Growth of lettuce, onion, and red beet. 1. Growth analysis, light interception, and radiation use efficiency

A field experiment was carried out to analyse the growth of lettuce, onion and red beet in terms of (i) canopy architecture, radiation interception and absorption, (ii) efficiency of conversion of absorbed radiation into biomass and (iii) dry matter partitioning. Growth analysis, total solar radiation interception, PAR interception and absorption by the crop canopy, ground cover, maintenance respiration of onion bulbs and red beet storage roots were measured. Models for different leaf angle distribution and ground cover were used to simulate light transmission by the crop canopy. The three crops are shown to have contrasting growth patterns from both a morphological and a physiological points of view. Lettuce showed very high light interception and growth after the early growth stages but, throughout the growth cycle, this leafy crop showed the lowest radiation use efficiency due to the respirational cost of the high leaf area. Onion showed a lower early relative growth rate than lettuce and red beet. This was due partly to the low light interception per unit leaf area in the later stages of growth and partly to the low initial radiation use efficiency compared with the other two crops. On the other hand, thanks to more uniform distribution of the radiation inside the canopy, to the earlier termination of leaf development and to the very low level of bulb respiration, onion showed high radiation use efficiency and was able to produce a large amount of dry matter. Red beet leaf posture and canopy structure resulted in high light interception and absorption. Its radiation use efficiency was lower than that one of onion, partly perhaps because of the more adverse distribution of the intercepted radiation fluxes within the canopy and partly because of the high respiration cost of a continuous dry-matter allocation to the leaves. However, this crop can accumulate a very large amount of dry matter as leaf blade development and storage root growth can both continue almost indefinitely, providing continuously available sinks. Ground cover gave a good estimate of the PAR interception only at low values of light interception but, in general, it underestimated PAR interception in any of the three crops. Ratios between attenuation coefficients established by considering PAR or total solar radiation and LAI or ground cover were calculated.