Influence of Drought, Rain and Artificial Irrigation on Photosynthesis, Gas Exchange and Water Relations of the Fynbos PlantProtea acaulos(L.) Reich at the End of the Dry Season

Abstract

Protea acaulos, a prostrate fynbos shrub, often experiences very low air humidity at leaf temperatures over 10 "C higher than mean air temperature. We determined to what degree this particular microclimate influenced photosynthetic performance, leaf conductance and water relations of non-irrigated and trickleirrigated plants. Measurements were made at the end of the dry summer season in the sand plain lowland fynbos on the west coast of South Africa. Independent of water supply, plants showed a pronounced midday depression of gas exchange. While in non-irrigated plants leaf water potential dropped to -2.0MPa around noon, it never fell below -1 .O MPa in irrigated plants. On the other hand minimum pressure potential was similar in irrigated and non-irrigated plants. The latter showed higher turgor after rain, due to osmotic acclimation, which resulted from a reduction in maximum water volume. The main osmoticum was 1,5-anhydro-D-glucitol. Leaf temperature, directly or via the vapour pressure deficit between leaf and air (Awl, rather than plant water status, was the determinant of the midday depression of gas exchange. High Aw caused stomata1 closure during times of saturating light, thus limiting photosynthetic C 0 2 uptake and availability and enhancing the susceptibility for photoinhibition. This, as well as high leaf temperature per se, decreased the efficiency of photochemistry of photosystem 11. Initial fluorescence remained constant until temperatures exceeded 35 "C, above which changes

Desert dogma revisited: coupling of stomatal conductance and photosynthesis in the desert shrub, Larrea tridentata

“…We also found that Larrea C i varies little if we only look at it when g @ g max , but over an entire day C i can vary by an order of magnitude (Figs 2e & 3e). Whereas Herppich, Herppich & von Willert (1994) measured diurnal C i / C a ratios for a semi‐arid fynbos shrub that ranged from 0·6 to 1·05, Larrea exhibits even greater variation ( C i / C a = 0·14–1·39). These C i dynamics can have profound consequences for Larrea 's daily carbon balance because this shrub operates near maximum A for only a short period during early mid‐mornings (Figs 2a & 3a).…”

“…We also found that Larrea C i varies little if we only look at it when g @ g max , but over an entire day C i can vary by an order of magnitude (Figs 2e & 3e). Whereas Herppich, Herppich & von Willert (1994) measured diurnal C i /C a ratios for a semi-arid fynbos shrub that ranged from 0·6 to 1·05, Figure 6. A-Season model predictions of conductance (g) as a function of VPD, based on fitting the g submodel (Eqns 7 & 8) to 1998 data.…”

Arundo donax L. (Poaceae) — a C3 Species with Unusually High Photosynthetic Capacity

“…Electron transport rates through PSII were determined according to Krause and Winter (1996). Since the emitter detector unit of the PAM 2000 shows temperature dependence (Herppich et al, 1994), all fluorescence parameters were corrected for temperature.…”

Diverse functional responses to drought in a Mediterranean‐type shrubland in South Africa

“…tall, deep-rooted proteoid shrubs; fine-leaved, shallow-rooted, ericoid shrubs; and graminoid restioids) (see Stock et al, 1992) that are likely to respond differently to drought. However, experimental studies manipulating water availability in the field have been rare (Herppich et al, 1994;Agenbag, 2006), and there have been no direct tests of the mechanistic basis for using these functional groups to categorize drought response in this flora.…”