Effect of defoliation on leaf physiology of sugar beet cultivars subjected to water stress and re-watering

Water stress causes defoliation, which can reduce yield and root quality of sugar beets (Beta vulgaris L.) through altered gas exchange characteristics of the leaves. In a two-year experiment, three sugar beet cultivars (Europa, Rival and Corsica) were subjected to three defoliation levels (control-C, moderate-MD, severe-SD) and re-watering after their exposition to drought for a month. Leaf physiological traits including net photosynthesis (A), transpiration rate (E), stomatal conductance (g(s)), intracellular CO2 (C-i), water use efficiency (WUEL-A/E and WUEi-A/g(s)), leaf N concentration, petiole NO3-N concentration, specific leaf area (SLA), leaf water potential (WP) and leaf water content (LWC), were determined before defoliation and 15, 30 and 40 days after defoliation (DAD). On contrary to previous reports, water-stressed cultivars differed significantly in their leaf physiology; the late-season cultivar Corsica had the lowest E and g(s) values without any significant reduction in A. Thus, Corsica was the most water-conservative cultivar. Re-watering rapidly restored leaf physiology but a gradual decline, with the progress of DAD, was evident for A, E, g(s) and C-i. After re-growth, cultivars differed only in WP and LWC with Europa, the early-harvested cultivar, to have the highest values. Thus, the better response (higher yield increase and lower root quality degradation) of Corsica to re-watering and the subsequent re-growth, as reported by Tsialtas et al. (2009), could be ascribed to its better LAI maintenance compared to Europa and Rival rather than to a better leaf physiological response. The SD plants showed the highest leaf physiological activity since they had the highest A, E, g(s) and leaf N concentration. The high leaf activity and the low respiration rate are the means by which sugar beets tolerate foliage losses up to 75% and compensate yield losses after defoliation.