EFFECT OF MALEIC HYDRAZIDE ON PHOTOSYNTHESIS, CARBOHYDRATE-METABOLISM, AND SENESCENCE OF BURLEY TOBACCO (NICOTIANA-TABACUM-L)

Maleic hydrazide (MH), a systemic agent used for control of axillary g rowth (suckers) in tobacco production, has been shown to alter cured-l eaf quality and increase yield compared to manual sucker control. The objective of this study was to determine how MH, applied at rates rang ing from zero (manual sucker control) to four times the usual commerci al 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 z ero to over 600 mug g-1. Leaf area was decreased and specific leaf wei ght was increased by increasing MH rates. Oven-dried leaf weight, howe ver, was similar for all treatments. CER was markedly decreased within 2 weeks after applying MH at rates above 1.68 kg ha-1. Chlorophyll co ncentration was also decreased by MH application, but not as rapidly o r to the same extent as observed for CER. Sucrose and starch levels we re significantly increased within 2 weeks after applying MH. As with C ER, 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 sucr ose-phosphate synthase (SPS) and adenosine diphosphate glucose (ADPG) pyrophosphorylase were not affected by any MH treatment. The results i ndicated that the increased sucrose and starch accumulation was due to decreased translocation of assimilate and not to increased photosynth esis. Because MH did not markedly alter total plant dry matter accumul ation, MH-induced decreases in CER and chlorophyll were possibly compe nsated for by leaves lower in the canopy. This conclusion was supporte d by the lack of effect of MH on starch and sucrose accumulation and c hlorophyll concentration for leaves in the middle of the canopy. While MH did inhibit photosynthesis of upper-canopy leaves, the results ind icated that, in general, plant senescence was not markedly affected.