Vincent Gutschick

Questions and Answers (11) View all

• Answer added in Drought
  31 What is the lethal dose of 'physiological drought' for plants?
  By Andreas Bolte · Thünen-Institut
  Vincent Gutschick · New Mexico State University

  Before addressing the equivalent of tolerance limits for physiological drought, we need to go more deeply into the concept of drought. The idea of ph... [more]

  Before addressing the equivalent of tolerance limits for physiological drought, we need to go more deeply into the concept of drought. The idea of physiological drought is certainly an advance over the raw concept of drought derived from rainfed cropping. It incorporates the twin ideas of Ernst Detlef-Schulze (1986, Annu Rev Plant Physiol) and of Grieu et al. (1988, Physiol Plant) who distinguished atmospheric drought (high evaporative demand, though "demand" is a problematic concept itself) from soil drought (lack of soil water per se). However, water shortage takes diverse forms as time series, so we should distinguish episodic drought from terminal drought (as occurs in Mediterranean climates). Different species have different physiological acclimation suites and different developmental programs to deal with terminal vs. episodic drought - you don't see species native to Iowa surviving in Western Australia. We should resolve stress (physiological drought) by at least three dimensions: duration, depth, and frequency. To return to tolerance limits for drought, the concept of drought frequency, especially, brings up the fact that tolerance itself acclimates. An earlier nonlethal drought commonly conditions a plant physiologically to the current drought, as by ABA accumulation, altered rooting patterns, etc. So, there is no LT50, unless one makes it a dynamic thing. Saying that there is an LT50 is like saying there is a fixed magnitude of stomatal conductance, gs. Since gs varies greatly but follows a control program, it is overwhelmingly more useful to view the control parameters (slope and intercept in the simple Ball-Berry model, variously amended in form and to incorporate water stress) as the fundamental descriptors and not gs values at some arbitrary environmental condition. Some regularities appear, such as that, in mesic plants, a Ball-Berry slope near 10 is almost universal. We need a few-parameter description of drought tolerance (not "few" as in oversimplified but few as in concise and fundamental). Extending this line of thought, we need to use an evolutionary context. Too much drought research harks back to irrigated agriculture of annual crops. Why are there species (nay, genotypes) that are so divergent in their tolerance of environmental conditions? Why isn't there a superstrategy to handle diverse environments? The idea of a superstrategy is unspoken in agricultural research, and it is very misleading. The selection pressures on wild plants and on crop plants are so different and this must be recognized (I wrote about this at length in my 1987 book, A Functional Ecology of Crop Plants). The evolutionary view of drought tolerance (DT) strategies has not been developed in a comprehensive framework. We have to recognize that drought tolerance is thrown in, for natural selection, with strategies for physiological competitiveness (Iphotosynthetic traits, rooting traits) for autecological performance and for traits for synecological performance (height competition, pollinator attraction, ...). In any one genotype, DT is not fully optimized (nor can it ever be perfect) when it must be in a joint optimization, traded off against degrees of optimization in other performance measures such as those I just mentioned. Mathematically, we have a case of constrained optimization for multiple objectives. It takes a great deal of quantitative formulation of plant performance to calculate this and comprehend its significance. Moreover, we have a system with stochastic drivers in the environment (rainfall stochasticity, e.g.). The proper formulation is one of risk management. A good start on this was made by Paltridge and Denholm, formulating optimal timing for the switch from vegetative growth to reproductive growth in an annual plant under the risk of an uncertain end to the growing season (frost, drought). We have to go up to many dimensions. Another interesting lead was made (Jones and Zur, 1984, J. Irrig. Sci.) in formulating plant performance in drought in terms of something like 20 parameters for stomatal control, rooting, etc., as I recall, and then running multiple simulations to see if an optimal set of parameters could be found. Plant performance optimization studies are common (Cowan and Farquhar, 1976, on to multiple studies by Farquhar and colleagues; Gutschick and Wiegel, 1988 and diverse related studies, including a particularly nice one by Schieving and Poorter). The mathematics of optimizing many parameters is not problematic (e.g., there are methods of simulated annealing or genetic algorithms); the problem is in discovering the fundamental parameters of plant physiology and development. We had a very fruitful week of discussion in 2010 that went into this topic repeatedly (a workshop at the Mathematical Biology Institute at Ohio State; we hope to publish our findings), but we have a long way to go. A final point is that we need a forensics of plant death, as Marilyn Ball of the ANU and I talked about long ago. Why does any one plant die? There has been some very good discussion in the past decade, such as work put out by Nate McDowell, Craig Allen, Dave Breshears, and others. They worked to distinguish carbon starvation from water-stress-induced cell death and other phenomena. The phenomena are linked, of course. Carbon starvation disallows a plant from expressing tolerance and recovery mechanisms such as growth of new vascular tissue or new roots. Cell death from stress is both a result of stress and (in key places, such as some vascular tissue) a cause of stress. Back to the first sentence - we need a coherent framework to discuss mortality of plants (and, as physicians also say, morbidity, a significant decline in function). When a human patient dies from complications of AIDS, such as pneumonia from an opportunistic infection, what is the cause? There are proximate causes (pneumonia...or the microbial species itself, or its availability...) and more nearly ultimate causes (AIDS...). Death is more of an ecosystem function (the patient, the HIV virus, the pneumonia microbe, the social milieu in which the patient acquired exposure to both) than an individual function or phenomenon. A view above the level of the organism is also made more appropriate when we consider that it's genes, not finite-lived individuals, that are the currency of the biosphere. I've laid out a very long roadmap into some terra incognita. However, like other explorers we have to go there. Otherwise we are in little parochial enterprises, seen in the large. Valuable as they may be in some area, our research efforts are then self-limiting. At least when we do explore in this mode, I think we won't be extinguishing native populations (oops - the Green Revolution did some of that, to small farmers; careful!).
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• Answer added in Plant Biology
  115 Why are plants green?
  By Ralf Niemann · University of Cambridge
  Vincent Gutschick · New Mexico State University

