Efficiency of light energy conversion in laboratory experiments and crop growth

Wassink, E.C.

Mededel Landboewhogesch Wageningen 64(16): 1-33


Accession: 024589324

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The driving force of the photosynthetic process is the absorption of light energy. The question of the efficiency of light energy has been discussed for a long time, e.g. by Liebig. Brown and Escombe established that the major part of absorbed energy goes in transpiration, a smaller part in reemission, and about 1% in photosynthesis. The first fundamental modern research led to assuming 4 quanta as necessary for the reduction of 1 molecule of CO2 (O. Warburg), later evidence (Daniels, Emerson) pointed more to ca. 8 quanta for positive, stationary photosynthesis. Recently 2 cooperative light reactions were recognized (Duysens). This not necessarily implies a theoretical minimum of 8 quanta per mol CO2 Attention is paid to the relation between quantum yield and the efficiency of solar energy conversion during growth, crop production depending on the latter. KOK showed that in small cultures of algae under optimal conditions and light limitation, an energy yield of 15-20% can be reached; Glas and Gaastra, for seedlings of higher plants under light limitation, found 10-15%. Gaastra, using data of Boonstra, found the energy yield of sugar beet growth in the field in early summer at closing canopy to be 7-9% while, over the whole season, good crop yield is only of the order of 1% (sugar beet 2%) (Wassink). Kamel found a similar situation in barley. GAASTRA determined quantitative photosynthesis-light curves for leaves, i.a. of sugar beet; at 0.03% CO2, CO2 generally limits light saturation. Using this type of data, DE WIT calculated "potential photosynthesis" of crop plants and arrived at yields of 6 to 9% for closed canopy; only under favorable conditions this is actually reached. Complicating phenomena were discussed. Gladiolus, e.g., later in the season, hardly shows any increase in dry weight while the new corm is mainly formed then; obviously most of the photosynthetic energy goes (directly or indirectly) in translocation processes. Other data (KAMEL, FOGG) point into the same direction. In aging algal cultures the daily dry weight increase diminishes, while carbohydrate and fat content increase. Other, even more spectacular examples of interaction between formative effects and photosynthesis are in factors influencing leaf development and, therefore, increase in dry weight. Such are (e.g. in lettuce, BENSINK): light intensity, night temperature, nitrogen supply, genetic factors, growth substances; the root-top ratio also is an important morphogenetic feature. In Gladiolus, a narrow relation was observed between leaf shape and dry weight increase (WASSINK); a morphogenetic agent must be active at high light intensities. Also water supply strongly affects dry weight production (ABD EL RAHMAN, KUIPER, C.S.). The importance of photoperiod was discussed; in photoperiods in which a plant layer remains vegetative, a greater dry weight is reached, both in long (VAN OORSCHOT) and short day plants (SMILDE). Also thermoperiodic effects (to be distinguished from direct effects of periodic temperature differences) may influence dry weight production (data of HARTSEMA and LUYTEN). Finally, total dry matter production on earth was discussed; uncertainties are here owing to lack of pertinent data which the I.B.P. (International Biological Program) may improve. The most plausible estimate now seems to be 0.27% for the land surface, 0.11% for the oceans, and 0.16% for the whole, the oceans covering about 70% of the total surface. For the inhabitable part of the earth's surface, the fraction of photosynthetically active radiation is around 10-4; for a densely populated area like the Netherlands this is around 10-3; for a thinly populated tropical area like New Guinea it is in the order of 10-6.