Drought photosynthesis

Photosynthesis and gas exchange of leaves are affected by many stresses including drought, flooding, salinity, chilling, high temperature, soil compaction and inadequate nutrition. Many, but not all, of these stresses have symptoms in common. For example, stomatal conductance and the rate of assimilation of CO2 per unit leaf area often decrease when stress occurs. Further, it is possible that several of the stresses may exert their effects, in part, by increasing the levels of the hormone abscisic acid (ABA) in the leaf epidermis. This hormone is known to close stomata when applied to leaves. 

An intriguing stress-induced alteration in gene expression occurs in a succulent plant, Mesembryanthemum crystallinum, which switches its primary photosynthetic CO2 fixation pathway from C3 type to CAM (Crassulacean acid metabolism) type upon salt or drought stress (Winter, 1974 Chapter 8). Ostrem et al. (1987) have shown that the pathway switching involves an increase in the level of mRNA encoding phosphoenol-pyruvate carboxylase, a key enzyme in CAM photosynthesis. 

Determination of such a quantum yield indicates how efficiently the photochemistry is performed versus all other deexcitation processes. For instance, the photochemical reactions of photosynthesis can be decreased by environmental stresses such as freezing temperatures, excessive light, and drought, leading to a decrease in < Photochem (Eq. 5.10), an important assessment in both the laboratory and in ecophysiological field studies. 

Above experiments indicated that protection of photosynthesis under salt/drought stress would be suitable target for the Improvement of plant productivity. Previously, we selected high NaCl-adapted photoautotrophic tobacco cells and characterized their characteristics of salt-tolerance (Sato et al., 1992 Murota et al., 1994). Our characterization indicated that photosynthetic activity of photosystem II (PSII),... 

Shah NH., Paulsen GM. Interaction of drought and high temperature on photosynthesis and grain-filling of wheat. Plant and Soil 2003 257(1) 219-226. 

Gu J., Yin X., Struik PC., Stomph TJ., Wang H. Using chromosome introgression lines to map quantitative trait loci for photosynthesis parameters in rice (Oryza sativa L.) leaves under drought and well-watered field conditions. Journal of Experimental Botany 2012 63(1) 455-469. 

Soil anaerobiosis results in plant stress symptoms similar to those caused by drought stress. Among these are stomatal closure and reduction in net carbon assimilation. Photosynthesis in plants susceptible to flooding rapidly declines under anaerobiosis. Reduction in photosynthesis in response to flooding has also been reported for flood-tolerant species. 

Particularly rich in B. are pollens (for Brassica napus ca. 100 xg/kg). B., especially brassinolide and 24-epibrassinolide, have unusually high phytohormonal activity. B. effect a general promotion of growth in plants (e. g., in roots), stimulate photosynthesis and protein biosynthesis, modulate the mode of action of other phytohormones, and delay senescence processes. B. exhibit antiecdysteroid effects on insects. For practical uses of B. the so-called anti-stress action on plants is of major interest and has been detected in field experiments under unfavorable cultivation conditions (e.g., drought, cold, nutrient deficiency, pest infestation). 

In the following it is shown that light-dependent nitrate reduction in leaves is in fact strictly coupled to net photosynthesis. When CO is not available, or when stomata close under water stress, nitrate reduction decreases drastically. This explains many earlier reports on a high sensitivity of nitrate reduction to water stress (5, and literature), and of nitrate accumulation in leaves under drought (5,6). 

Some authors(1) concluded that photosynthesis at low Fw was more limited by the loss of chloroplast activity than by increased difussive resistance. RBPC activity decreased in bean and cotton plants at water stress(2)(3) (4). PEPC and RBPC activities decreased at water stress in barley plants(5). In mesophyll cells from bean and tomato plants there was decreased in CO2 fixation at fairly low osmotic potentials which simultaneous with stomatal closure(6). Recently it was reported(7) that in soybean leaves a non stomatal limitations of leaf photosynthesis under drought stress conditions appears to be due in part to a reduction in the in vivo activity of RBPC. On the other hand it has been reported that chlorophyll content (8) (9) (10) shows alterations due to water stress. In the present work we pretend to compare the responses of carboxylase activities and chlorophyl content to water deficit in two maize hybrids (Ci ) (CPB2 and CPB8), two tomato cultivars (C3) (Pera Quibor, PQ and Rio Grande, RG) and two bean cultivars CC3)(Tacarigua,T and VUL-73-401,V). 

Previous reports have shown that the destructive effect of photosynthetic apparatus can be proved by measuring some fluorescence parameters (Fo, Fv/Fm) (7). In our experiment the Fv/Fm ratio was reduced from 0.8 in the control to 0.6 on the day 6 of the drought, and was similar in both maize lines (Fig. 4.A). In parallel with reduction of PSII photochemical efficiency under strong stress conditions (RWC=50 ), basic fluorescence level decreased in both lines (Fig. 4.B). In addition to dehydration, some exogenous factors, such as a high intensity of the light, under normal drought conditions may also inhibit photosynthesis (8,9). 

The results have shown that under the given drought conditions. In addition to the altered water capacity, both maize lines suffered the changed photosynthetic processes. It was further shown that the fluorescence may be a very sensitive parameter of water stress and thus be used successfully for the determination of plant drought susceptibility. Further Investigations Into relationships of fluorescence parameters and photosynthetic activity are necessary, so as to acquire a quantitative dimension of the photosynthesis by means of a fast and undestructive measurements of chlorophyll fluorescence in vivo. 

Under drought Clare took a clear advantatge in photosynthesis rate with g values close to Seaton Park ones either bellow or above 1000 pE m s y so the superior A values must be atributed to nonstomatal effects that able Clare cvar to maintain higher CO2 assimilation rates (table 3). 

These data show that morphological and physiological modifications associated with water stress able cvar Clare to maintain higher rates of photosynthesis and plant production, but also that the water economy indexes were superior in this cultivar even under irrigation, showing the efficiency of this indexes and also the existence of intraespecific variability in photosynthesis rates under drought that able cultivar Clare to maintain better CO2 assimilation rates by water stressed leaves. 

---