The effect was estimated of light intensity, leaf temperature, and C0 2 concentration on photosynthetic rate in leaves of crop plants. The potential capacities of photochemical and biochemical processes and of C0 2 transport were compared. Resistance to C0 2 transport in various parts of the pathway from external air to reaction centre in the chloroplasts was estimated in absolute units. This allowed quantitative comparison of micrometeorological and physiological effects on photosynthesis. C0 2 transport towards the leaf is affected by the sum of resistances in external air (r a ), stomata (r s ), and mesophyll cells (r m ). Since r m is usually larger than r s of open stomata, r m is probably an important yield determining factor. With adequate water-supply, transpiration depends on r s + r a , but not on r m . Therefore higher values of r s and lower values of r m favour the water balance of crops. With strong light and leaf temperature around 22°C, C0 2 saturation of photosynthesis was reached with C0 2 concentrations between 01 and 0.15 %. Techniques were described for accurate measurement of C0 2 and water vapour, leaf temperature, light intensity and diffusion resistance. The importance of different types of light bulbs for photosynthesis was evaluated, and much attention was paid to accurate assessment of effective C0 2 concentration in the air around leaves in assimilation chambers.

Photosynthesis of crop plants as influenced by light, carbon dioxide, temperature, and stomatal diffusion resistance

Chloroplasts are a key feature of most guard cells; however, the function of these organelles in stomatal responses has been a subject of debate This review examines evidence for and against a role of guard cell chloroplasts in stimulating stomatal opening Controversy remains over the extent to which guard cell Calvin cycle activity contributes to stomatal regulation However, this is only one of four possible functions of guard cell chloroplasts; other roles include supply of ATP, blue-light signalling and starch storage Evidence exists for all these mechanisms, but is highly dependent upon species and growth/measurement conditions, with inconsistencies between different laboratories reported Significant plasticity and extreme flexibility in guard cell osmoregulatory, signalling and sensory pathways may be one explanation The use of chlorophyll a fluorescence analysis of individual guard cells is discussed in assessing guard and mesophyll cell physiology in relation to stomatal function Developments in transgenic and molecular techniques have recently provided interesting, albeit contrasting, data regarding the role of these highly conserved organelles in stomatal function Recent studies examining the link between mesophyll photosynthesis and stomatal conductance are discussed An enhanced understanding of these processes may be fundamental in generating crop plants with greater water use efficiencies, capable of combating future climatic changes

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2008.02685.x

Guard cell photosynthesis and stomatal function

Chapter 4 – water deficits and photosynthesis

Low leaf water potentials result in large reductions in photosynthesis. In higher plants, the reductions are caused both by decreases in the photosynthetic activity of a unit of leaf and in the production of new leaf surface. Photosynthetic activity declines because of decreased stomatal opening and the inhibition of chloroplast activity, either of which may control photosynthesis depending on which is more limiting at low leaf water potentials. The production of new leaf area is highly sensitive to water deficits and is usually reduced before photosynthetic activity decreases. This may be attributed to the high responsiveness of leaf enlargement to turgor, which expands the cells. When low leaf water potentials are prolonged, leaf senescence often occurs and the quantity of existing leaf area may decline. There is evidence that translocation is less sensitive than photosynthesis to low leaf water potentials. Consequently, grain yield, which depends on both photosynthesis and translocation, is more likely to be limited by photosynthesis than translocation. Since substantial translocation to the grain may occur from parts of the plant other than the leaves during desiccation, the total photosynthate accumulated during the growing season is more important than that produced during the grain-filling period alone when plants have had low water potentials.

Photosynthesis at low water potentials

High-resolution images of the chlorophyll fluorescence parameter Fq'/Fm' from attached leaves of commelina (Commelina communis) and tradescantia (Tradescantia albiflora) were used to compare the responses of photosynthetic electron transport in stomatal guard cell chloroplasts and underlying mesophyll cells to key environmental variables. Fq'/Fm' estimates the quantum efficiency of photosystem II photochemistry and provides a relative measure of the quantum efficiency of non-cyclic photosynthetic electron transport. Over a range of light intensities, values of Fq'/Fm' were 20% to 30% lower in guard cell chloroplasts than in mesophyll cells, and there was a close linear relationship between the values for the two cell types. The responses of Fq'/Fm' of guard and mesophyll cells to changes of CO2 and O2 concentration were very similar. There were similar reductions of Fq'/Fm' of guard and mesophyll cells over a wide range of CO2 concentrations when the ambient oxygen concentration was decreased from 21% to 2%, suggesting that both cell types have similar proportions of photosynthetic electron transport used by Rubisco activity. When stomata closed after a pulse of dry air, Fq'/Fm' of both guard cell and mesophyll showed the same response; with a marked decline when ambient CO2 was low, but no change when ambient CO2 was high. This indicates that photosynthetic electron transport in guard cell chloroplasts responds to internal, not ambient, CO2 concentration.

http://www.plantphysiol.org/content/plantphysiol/128/1/52.full.pdf

Responses of photosynthetic electron transport in stomatal guard cells and mesophyll cells in intact leaves to light, CO2, and humidity.

This paper describes the second half of a series of tests of the photosynthetic theory of stomatal movement. A corollary of this theory is that any agent that inhibits photosynthesis should prevent opening or cause closure in light?unless some other action prevents this. It has been reported that this relationship does not hold either in wilting or under the action of chloroform (Wilson, 14). The effects of these factors are investigated in the present paper. As before (11), stomatal aperture was measured by means of a modified porometer method, photosynthesis by infrared absorption, and, except in a few experiments, the leaf material all belonged to the same clone of Pelargonium zonale var. Barney.

Stomatal movement and photosynthesis in pelargonium. ii. effects of water deficit and of chloroform: photosynthesis in guard cells.

The old hypothesis that light induces stomatal opening through its photosynthetic activity has been variously tested in a number of recent investigations. The results are strangely contradictory. First, reference may be made to tests based on the relative efficiency of different regions of the spectrum in promoting stomatal opening, though these tests are not very recent. The only systematic studies in which strictly quantitative methods have been used to equalize light intensity are those of Paetz (11), Sierp (15), and Harms (?). Paetz found that the order of activity was red light > blue > green, whereas Sierp and Harms found that red light was regularly the least active and that in most species green light was as active as blue. It has long been known that a reduction in the normal C02 content of the air tends to induce stomatal opening and an increase in CO2, up to a point, above normal has the opposite effect. It has, however, only recently been demonstrated (Heath, 7, 9) that the sensitivity of stornata to small changes in C02 concentration below normal is such that stomatal response to variations in light intensity could conceivably be due primarily to the effect of the latter on C02 concentration within the leaf. The only investigation in which measurement of photosynthesis rate has been combined with estimation of stomatal aperture is that of Wilson (18). Approaching the problem from various angles, he obtained results which seem quite opposed to the photosynthetic theory. Both in the behavior of plants in the open and under certain artificial conditions, stomatal opening and photosynthesis often showed no correlation, while complete absence of chlorophyll from leaves, including guard cells, did not entirely prevent stomatal response to light. On the other hand, regarding the long unsettled question of whether guard cells are capable of photosynthesis, the most

Stomatal movement and photosynthesis in pelargonium. i. effects of light and carbon dioxide.

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Stomatal movement and photosynthesis in pelargonium. ii. effects of water deficit and of chloroform: photosynthesis in guard cells.

Stomatal movement and photosynthesis in pelargonium. i. effects of light and carbon dioxide.