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Influence of Drought, Rain and Artificial Irrigation on Photosynthesis, Gas Exchange and Water Relations of the Fynbos Plant Protea acaulos (L.) Reich at the End of the Dry Season
Author(s) -
Herppich Margaretha,
Herppich W. B.,
Willert D. J.
Publication year - 1994
Publication title -
botanica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.871
H-Index - 87
eISSN - 1438-8677
pISSN - 0932-8629
DOI - 10.1111/j.1438-8677.1994.tb00819.x
Subject(s) - vapour pressure deficit , photoinhibition , shrub , photosynthesis , transpiration , stomatal conductance , microclimate , dry season , dew , irrigation , turgor pressure , perennial plant , chlorophyll fluorescence , agronomy , acclimatization , horticulture , environmental science , botany , biology , ecology , photosystem ii , condensation , physics , thermodynamics
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 trickle‐irrigated 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 − 2.0 MPa around noon, it never fell below −1.0 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 (Δw), rather than plant water status, was the determinant of the midday depression of gas exchange. High Δw caused stomatal closure during times of saturating light, thus limiting photosynthetic CO 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 II. Initial fluorescence remained constant until temperatures exceeded 35 °C, above which changes in fluorescence indicated both photoinhibition and heat stress. Unlike other fynbos plants, Protea acaulos could not use the improved soil water supply to increase carbon gain under hot summer condition.