Closure of stomata

Ward, D.A. Drake, B.G. (1988). Osmotic stress temporarily reverses the inhibitions of photosynthesis and stomatal conductance by abscisic acid - evidence that abscisic acid induces a localized closure of stomata in intact, detached leaves. Journal of Experimental Botany, 39, 147-55. 

Several physiological changes induced by drought have been documented, including an increase in ABA levels, the closure of stomata and the increase in cellular osmolarity. The increase in ABA levels is probably due to the de novo synthesis of this hormone, and the process requires... 

The physiology of stomatal movement has been extensively researched. Early studies using phenylmercuric acetate demonstrated that the induced closure of stomata could conserve water significantly up to one month from the time of treatment. Although reductions in transpiration would probably carry a yield penalty, the long-term benefits of increased land use are desirable. However, no commercially viable anti-transpirants have been produced. 

One can expect the same types of mechanisms, which can operate at other levels, to explain thresholds and their variability. The numbers and condition of the stomata will determine how much of the pollutant will reach the mesophyll cells of the leaf. Indeed, the reflex-type resistance (4) of the plant is attributed to the closure of stomata in response to 03, which thereby impedes further entrance of these pollutants. Young or old tobacco leaves are very resistant to oxidants. When young, a leafs resistance may be attributed to the density of the cells, which limits penetration of gases into the mesophyll. In the mature and aging leaf, suberization of the cell walls blocks the penetration of the pollutants (28). The cells of the leaf and their susceptibility to oxidants illustrate another aspect of the threshold and response—they are determined by the stage of development of the receptor, and different mechanisms operate at different times. 

Recently, another role for ABA-cytoskeleton interactions was discovered in guard cells of Vida faba leaves [150]. ABA specifically disrupted the MTs in guard cells but not in other epidermal cells. This effect resulted in the closure of stomata. When MTs repolymerized, the stomata reopened. Interestingly, no other plant hormone could elicit such a response in the guard cells. Actually, a role for MTs in the stomata closure-opening mechanism had been proposed earlier on the grounds that the MT-disrupter, colchicine, inhibited stomatal opening in Tradescantia leaves [151]. 

However, in certain cases, though the partial closure of stomata occurred with the appearance of midday depression, the concentration of COp in the intercellular space of the leaves was increased rather than aecreased(Fig.6.7)-... 

Dormancy hormones inhibiting growth and metabolism, causing closure of stomata... 

In a recent review Cote and Crain [5] have pointed to three phenomena which they thought particularly convenient to study the physiological role of phosphoinositides in plants. These were 1) the turgor changes in stomatal guard cells, paiticulaiiy after a treatment with abscisic acid which provokes the closure of stomata, 2) the osmotic stress in Dunaliella salina or carrot cell cultures and 3) the deflagellation of Chlamydomonas reinhardtii after exposure of the alga to acid pH, elevated temperature or ethanol. Nevertheless the rates of apparition of inositolphosphates after stimulation have been found very different in various plant tissues (from 30 sec to 30 min). 

Abseisie aeid - a growth inhibitor assoeiated with dormant buds and some seeds, and the closure of stomata. 

Adaptive features of stomata - rapid closure in response to turgor loss (and perhaps other stimuli) - which reduce transpiration when plants become stressed. Alternatively, for ephemer-als, maintenance of high stomatal conductance and photosynthetic capacity to permit maximum growth rate during the limited time that water is available. 

Unnatural (—)-ABA shows one-half to one-third of the activity of (+)-ABA in many bioassays,634 and this small difference in activity between the enantiomers has been explained by the pseudosymmetry of the molecule, which is derived from the 2,6,6-trimethyl-cyclohex-2-en-4-one.635 Figure 26 shows the steric structures of (+)- and (—)-ABAs with the preferable conformation, a half-chair with the pseudoaxial side chain, viewed from the carbonyl group at C-4. In the molecule of (—)-ABA, the C-7 corresponds to the C-9 of (+)-ABA, the C-9 corresponds to the C-7 of (+)-ABA, and the C-8 occupies the space facing the rtf-face of the C-2 in (+)-ABA, whereas a methyl group corresponding to the C-8 of (+)-ABA is absent. This hypothesis has been supported by the achiral analog (38), which shows activity intermediate between (+)- and (—)-ABAs.592 The activity of (—)-ABA is low in the assay of stomatal closure,618 which suggests that the receptor of stomata is more specific to (+)-ABA than to (—)-ABA. 

