaves decreased from a maximum of 0-9 cm s~1 at full leaf expansion to zero about 30 d later when leaves were very senescent. Conductances declined more slowly with age in unshaded leaves. Photosynthesis of leaf slices also declined with age from a maximum at full leaf expansion until about 30 d later when no 02 exchange was detectable. Exogenously applied ABA (0-1 mol m~3) did not affect respiration or photosynthesis. In epidermal strips taken from ageing leaves the widest stomatal apertures occurred about 10 d after full leaf expansion (just before flowering began) and then decreased with age; this decrease was less dramatic in unshaded leaves. The inhibitory effects of ABA on stomatal opening in epidermal strips decreased as leaves aged and was greater in the presence of C02 than in its absence. When leaves were almost fully-senescent stomata were still able to open. At this stage, guard cells remained healthy-looking with green chloroplasts while mesophyll cells were senescing and their chloroplasts were yellow. Similar data were obtained for stomata in epidermal strips taken from the same-aged leaves of ageing plants. The inhibitory effects of ABA on stomatal opening also decreased with plant age. In ageing leaves both free and conjugated ABA concentrations remained low before increasing dramatically about 30 d after full leaf expansion when senescence was well advanced. Concentrations of free and conjugated ABA remained similar to each other at all times. It is concluded that the restriction of stomatal movements in intact leaves as the leaves and plants age is due mainly to a fall in photosynthetic capacity of the leaves which affects intracellular C02 levels rather than to an inherent inability of the stomata to function normally. Since stomatal aperture in epidermal strips declines with plant and leaf age and stomata become less responsive to ABA (while endogenous leaf ABA levels remain fairly constant until leaf senescence) it is suggested that some signal, other than ABA, is transmitted from the leaf or other parts of the plant to the stomata and influences their behaviour.