An ecophysiological study on the moss hydrogoniuh fontanum from the Asir mountains, Saudi Arabia

Abstract

The thesis describes a study on the ecophysiology of the moss Hydrogonium fontanum (C. Mail.) Jaeg., the dominant plant at a waterfall in Saudi Arabia. The influence of environmental variables and water stress on the growth, stress metabolite accumulation and phosphatase activities of the moss was studied in laboratory axenic culture along with observations and experiments conducted in the field. The variables chosen for growth experiments were light flux, flooding, nutrient concentrations and water stress. For phosphatase activities, the influence of temperature, pH, ions, water stress were studied. Differences were found in phosphatase activities for rhlzoids, protonema and leafy shoots of the moss and, therefore, the phosphomonoesterase (PMEase) and phosphodiesterase (PDEase) activities of these fractions were also investigated. H. fontanum was originally collected from the tufa-depositing waterfall (Water chemistry - 44 mg 1(^-1) Na, 44 mg 1(^-1) Ca). High Na and Ca had significant positive effect on yield of the protonema under the laboratory conditions. Low light intensity (10 µmol photon m(^-2) s(^-1)) decreased the yield, but high light intensity (90 µmol photon m(^-2) s(^-1)) increased the yield of the protonema. The moss showed no response to water stress in respect to praline accumulation. Protein content decreased significantly over 48 h with increase in water stress. The Influence of water stress was greater in terms of dry weight and chlorophyll content changes in protonema than in leafy shoots. The protonema was capable of using various organic P substrates as sources of phosphorus and showed both PMEase and PDEase activities. PMEase and PDEase activities were detectable in all moss fractions (rhizoids, protonema, leafy shoots). Laboratory grown material showed higher activities than field grown material. Rhizoids produced the highest PMEase and PDEase activities among the moss fractions. Some leafy shoots collected from the field had low phosphorus content with high phosphatase activities, while others had high phosphorus content with low phosphatase activities. Changes in phosphatase activities in batch culture were studied in relation to growth rate. PMEase activity was first evident when cellular P was 1.15% with low activity (0.117 µmol pNP mg d. wt(^-1) h(^-1)) and PDEase appeared 4 days later when cellular P was 0.54%. The activities increased up to day 12 after which the activities maintained this level. The optimum temperatures, measured over a period of 1 h, for PMEase and PDEase activities were 60 ºC and 65 ºC with pH optima of 5.5-6.0 and 6.4-6.8, respectively. Of the six ions tested, Ca, Zn and P had significant inhibitory effects on the activities at the highest concentration used (10 mM).Drying the moss decreases PMEase and PDEase activities by about 23% and 21% (5-d) and 3.7 and 2,8 times (3 months), respectively. Water stress (PEG treatment) also reduced significantly the activities of PMEase and PDEase with a greater effect on the activity of the latter. A brief comparison in PMEase activity using two different substrates p- nitrophenyl phosphate (pNPP) and 4-methylumbelliferyl phosphate (4-MUP) was made to investigate the pH optima and time course. PMEase activity measured using 250 µM 4-MUP was about 60% of that measured using the same concentration of pNPP.