Salinity and Extreme Temperature Effects on Sprouting Buds of Sugarcane (Saccharum officinarum L.): Some Histological and Biochemical Studies

Abstract

Sugarcane shows reduced crop stand under relatively suboptimal conditions, the main reason for this is sensitivity of bud tissue to temperature fluctuations and salinity at sowing time. The aim of these studies was to explore the physiological, developmental and molecular changes occurring in the immature sugarcane buds under heat, cold and salt stresses, and possible role of proline and glycinebetaine in mitigating the changes in a time course manner during sprouting of nodal buds. All the stresses reduced bud fresh and dry weight, led to the generation of H 2 O 2 , reduced the tissue levels of K + and Ca 2+ , but enhanced the synthesis of osmolytes in a time course manner. Heat stress mainly produced oxidative damage and acted as a dehydrative force, whereas cold stress caused oxidative stress and slowed down the physiological activities. Salinity was the most damaging of all the stresses. The main effects of salinity were the accumulation of Na + and Cl - , reduced tissue contents of Ca 2+ and K + and enhanced synthesis of H 2 O 2 in the developing sugarcane bud. As for histological changes, all the stresses delayed and reduced the formation of new bud leaves and their expansion, which was mainly because of reduction in the number and area of mesophyll cells and poor development of vascular bundles. The pretreatment of bud chips with proline and GB effectively reduced stress effects being more effective under heat stress followed by cold stress and the least under salinity stress. As revealed from the correlation studies, although pretreatment with proline and GB appeared to have no direct role in stress tolerance, main effects were the reduced generation of H 2 O 2 , improvement in the K + and Ca 2+ nutrition and further enhancement in the levels of free proline, GB and soluble sugars under heat and cold stress. Under salt stress, the pretreatment reduced Na + and Cl - , in addition to the observed effects under heat and cold stresses. Histological changes revealed that the pretreatment with osmoprotectants increased the mesophyll cell area leading to expansion in the bud leaves and led the development of elaborated vascular tissues. Of the two, GB was more effective than proline for all stress treatments but for different measured variables. Detailed studies on the molecular and metabolic responses indicated the enhanced expression of Asn, dehydrins, LEA and GAPDH genes, stronger accumulation of proline and alanine followed by glycine, glutamic viiiacid and 5-oxo-proline. Among the sugars, sucrose followed by diethylene glycol, fructose, glucose and glycerol, and nucleic acids, adenosine and uracil, as well as organic acids, aconitate, chlorogenate, ribonate_put, quinate, pipecolate, erythronate, GABA, glucoronate, gluconate, glucarate, glucoranate and octadecanoate showed accumulation at all temperature treatments and time periods. HCA and LVL revealed that out of 108 metabolites, chlorogenate, putrescine, octadecanoate, fructose, proline, glycine, sucrose, quinate, trans-aconitate, guanine, GABA and ethanolamine showed greater accumulation under high temperature. In crux, albeit all stresses deterred the transition of bud from immature to mature state, the salinity was the most damaging. In most cases, improvement produced by proline was greater than GB. Heat stress revealed distinct patterns of gene expression and metabolites synthesis. The correlation data showed that pretreatment with the osmoprotectants improved bud growth under stress regimes; the roles are indirect in improved Ca 2+ and K + nutrition and reduced production of H 2 O 2 . Nevertheless, these findings suggest that in low to moderately hot, cool and saline areas sugarcane bud sprouting can be improved by pretreatment of bud chips with the 20 mM levels of proline and GB and requisite crop stand can be achieved