Seasonal and diurnal courses of water relations of the arido-active plant Hammada scoparia in the Negev desert.

The photosynthesizing branches of Hammada scoparia, one of the typical dwarf shrubs of the Negev desert, undergo a seasonal change from succulent to xeromorphic anatomy. This trend is accompanied by a marked decrease of water content and of total water Ψ and osmotic Ψ potential. Irrigated plants do not show such transitions. The daily courses of Ψ and Ψ showed minima around noon and a tendency for maxima before sunrise. Turgor pressure Ψ reached minima around noon and became negative (until ca.-10 bars). Generally, Ψ decreases with increasing water vapour concentration difference between plant and air (WD) in the first half of the day, and in the second half the reversal of this trend occurs. Mostly smaller increments of Ψ were correlated with larger increases in WD which lead to the conclusion that stomates closed enough to maintain transpiration at a constant value. Non-irrigated and irrigated plants showed different hysteresis loops of relation between Ψ and WD. Regulatory reduction of transpiration appears largely independently of Ψ which is in spring and with irrigated plants on a high level, with non-irrigated plants in summer on a low level. In summer the continous but decreasing drop of Ψ with increasing WD was interpreted as caused by a change in soil or root resistance. Independent of the seasonal state and of the Ψ level, H. scoparia regulates its water status within limited ranges of Ψ changes: the irrigated plants on a higher level, the non-irrigated on a lower level of Ψ . The water contents of the tissues of H. scoparia are linearily related to Ψ as well as Ψ . Steeper slopes with non-irrigated plants in summer than with spring palnts and with irrigated plants during the whole season signify that in the latter a certain increment in turgor pressure corresponds to a large gain in water content while in the non-irrigated summer plants it varies only little for an identical change in Ψ . This behaviour of non-irrigated wild plants apparently is due to the change of the elastic properties of the tissues in the assimilating branches.