Influence of transplant size on the above- and below-ground performance of four contrasting field-grown lettuce cultivars

Abstract

Background and aims: Modern lettuce cultivars underperform under conditions of variable temporal and spatial resource availability, common in organic or low-input production systems. Information is scarce on the impact of below-ground traits on such resource acquisition and performance of field-grown lettuce; exploring genetic variation in such traits might contribute to strategies to select for robust cultivars, i.e., cultivars that perform well in the field, even under stress.Methods: To investigate the impact of below-ground (root development and resource capture) on above-ground (shoot weight, leaf area) traits, different combinations of shoot and root growth were created using transplants of different sizes in three field experiments. Genetic variation in morphological and physiological below- and above-ground responses to different types of transplant shocks was assessed using four cultivars.Results: Transplanting over-developed seedlings did not affect final yield of any of the four cultivars. Small transplant size persistently impacted growth and delayed maturity. The cultivars with overall larger root weights and rooting depth, “Matilda” and “Pronto,” displayed a slightly higher growth rate in the linear phase leading to better yields than “Mariska” which had a smaller root system and a slower linear growth despite a higher maximal exponential growth rate. “Nadine,” which had the highest physiological nitrogen-use efficiency (g dry matter produced per g N accumulated in the head) among the four cultivars used in these trials, gave most stable yields over seasons and trial locations.Conclusions: Robustness was conferred by a large root system exploring deep soil layers. Additional root proliferation generally correlates with improved nitrate capture in a soil layer and cultivars with a larger root system may therefore perform better in harsh environmental conditions; increased nitrogen use efficiency can also confer robustness at low cost for the plant, and secure stable yields under a wide range of growing conditions.

Diverse concepts of breeding for nitrogen use efficiency. A review

“…When N stress occurs in a transplanted lettuce crop in the upper soil layer (0-20 cm), roots can elongate and increase root length density at deeper soil layers to capture more N (Kerbiriou et al 2013a). Kerbiriou et al (2013b) showed that lettuce cultivars with higher root weight and rooting depth resulted in higher yields. However, one of the tested cultivars had the highest physiological use efficiency (g dry matter produced per g N accumulated in the head) and gave the most stable yield across trials despite a relatively small root system.…”

Genetic Control of Water and Nitrate Capture and Their Use Efficiency in Lettuce (Lactuca sativa L.)

“…Nitrate content (soil [NO 3 ], assessed in ppm) in each 0.1 m soil layer was measured using an Ion Selective Electrode (ThermoFisher™, Waltham, MA, USA) using the method described previously by Sibley et al ( 2009 ) and also used in Kerbiriou et al ( 2013a ).…”

“…Kerbiriou et al ( 2014 ) showed that in lettuce the relationship between root mass and nitrate capture does not follow the relationship found by King et al in barley (King et al, 2003 ), where the non-captured resource logarithmically declines with an increase in the amount of roots or with the root length density. Although nitrate capture in lettuce is generally fairly correlated to root mass or root length density when field conditions are conducive to growth (Kerbiriou et al, 2013a ), in lettuce localized root growth is related to specific, localized resource availability as demonstrated by Kerbiriou et al ( 2013b ) in a pot trial. In case localized nitrate shortage was applied, root growth was more abundant in N rich soil layers—as previously noted by Hodge ( 2004 ) in grass species under various conditions—whereas when localized drought was applied, root growth occurred in the dry compartment (as opposed to the moist compartment).…”

“…Agronomic research has contributed to the design of lettuce cropping systems that maximize yields and optimize quality by supplying abundant water and nutrients, avoiding stress conditions (Gallardo et al, 1996a , b ; Broadley et al, 2000 ; Frantz et al, 2004 ). In lettuce, drought induced by a shortage in water supply, even temporary, significantly reduces yields, as drought limits shoot growth rate (Biddington and Dearman, 1985 ; Kerbiriou et al, 2013a ). With costs of fossil fuel-based inputs forecasted to increase steadily in the future (Mou, 2011 ), the environmental and economic sustainability of such intensive systems is becoming more and more questionable, calling for the design of more resilient systems.…”

Root and rhizosphere processes–high time to dig deeper

“…The study by Kerbiriou et al (2013) provides first evidence that “robustness” and head growth rates of lettuce cultivars are related to the size of the root system. Terzaghi et al (2013) investigated C and N concentrations in Fagus sylvatica fine roots in relation to different stand characteristics resulting from conversion of coppiced forests to high forests.…”