What is the optimal behavior of stomata over the course of a day WUE (Eq. 8.39) is maximized by minimal stomatal opening because transpiration is decreased more than photosynthesis by partial stomatal closure that is, changes proportionally more than does 7c02 as changes (Fig. 8-23 also Section 8.4E). However, minimal stomatal opening can lead to very little net CO2 uptake. Thus a more pertinent consideration might be the maximal amount of CO2 that can be taken up for a certain amount of water transpired. The amount of water lost depends on the plant condition and environmental factors and should be considered over the course of a whole day. 

The photosynthetic efficiency mainly depends on the openness of stomata, particularly in C3 crops, while their closure tends to avoid excessive water loss. Abscisic acid (ABA) mediates water loss from the guardian cells of the stomata, which is triggered by a decrease in the water content of the leaf and inhibits leaf expansion. In muskmelon seedlings, ABA could improve the maintenance of the leaf water potential and relative water content, and reduce electrolyte leakage [55]. 

Investigation of the response of guard cells to ABA has yielded important information about ABA signal transduction. ABA induces a cascade of signalling events in guard cells resulting in closure of the stomata [93]. The final output, stomatal closure, is restricted to... 

Atrazine enters plants primarily by way of the roots and secondarily by way of the foliage, passively translocated in the xylem with the transpiration stream, and accumulates in the apical meristems and leaves. The main phytotoxic effect is the inhibition of photosynthesis by blocking the electron transport during Hill reaction of photosystem II. This blockage leads to inhibitory effects on the synthesis of carbohydrate, a reduction in the carbon pool, and a buildup of carbon dioxide within the leaf, which subsequently causes closure of the stomata, thus inhibiting transpiration. 

Foliar application of chitosan reduced transpiration of plant by inducing closure of plant stomata (Bittelli et al. 2001). It was found that the foliar application reduced water use of pepper by 26%-43% while maintaining biomass and yield of pepper in growth chamber as well as field condition. Reducing water use by chitosan foliar application is the best way in helping plants against dronght... 

Fig. 2. Reduction of stomatal conductance for gases by a factor of 0.5, and its effect on photosynthesis, accomplished either through proportional responses of all stomata left), or by closure of one-half of all stomata (right). A C0.2-assimilation rate, E transpiration rate g conductance for COo or H.>0 intercellular partial pressure of CO subscripts o initial p after proportional response b after binary response...

In real leaves, there will be lateral diffusion of CO2, and correlated closure may not occur among opposite stomata in the upper and lower epidermis. An exact equality of the fraction of closed stomata with the relative reduction of the rate of photosynthesis is therefore not to be expected nevertheless, discrepancies between measured rates of photosynthesis and computed values of Cj would appear. In the case of stomata responding proportionally, computed values of Cj will be applicable, and measured rates of photosynthesis represent working points on the saturation curve of COo assimilation. 

Similarly, Portulacaria afra, a succulent termed weak CAM by Neales (1975) is shifted from day-time CO2 uptake to nocturnal CO2 fixation and (relatively little) acid fluctuation by NaCl stress. This shift is concomitant with a day-time closure and nocturnal opening of stomata and an increased diurnal fluctuation of organic acids (Fig. 4.8). Since water stress was accompanied by the same shift in malic acid metabolism, the NaCl effect here may be indirect and simply related to physiological drought. 

There are numerous reports in the literature of a depression in daytime CO2 uptake in non-CAM plants (see Stocker, 1960 Larcher, 1973). The occurrence of noon depression of photosynthesis (Stocker, 1960) seems to be especially common in plants growing in arid habitats, the same habitats in which CAM plants exist. However, there is a fundamental difference between the noon depression of CO2 uptake in CAM and non-CAM plants. In species without CAM, the depression is caused by stress factors such as temporary water deficit, high air temperature, or reduced atmosphere moisture. These factors cause stomata closure during midday, thus inhibiting photosynthesis. In CAM plants, however, the occurrence of the CO2 uptake depression is an essential consequence of malic acid metabolism which characterizes CAM (Kluge, 1968 b). For example, in CAM plants the depression occurs independently of the actual water status of the leaves, i.e., it occurs in nonstressed plants as well as stressed. An explanation of the coupling between malate metabolism of CAM and the daytime closure of the stomata in CAM plants is suggested in Chapter 5.3.2.2. 

The stimulation of the closure of abaxial stomata after the application of abscisic acid of Commelina communis was inhibited by La (De Silva et al. 1985). The action of cytokinin in stimulating the adventitious bud initiation in Torenia step segments was inhibited by 0.3 mM LaClj (Tanimoto and Harada 1986). The consequences of this effect on crop yield is theoretically quite severe. 

---