Effect of maleic hydrazide on photosynthesis, carbohydrate metabolism, and senescence of burley tobacco (Nicotiana tabacum L.)

Abstract

Maleic hydrazide (MH), a systematic agent used for control of axillary growth (suckers) in tobacco production, has been shown to alter cured-leaf quality and increase yield compared to manual sucker control. The objective of this study was to determine how MH, applied at rates ranging from zero (manual sucker control) to four times the usual commercial rate of 3.36 kg ha−1, influenced the carbon dioxide exchange rate (CER), carbohydrate metabolism, and senescence of upper-canopy leaves. Leaves treated with various MH rates had residue levels ranging from zero to over 600 μg g−1. Leaf area was decreased and specific leaf weight was increased by increasing MH rates. Oven-dried leaf weight, however, was similar for all treatments. CER was markedly decreased 2 weeks after applying MH rates above 1.68 kg ha−1. Chlorophyll concentration was also decreased by MH application, but not as rapidly or to the same extent as observed for CER. Sucrose and starch levels were significantly increased within 2 weeks after applying MH. As with CER, MH effects on sucrose and starch accumulation were maximum at 3.36 kg ha−1 MH with no further change at higher rates. Activities of sucrose-phosphate synthase (SPS) and adenosine diphosphate glucose (ADPG) pyrophosphorylase were not affected by any MH treatment. The results indicated that the increased sucrose and starch accumulation was due to decreased translocation of assimilate and not to increased photosynthesis. Because MH did not markedly alter total plant dry matter accumulation, MH-induced decreases in CER and chlorophyll were possibly compensated for by leaves lower in the canopy. This conclusion was supported by the lack of effect of MH on starch and sucrose accumulation and chlorophyll concentration of leaves in the middle of the canopy. While MH did inhibit photosynthesis of upper-canopy leaves, the results indicated that, in general, plant senescence was not markedly affected.