Uptake of nitrogen by flue-cured tobacco during maturation and senescence

Abstract

A field experiment was conducted with flue-cured tobacco,Nicotiana tabacum L., to examine the hypothesis that after removal of the reproductive tissues (topping), which is coincident with rapid depletion of soil nitrate, the growing apical leaves receive their N primarily from older, senescing leaves. On the 83rd day after transplanting (crop day 83) the nitrate remaining in the plow layer was leached downward and replaced with an equivalent quantity of15N-labeled nitrate. Theoretically, any N taken up by the plants thereafter could be distinguished from the endogenous N present in the plant on crop day 83, thus allowing remobilization of the latter to be estimated. However, some soil N remained accessible to the plants throughout the remainder of the growing season, and as a consequence may have limited the remobilization of endogenous N. Plants were harvested and sectioned into 11 different plant parts at six intervals up to crop day 127. The nitrogen in each plant tissue was fractionated into nitrate-N, soluble reduced-N (SRN), and insoluble N (IN). Tissue nitrate, which accounted for less than 0.7% of total N in the tissue, remained relatively constant throughout the 44 day period. In contrast, SRN and IN fluctuated appreciably. During a 6-day period of water stress, 25% of the IN in the root and leaf lamina was hydrolized to SRN. It was postulated that the drought initiated a type of reversible senescence in which protein was hydrolyzed to amino acids, thereby increasing the osmotic potential of the tissues and improving cell survival. During recovery from water stress, SRN was reincorporated into IN in all tissues except the lower leaves, which were senescing. Root growth was predominant during this period and was supported by N remobilization from the lower leaves and root stump. After subsequent harvest of the lower and middle leaves, growth of the remaining upper leaves was supported in part by remobilization of N from the adjacent stalk tissue.