How to define yield with energy crops? This is both more flexible and less precise than with, for example, crop yield as measured by grain size or weight per plant or per unit area. For a feedstock such as wheat straw, increasing plant biomass accumulation per plant may suffice, and this implies a change in nutrient utilization or absorption from the soil, but this may paradoxically reverse the trend toward “dwarf” crops (i. e., more grain, less stem/stalk) that has been emphasized in the green revolution type of agronomy.290 In principle, simply achieving higher leaf, stalk, and stem mass per plant is a straightforward target that is not limited by considerations of morphology for crops dedicated to energy supply and/or biofu­els. Mass clonal propagation of commercial trees is now well advanced, and gene transfer technologies have been devised for conifers and hardwoods, that is, forest biotechnology has emerged (probably irreversibly) out of the laboratory and into the global ecosystem.291

Traditional breeding and marker-assisted selection can identify genes involved in nutrient use efficiency that can then be used in gene transfer programs to improve features of plant nutrition — for crop plants in intensive agriculture, nitrogen assimilation and recycling within the plant over the stages of plant development are crucial.292 Although plant biochemistry is increasingly well understood at the molecular level, what is much less clear is how to accurately modulate gene expres­sion (single genes or whole pathways) to achieve harvestable yield increases.290 As with other “higher” organisms, a greater understanding of how regulatory circuits and networks control metabolism at organ and whole-plant levels as an exercise in systems biology will be necessary before metabolic engineering for yield in crop plants becomes routine.294

Nevertheless, successes are now being reported for “gene therapy” with the goal of improving the assimilation of CO2 into biomass, however this is defined:

• Transgenic rice plants with genes for phosphoeno/pyruvate carboxylase and pyruvate, orthophosphate dikinase from maize, where the two enzymes are key to high photosynthetic carbon fixation under tropical conditions (“C4 metabolism”), increases photosynthetic efficiency and grain yield by up to 35% and has the potential to enhance stress tolerance.295

• Once CO2 is “fixed” by green plants, some of the organic carbon is lost by respiratory pathways shared with microorganisms, and there are several reports that partially disabling the oxidative pathways of glucose metabo­lism enhances photosynthetic performance and overall growth: for exam­ple, in transgenic tomato plants with targeted decreases in the activity of mitochondrial malate dehydrogenase, CO2 assimilation rates increased by up to 11% and total plant dry matter by 19%.296

• Starch synthesis in developing seeds requires ADP-glucose phosphorylase; expressing a mutant maize gene for this enzyme in wheat increases both seed number and total plant biomass, these effects being dependent on increased photosynthetic rates early in seed development.297

Much attention has been given to improving the catalytic properties of the primary enzyme of CO2 fixation, ribulose 1,5-bisphosphate carboxylase (Rubisco), the most abundant single protein on Earth and one with a chronically poor kinetic efficiency for catalysis; although much knowledge has been garnered pertaining to the natural variation in rubisco’s catalytic properties from different plant species and in developing the molecular genetics for gene transfer among plants, positive effects on carbon metabolism as a direct result of varying the amounts of the enzyme in leaves have proved very slow to materialize.298 A more radical approach offers far greater benefits: accepting the inevitable side reaction catalyzed by Rubisco, that is, the formation of phosphoglyceric acid (figure 4.15), transgenic plants were constructed to contain a bacterial pathway to recycle the “lost” carbon entirely inside the chloro — plast rather than the route present in plant biochemistry that involved the concerted actions of enzyme in three plant cell organelles (chloroplast, mitochondrion, and peroxisome); transgenic plants grew faster, produced more biomass (in shoot and roots), and had elevated sugar contents.299

Is there an upper limit to plant productivity? A temperate zone crop such as wheat is physiologically and genetically capable of much higher productivity and efficiency of converting light and CO2 into biomass than can be achieved in a “real”

environment, that is, in hydroponics and with optimal mineral nutrition; again, most studies focus on yield parameters such as grain yield (in mass per unit area) but total leaf mass (a component of straw or stover) will also increase under such ideal conditions.300 The higher the light intensity, the greater the plant response, but natural environments have measurable total hours of sunshine per year, and the climate imposes average, minimum, and maximum rainfall and temperatures; supplementary lighting is expensive — but, given unlimited renewable energy resources, substantial increases in plant productivity are theoretically possible. From a shorter-term perspective, however, the choice of biomass refuses to be erased from the agenda, and agricultural wastes simply cannot compete with the “best” energy crops. For example, using biofuel yield as the metric, the following ranking can be computed from relative annual yields (liters per hectare):301

Corn stover (1.0) < poplar (2.9) < switchgrass (3.2) < elephant grass (Miscanthus, 4.4)

Rather than mobilizing molecular resources against the vagaries of climate, concen­trating effort on maximizing biomass supply from a portfolio of crops other than those most presently abundant would pay dividends. Overreliance on a few species (vulnerable to pests and climatic variability) would be minimized by expanding the range of such crops.