No abstract

The water balance of soybean (Glycine max), cowpea (Vigna unguiculata), black gram (Vigna mungo), and pigeonpea (Cajanus cajan) grown in pots was studied during a soil drying cycle. The response of the plants was analysed for three distinct stages of dehydration. In stage I, the rate of transpiration remained constant and equal to that of well watered plants even though soil water status fell by more than 50%. Stage II began when the rate of soil water supply to the plant was less than potential transpiration and stomates closed resultingjn the maintenance of plant water balance. When soil water content was expressed as a fraction of transpirable soil water, all species showed a transition from stage I to stage II at a fraction of transpirable soil water of about 0.3 to 0.2. As the soil water declined further, all species had a similar decrease in relative transpiration rate. Consequently, the responses of the four species in stages I and II were essentially identical, except that pigeonpea extracted a slightly greater amount of soil water. Stage III occurred once stomates had reached minimum conductance and water loss was then a function of the epidermal conductance and the environment around the leaf. Substantial differences were found among the four grain legumes in epidermal conductance. Soybean had the highest conductance, followed by black gram, cowpea and pigeonpea. Substantial variation in dehydration tolerance among the four grain legumes was also found: the ranking of dehydration tolerance based on the relative water content was pigeonpea > cowpea > mungbean > soybean. Differences among the four grain legume species in the duration of stage III which finished when plants died, were consistent with differences in epidermal conductance and in dehydration tolerance of leaves.

The thermodynamic properties of water have been of interest to plant physiologists since Dixon (1914) first used a pressure chamber to study the 'osmotic tension' of tree sap. With the development of the psychrometer (Spanner 1951) and the modern pressure chamber (Scholander et al. 1965), data have been reported regularly on the chemical potential of water and its components. Apart from the relationship between turgor potential and abscisic acid accumulation (Pierce and Raschke 1980), no unique, fundamental relationships have been established relating the thermodynamic state of water to physiological performance (Ritchie 1981;Turner et al. 1985;Van Volkenburgh and Boyer 1985).From time to time, doubts have been raised about expectations that thermodynamic state variables would lead to explanations of physiological behaviour. Hsiao (1973) and Oertli (1976) both argued strongly that there did not appear t o be any transducers to translate changes in either total water potential ($) or osmotic pressure (a) into physiological responses. For example, in plant tissue suffering severe stress the chemical potential of water is reduced only a few per cent below that of free water. Moreover, the only effect of osmotic pressure appears to be via local concentrations of deleterious compounds in specific compartments within the cell.One justification frequently raised for measuring water potential is that it influences water flow. In fact, however, much of the water flow in the soil-plant system is governed by hydrostatic pressure (P). Water flow in the soil, xylem, phloem, and apoplasm is driven by hydrostatic pressure gradients and not total water potential gradients (Nobel 1974;Passioura 1984). Water potential gradients are probably only of major importance for water flow across membranes between the apoplasm and symplasm. Since equilibrium or near equilibrium of water potential is usually assumed over these short distances and is therefore not measured, characterization of water flow by water potential gradients is of little practical relevance.Not only does hydrostatic pressure seem relevant to the problem of water flow, but it is also the thermodynamic component, in the form of turgor pressure, that recently has received the most attention as the variable influencing the physiology of plants. However, for plants to transduce turgor pressure into a physiological signal, the mechanical potential energy of hydrostatic pressure must be gauged.

SummaryThe relation between the ratio of the natural 12C and 13C isotopes of carbon in the feed and resultant faeces of animals was studied to develop a technique for estimating the proportion of C3 species (tropical legumes) and C4 species (tropical grasses) selected by grazing animals.In general, theδ13C values (see text for definition) of faeces from rabbits, sheep, goats and cattle were lower (more negative) than those of the corresponding feeds by from 0·4 to 2·0. This was possibly due to contamination in the gut by tissues or fluids with lower δ13C values. When C4 and C3 feeds were alternated, cattle took about a week to fully achieve the new level (δ13C of – 28·7 on the C3 feed and – 13·1 on the C4 feed) in the faeces. This time lag is associated with the time taken for the feed to move through the digestive tract.When mixed C3 and C4 feeds were fed to rabbits, sheep, goats and cattle there was a negative linear relation between percentage legume (C3) in the feed and the δ13C of the faeces (P < 0·01). A decrease in one unit in the δ13C value was associated with an increase of 7·0–8·5% legume in the diet.Estimation of the percentage legume in the feed from the δ13C value of the faeces and of the C3 and C4 components of the diet, resulted in a consistent over estimation of the legume component because the faeces had lower values than the corresponding feeds. This bias was removed if the prediction was based on the δ13C of the feeds minus 1 unit; the legume percentage in the diets of the sheep, goats and cattle could then be estimated with a precision of about ± 5%.Differences in digestibility between the C3 and C4 components greatly bias the estimations. This bias in the diets fed to rabbits was effectively removed by using in vitro organic matter digestibility values of the two components to correct for the differences. Legume percentage in the diet could then be estimated with a RSD of ± 3%.Advantages and disadvantages compared with alternative methods of estimating the diet of grazing animals are discussed.