  Hi, everyone, Infrared is forgone as an energy source by plants, algae, and bacteria for good reason. If there's one reaction center (OK, two tha... [more]

  Hi, everyone, Infrared is forgone as an energy source by plants, algae, and bacteria for good reason. If there's one reaction center (OK, two that are very similar in energy level), then the energy of all absorbed photons is degraded to the energy level of the reaction center. This happens in all photosynthesis. To be able to use infrared while having one reaction center type, the energy of all photons, even in the PAR, would have to be degraded to the level of some infrared photon. E.g., if the reaction center had the energy of an 1000 nm photon, then the energy in a blue-light photon would be degraded by 60% (given that E = hc/lambda). In addition to making the use of PAR inefficient, the energy at the reaction center might be insufficient to do CO2 reduction for PS in the style of vascular plants. The other option is to have a separate set of reaction centers for handling lower energy photons. This is akin to multilayer solar cells (very efficient, very costly, only usable with concentrated solar power), and no organism has managed the feat. We're "stuck" with using the PAR only, and that's not so bad. Note that plants are not often energy-limited! In full sun, most PS is supersaturated; plants more often have to deal with excess energy than insufficient energy, as an ecosystem (understory shade plants have it harder, of course). Having enough N or water is more of a resource challenge for plants. There's also a packing problem: it's not possible to pack in enough photosynthetic apparatus to make use of full sun, if only because Rubisco is so slow as the rate-limiting enzyme.
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• Answer added in Plant Biology
  115 Why are plants green?
  By Ralf Niemann · University of Cambridge
  Vincent Gutschick · New Mexico State University

  Hi, everyone, Yes, resonant energy transfer is a bit off-track, but I'd like to add some counterpoint to Marco's answer. Foerster energy transfer... [more]

