Anil P. Ranwala

Preventive mechanisms of gibberellin4+7 and light on low-temperature-induced leaf senescence in Lilium cv. Stargazer

Abstract Lilium cv. Stargazer plants were held for 2 weeks at 4°C, either in darkness or in light (40 μmol m −2 s −1 ). Another series of plants was held in darkness after pre-treatment with 100 mg l −1 GA 4+7 . Changes in major senescence parameters were determined during storage at 4°C and during 6 days after transferring the plants to 22°C. Foliar sprays of GA 4+7 or supplemental light prevented rapid leaf senescence induced by dark low-temperature storage. During storage at 4°C, basal leaves showed no significant changes in concentrations of chlorophyll, soluble proteins, lipid peroxidation, and activity of catalase. However, shifting from 4 to 22°C induced a series of changes in basal leaves of dark-held plants including rapid loss of chlorophyll, proteolysis, increased lipid peroxidation and loss of catalase activity. Both light and GA 4+7 treatments prevented these changes. Total soluble carbohydrates decreased gradually during 4°C dark storage and after transferring to 22°C. GA 4+7 treatments did not prevent the decline in carbohydrate levels at 4°C, but prevented it upon transferring to 22°C. Supplemental light during 4°C storage significantly increased soluble carbohydrate concentration. The abrupt increases in metabolic activities by shifting from 4 to 22°C accompanied by oxidative stress in leaves already depleted in reserves during 4°C storage seem to induce leaf senescence in dark-held plants.

Gibberellin-mediated changes in carbohydrate metabolism during flower stalk elongation in tulips

We investigated the effects of a gibberellin synthesis inhibitor (ancymidol) and gibberellin (GA4+7) on carbohydrate metabolism and elongation in internodes of the tulip (Tulipa gesneriana L.) flower stalk during greenhouse growth. During the initial stages of flower stalk growth, the lowermost internode was mainly responsible for total flower stalk length, whereas the uppermost internode mostly contributed to total length during later stages. High concentrations of hexose sugar (mainly glucose) and increased activity of acid invertase were observed when internodes were rapidly elongating. Inhibition of gibberellin biosynthesis with ancymidol reduced the elongation rate of internodes, and inhibited the hexose sugar accumulation and acid invertase activity. Application of GA4+7 to ancymidol-treated plants reversed these effects. The degree of response to ancymidol and GA4+7 was, however, different in different internodes such that the lowermost internode was most responsive and the uppermost internode was least responsive. The results indicate that de novo biosynthesis of gibberellins is a requirement for expression of high acid invertase activity during the rapid elongation phase in tulip internodes which enables cleavage of imported sucrose to hexoses that can be readily utilized in elongating cells.

Changes in soluble carbohydrates during phytochrome-regulated petiole elongation in watermelon seedlings

Changes in soluble carbohydrate composition and concentration in leavesand petioles of watermelon (Citrullus lanatus (Thunb)Matsum and Nakai cv. Sugar Baby) seedlings during early stages ofphytochrome-regulated petiole elongation were investigated. Watermelon seedlingswere grown in a controlled environment with 350 μmolm−2 s−1 photosynthetically activeradiation (PAR) during a 12-h photoperiod. Low intensity end-of-day(EOD) light treatments (for 15 min) of red (R), far-red (FR) and FRfollowed by R (FR/R) were initiated when the seedlings were 14 days old.Seedling growth, and soluble carbohydrate concentration and composition inleaves and petioles were determined after 3 and 6 days of EOD light treatments.The EOD FR increased the petiole length and dry mass partitioned to petioles asearly as 3 days into the treatment. This increased petiole dry mass inFR-treated plants was accompanied with an increase in reducing sugar (glucoseand fructose) concentration in the petioles. Although both leaves and petiolesshowed this effect, the relative increase was greater in petioles than leaves.While the most abundant sugars in petioles were fructose and glucose, thepredominant sugars in leaves were sucrose, raffinose, and stachyose. Thephotoreversion of FR induced changes in growth and sugar concentrations by Rindicates the involvement of phytochrome in these processes.