Effects of NO – Availability on Total Productivity, Root Morphology, Photosynthesis and Nitrogen Metabolism of Lettuce ( L.) Recombinant Inbred Lines

ABSTRACT

We have identified certain heat-resistant (HR) and heat-sensitive (HS) lettuce (Lactuca sativa) recombinant inbred lines (RILs) from 113 lines under hot ambient temperature by studying the root morphology, shoot and root productivity. Except for temperature, one of the other major determinants of root morphology is nitrate (NO₃^–) availability. In this study, total productivity, root morphology, photosynthesis and nitrogen (N) metabolism of two RILs, 168 HS and 200 HR were studied under full N (100% NO₃^–), +N (125% NO₃^–) and –N (50% NO₃^–). The shoot and root productivity of both RILs under +N and –N treatments declined compared to those of full N plants. Reductions in root length, root surface area and total number of root tips were observed in 168 HS plants under both +N and –N treatments. For 200 HR plants, they all had similar values of root parameters regardless of N treatments. There were no significant differences in the light saturated CO2 assimilation (Asat) and stomatal conductance (gs sat) between two RIL plants For each lettuce RIL, no differences in total chlorophyll (Chl ) content and Chl a/b ratio were observed among the different N treatment. For both lettuce RILs, shoot NO₃^– concentration was highest in +N followed by full N plants and –N plants had the lowest values. There were no differences in root NO₃^– concentration between +N and full N plants but root NO₃^– concentration was significantly lower in –N plants than in +N and full N plants. For shoot total reduced N, +N plants had significantly higher concentration in both RILs compared to those of full N and –N plants. All plants had similar root total reduced N concentrations except for 168 HS under –N condition, which had significantly lower total reduced N concentrations. Differences in shoot maximal nitrate reductase (NR) activity among the different N treated plants were similar to those of total reduced N concentration. The relationships among NO₃^– availability, root morphology, productivity, photosynthesis and N metabolism were discussed.

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