Sugarcane is an important sugar crop of Pakistan, but shows sensitivity to supra-optimal growth temperature. Different varieties of this crop show differential sensitivity to prevailing heat stress. The responses to high temperature are elicited at physiological and molecular levels. In this project two differentially heat tolerant sugarcane varieties were compared for some growth, physiological and biochemical responses to heat, with particular reference to tissue-specific expression and immunolocalization of dehydrins, proteins synthesized in response to dehyrdative forces. The data for various attributes were recorded at seedling, grand growth and maturity stages of growth.
Data revealed that applied heat stress over the period of three days changed most of the physiological attributes, although growth attributes were minimally affected. Photosynthetic attributes and water relations of sugarcane leaves are greatly modulated under heat stress, although both the varieties revealed differences due to the differences in their heat tolerance potential. Little differences in the growth parameters appear due to the short exposure to the heat stress, although the reduction in growth takes place at prolonged exposures. Correlation of gas exchange attributes with growth and water relations attributes revealed that heat stress decreased the Pn by decreasing the \j/w and \I/ potential and P n:E, as well as the decreased capacity of Rubisco to assimilate the absorbed C02. From a greater decrease in HSF-240 (heat sensitive variety), it appeared that reduction in these parameters was a great manifestation of heat sensitivity response in sugarcane
Both the varieties indicated the differential expression of dehydrins at all growth stages. CP-4333 indicated the expression of three, while HSF-240 two dehydrins peptides at formative and maturity stages. At grand growth stage, two proteins were expressed in CP-4333 and just one in HSF-240. Furthermore, the expression of dehydrins was much stronger in CP-4333 than HSF-240. This indicated that better heat tolerance ability of CP- 4333 in terms of improved water relations and sustained photosynthetic attributes was due to enhanced ability of this variety to accumulate dehydrins and their persistence for a longer time even under recovery from stress. However, this ability was lacking in HSF- 240.
Immonohistochemical staining showed that dehydrins were associating to the vascular bundles sheath and phloem parenchyma. Furthermore scattered dehydrins accumulation was seen in the epidermal tissue. This revealed that under heat stress dehydrins play a protective role. Their accumulation in these tissues has specific implications. When associate to epidermal cell, they appear to protect the underlying tissues from the high temperature. When associate to vascular tissue, they tend to ensure the normal photoassimilate loading to the sieve element-companion cell (SE-CC) complex and partitioning to other plant parts under heat stress. Their disappearance during recovery from stress indicates that dehydrins accumulation was transitory and specifically accumulated under heat stress. This may be an adaptive advantage of sugarcane to cope with high temperature stress.
The above findings confirm that like other environmental stresses dehydrins are also expressed under heat stress. Further studies on structural characterization of the expressed dehydrins are imperative. It is also very important that other high yielding sugarcane and other crops may be screened for the expression of dehydrins in order to cope with adversaries of high temperature, which may be a great threat to crop production in the near future.