Responses Of Ecophysiological Characteristics And Leaf Ultrastructure To Water Stress In 15 Grass Species

The subtropical region of South China has great potential of forage production because of the high rainfall and temperature. However, the climate in the region is a transitional type characterized by cold winter, hot summer, and often a period of both high temperature and drought, limiting the growth of many forage species. A pot experiment was conducted in Dushan, Guizhou of Southwest China to study the response of ecophysiological characteristics and leaf ultrastructure to water stress and rewatering treatment in 15 forage grasses including 6 warm-season species (WSS) and 9 cool-season species (CSS). The 15 grass species studied were Setaria anceps, Hemarthria compressa (L.f.)R.Br., Paspalum urville S., P. vitgatum L., P. notatum Flugge, Paspalum dilatatum Poir, Bromus catharticus Vahl, Phalaris tuberose L., Roegneria kamoji Ohwi, Lolium perenne L., Phleum oratense L., Festuca arundinacea Schreb., Dactylis glomerata L., Festulolium, and Bromus inermis Leyss. The plants of various species were treated by stopping watering continuously for 11days and then rewatering on the 12nd day. Measurements were taken on the 0, 4th, 8th, 12th and 16th day (4 day after rewatering). The results are summarized below.1. Responses of leaf water condition (LWC) and osmo-regulating substances (ORS). Significant differences (P<0.05) were detected among the species in the degree of reduction in LWC with the progress of water stress. ORS changed significantly in all the species, with an increase in the content of free praline (FP) and the soluble sugar (SS) and a decrease after rewatering. The WSS and CSS were not significantly different in the pattern of changes in leaf water condition and osmo-regulating substances (ORS), although the degree of changes in FP varied remarkably between the two grass types. The FP accumulation was much higher in CSS than in the CSS, and The WSS reached their peaks of SS contents later, and accumulated less SS, than the CSS. At the later stage of water stress, the increase in malondialdehyde (MDA) content was significantly higher in the CSS than in the CSS, indicating a more severe damage of membrane system in CSS than in WSS.2. Responses of active oxygen scavenging system (AOSY). MDA continuously increased in all species under water stress, and the longer water stress lasted the more MDA increased. The activity of Superoxide Dismutase (SOD) and Peroxidase (POD) changed in variable patterns dependent on species. In general, the activity of SOD and POD increased to a certain level under which plants were still tolerant of the stress and then decreased. This is particularly true for POD activity, indicating that various species had different adjustment mechanisms. The WSS and CSS showed different adjustment mechanisms of enzyme Recovery System. The leaf CAT activity of the WSS increased and then decreased while that of CSS continuously decreased over the period of water stress. The CAT value of CSS was always lower than that of the WSS over the experimental period, indicating lower CAT contribution to drought tolerance in CSS. SOD had a similar change pattern in both WSS and CSS, with an increase and then a decrease, except that the SOD value was always higher in CSS than in WSS over the time. 3. Adaptive mechanism of photosynthesis under water stress. The net photosynthetic rate (Pn) and transpiration rate (Tr) declined with the time of water stress in all species, while the stomatal conductance (SC), water use efficiency (WUE) , internal CO2 concentration (Ci) and chlorophyll content (Chl) had variable changes. There was no significant difference in the tendency of change in Pn between the WSS and CSS, although Pn of CSS was slightly higher than that of WSS. Tr decreased with the time of water stress in all species, with the decrease in CSS being much more than in WSS. There were no significant difference in Gs, WUE and Chl between WSS and CSS.4. Overall evaluation on drought tolerance. A overall evaluation on drought tolerance of the 15 species, using the method of fuzzy subordination based on physiological and biochemical indicators, resulted in the following rank of the 15 species according to their drought tolerance: P. vitgatum L.> Roegneria kamoji Ohwi> Festuca arundinacea Schreb.> P. notatum Flugge> Bromus catharticus Vahl> Festulolium> Lolium perenne L.> Phleum oratense L.> Phalaris tuberose L.> Bromus inermis Leyss> Paspalum dilatatum Poir> Setaria anceps> Dactylis glomerata L.> Hemarthria compressa (L.f.)R.Br> Paspalum urville S.5. Changes in ultrastructure of leaf tissue. Under water stress, the shape and internal structure of chloroplast, mitochondrion and cell membrane in mesophyll cells underwent significant changes, more or less dependent on species. The cell structure of plant species with higher tolerance was well maintained while that of the species with poor tolerance was severely damaged by water stress. Based on the observation on ultrastructure of leaf tissues, P. vitgatum L. and P. notatum Flugge had the highest tolerance to water stress, followed by Festuloliu,Phalaris tuberose L,Paspalum dilatatum Poir and Lolium perenne L.