Identification of Stress Tolerance Traits in Sugar Beet

Abstract

Drought stress is probably the largest single cause of lost potential yield in UK sugar beet production and global warming seems certain to exacerbate this effect. Irrigation, the obvious corrective measure for drought stress, is only used to a limited extent, and this is unlikely to change. Modern varieties of sugar beet are derived from seashore beet, which is accustomed to a saline environment and has specific mechanisms to combat water stress. Drought-tolerance has, however, been a low priority in the selection and breeding of European sugar beet varieties in the past. We are trying to identify traits responsible for stress tolerance and develop suitable screening methods. During drought, beet leaves are subjected to both heat and water stress. We are using measurements of photosynthetic efficiency (principally chlorophyll fluorescence) to measure the effects of both of these stresses upon the plant. A portable plant efficiency analyser (Hansatech Instruments Ltd) is being used to analyse the chlorophyll-fluorescence induction kinetics (Kautsky effect) of dark-adapted leaves. A number of parameters are recorded, but the principal area of study is the use of the ratio of the variable fluorescence (F_v) to the maximal fluorescence (F_m) as a measure of photosynthetic efficiency. During water stress, sugar beet accumulates amino acids (mainly glutamine) and glycine betaine. Unfortunately these, together with sodium and potassium, are the principal impurities which reduce beet quality for processing. An HPLC system is being used to separate and measure sucrose, invert sugars and glycine betaine in stressed plants. The content of these components, other quality parameters (amino acids, sodium and potassium) and total sap osmotic concentration are being correlated with the estimated varietal stress resistance with the aim of identifying the characteristic(s) responsible for stress tolerance. Similarly, plants are being screened for the presence and quantity of the major classes of desiccation stress and heat shock proteins.