Introduced African grasses are invading the grasslands of the Venezuelan savannas and displacing the native grasses. This work, which is part of a program to understand the reasons for the success of the African grasses, specifically investigates whether introduced and native grasses differ in some photosynthetic characteristics.The responses to photon flux density, leaf temperature, leaf-air vapour pressure difference and leaf water potential of leaf photosynthetic rate of two introduced African C grasses (Hyparrhenia rufa and Melinis minutiflora) and of a lowland and a highland population of a native Venezuelan grass (Trachypogon plumosus) grown under controlled conditions were compared. These responses in all three species were typical of tropical C pasture grasses. The introduced grasses had higher maximum leaf conductance, net photosynthetic rates, and optimum temperature (H. rufa only) for photosynthesis than T. plumosus. However, T. plumosus was able to continue photosynthesis to lower leaf water potentials than the two introduced grasses, and the efficiency which it utilized water, light and mineral nutrients to fix carbon were similar to those of the introduced grasses.The higher rates of leaf photosynthesis of the introduced grasses contributed to, but only partially explained, the higher growth rates compared to T. plumosus. The higher growth rates and nutrient concentration of the introduced grasses are consistent with their ability to establish rapidly, compete successfully for resources, and displace T. plumosus from moist, fertile sites. Conversely, the slower growth rate, lower nutrient concentrations, and superior water relations characteristics are consistent with the capacity of T. plumosus to resist invasion by introduced grasses in poorer sites.

No abstract

Damage to primary photosynthetic reactions by drought, excess light and heat in leaves of Macroptilium atropurpureum Dc. cv. Siratro was assessed by measurements of chlorophyll fluorescence emission kinetics at 77 K (-196°C). Paraheliotropic leaf movement protected waterstressed Siratro leaves from damage by excess light (photoinhibition), by heat, and by the interactive effects of excess light and high leaf temperatures. When the leaves were restrained to a horizontal position, photoinhibition occurred and the degree of photoinhibitory damage increased with the time of exposure to high levels of solar radiation. Severe inhibition was followed by leaf death, but leaves gradually recovered from moderate damage. This drought-induced photoinhibitory damage seemed more closely related to low leaf water potential than to low leaf conductance. Exposure to leaf temperatures above 42°C caused damage to the photosynthetic system even in the dark and leaves died at 48°C. Between 42 and 48°C the degree of heat damage increased with the time of exposure, but recovery from moderate heat damage occurred over several days. The threshold temperature for direct heat damage increased with the growth temperature regime, but was unaffected by water-stress history or by current leaf water status. No direct heat damage occurred below 42°C, but in water-stressed plants photoinhibition increased with increasing leaf temperature in the range 31-42°C and with increasing photon flux density up to full sunglight values. Thus, water stress evidently predisposes the photosynthetic system to photoinhibition and high leaf temperature exacerbates this photoinhibitory damage. It seems probable that, under the climatic conditions where Siratro occurs in nature, but in the absence of paraheliotropic leaf movement, photoinhibitory damage would occur more frequently during drought than would direct heat damage.

Pigeonpea is a tropical grain-legume, which is highly dehydration tolerant. The effect of drought stress on the carbohydrate metabolism in mature pigeonpea leaves was investigated by withholding water from plants grown in very large pots (50 kg of soil). The most striking feature of drought-stressed plants was the pronounced accumulation of D-pinitol (lD-3-methyl-c/i/ro inositol), which increased from 14 to 85 mg g" 1 dry weight during a 27 d stress period. Concomitantly, the levels of starch, sucrose and the pinitol precursors myo-inositol and ononitol all decreased rapidly to zero or near-zero in response to drought. The levels of glucose and fructose increased moderately. Drought stress induced a pronounced increase of the activities of enzymes hydrolysing soluble starch (amylases) and sucrose (invertase and sucrose synthase). The two anabolic enzymes sucrose phosphate synthase (sucrose synthetic pathway) and myo-inositol methyl transferase (pinitol synthetic pathway) also showed an increase of activity during stress. These results indicate that pinitol accumulated in pigeonpea leaves, because the carbon flux was diverted from starch and sucrose into polyols.