  Hi, everyone, Yes, resonant energy transfer is a bit off-track, but I'd like to add some counterpoint to Marco's answer. Foerster energy transfer is simply an expression of physics, independent of any molecular orbital model of molecular electronic states (HUMO, LUMO). It expresses the energy transfer probability between two molecules as the product of a density of rovibronic states and transition dipole moments. Bandgap energy models apply to regular arrays, as in solid-state physics, but using effectively the same formulation of pairwise interactions of molecules. Molecules in biological systems don't have those high-level spatial symmetries. One can use intermediate concepts, such as excitons in a semi-ordered array of molecules. Excitons are useful in discussing transfer among identical molecules, such as Chl in a PSU, but the pairwise view of Foerster transfer is useful to formulate rates between dissimilar molecules that have big differences in electronically excited states at the ground state in vibration and rotation. At the base, however, the formulations are different ways to similar answers. Getting back to using green light: isolated Chl molecules in solution are very poor absorbers of green light, it is true. However, nearby molecules perturb the Chls and their absorption is broadened to fill in some of the green-light gap, though not too much. The carotenoids, as auxiliary pigments, absorb much green light and pass energy to the Chls. Overall, leaves absorb 50% (typical annuals or crops) to 85% (e.g., Ficus) of light at the wavelength of minimum absorption, near 550 nm. That's not too bad, considering that there is no molecule that can absorb well over the whole PAR spectrum, and that molecules are very rare (really, only chlorophylls) that have the right combination of traits for energy capture and efficient transfer: strong absorption over a good part of the spectrum; chemical stability; high rates of internal conversion (from second excited singlet state S2, which covers the higher-energy part, the blue region, to first excited singlet state S1); weak rates of competing processes of radiationless transitions (dumping energy as heat) and intersystem crossing (creating triplet T1 that can exchange energy with ubiquitous oxygen, O2, in its triplet ground state, to make excited singlet oxygen that is a dangerous nonspecific oxidant); and redox states that allow photochemistry to start the whole process of photosynthesis. I am in awe of chlorophyll and of the wild coalescence of photochemistry and evolution that made photosynthesis possible, and so early in the history of life!
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• Answer added in Plant Physiology
  6 Is pheophytin one compound or is it group of compounds?
  By Tomáš Hluska · Palacký University of Olomouc
  Vincent Gutschick · New Mexico State University

  For each single chlorophyll (e.g., Chl a, Chl a, Bchl a, ...), there is a single pheophytin, in which the Mg atom in the center is replaced by 2 H ato... [more]

  For each single chlorophyll (e.g., Chl a, Chl a, Bchl a, ...), there is a single pheophytin, in which the Mg atom in the center is replaced by 2 H atoms covalently attached to two of the pyrrole rings. So, pheophytins may be taken as a class, but if you're talking about a specific chlorophyll molecule, there is only one corresponding pheophytin.
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• Answer added in Plant Biology
  115 Why are plants green?
  By Ralf Niemann · University of Cambridge
  Vincent Gutschick · New Mexico State University

  Hi, Satyajit and Xavier. While some phenolics / flavonoids fluoresce under UV irradiation, molecules don't need to emit light to transfer energy to C... [more]

  Hi, Satyajit and Xavier. While some phenolics / flavonoids fluoresce under UV irradiation, molecules don't need to emit light to transfer energy to Chl; Foerster transfer is a radiationless process, in which the quantum state (rovibronic state) of each molecule shifts, down in energy for the donor molecule and up in energy for the acceptor. That's how carotenoids pass energy to Chl and how Chl molecules pass it to each other in the PSU. I believe that there's no evidence of the phenolics doing Foerster transfer to Chl. Furthermore, the UV protectants are too far away to do the transfer (transfer rate falls off as distance to the 6th power), and there are a lot of constraints. One constraint is that the quantum states have to have the proper wavefunction symmetries. OK, now I raised one possibility for energy transfer and quashed the idea. Still, for either fluorescence or Foerster transfer to work well in getting energy to Chl, the energy of excitation has to be retained well against competing processes (radiationless relaxation, internal conversion to states too low in energy, intersystem crossing). Some phenolics do retain or partition the energy well (e.g., they fluoresce), but others do not. Finally, fluorescence of molecules that aren't strongly ordered in orientation, such as the phenolics in the epidermis, is omindirectional - only a portion goes from epidermis to mesophyll where the Chl is located Overall, then, with UV quantum flux density being only about 3% that of PAR and with the only moderate fluorescence quantum yields and with fluorescence "mis-aiming" toward the mesophyll cells, I think that we should consider that energy absorbed by UV protectants is an insignificant source of energy that is transferred to Chl.
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