Seasonal trends and relationships of light, temperature and leaf physiological traits of sugar beets (Beta vulgaris L.) grown under semi-arid, Mediterranean conditions

The seasonal changes of leaf physiological traits and its relationship with abiotic factors (photosynthetic active radiation-PAR, leaf temperature-Tleaf, air temperature-Tair, Tleaf - Tair- ΔT) were studied on sugar beets grown under the semi-arid conditions of central Greece. Sugar beet (Beta vulgaris L.) cv Rizor was established in a Randomised Complete Block design experiment for two years (2004-2005). From early June till the end of October, 11 determinations of leaf physiological traits (intracellular to ambient CO2 concentration-ci, transpiration rate-E, stomatal conductance-gs, net photosynthesis-A, instantaneous water use efficiency-A/E, photosynthetic water use efficiency-A/gs, chlorophyll content-SPAD, carbon isotope discrimination- Δ, N concentration and specific leaf area-SLA) took place. Temperatutes (Tleaf and Tair) and ΔT were higher in 2004 when water budget (WB, precipitation + irrigation – evapotranspiration) was more negative. Also, seasonal mean of carbon isotope discrimination (Δ) and net photosynthesis (A) were lower in 2004 to compare to 2005. A gradual decline with the progress of the growing season was found for A, Δ, gs, E, SLA and leaf N concentration while the adverse trend was evident for water use efficiencies (WUE, A/E and A/gs). PAR was positively related with gs and E and negatively with A/E. Temperatures (Tair and Tleaf) had a positive effect on E and a negative one on A/E. High temperatures (35-37 ºC) degraded leaf chlorophyll but temperatures higher than 37 ºC tended to increase chlorophyll content. ΔT (transpirational cooling) was negatively related with gs, E, and A and positively with A/E and A/gs. Both E and A were regulated by gs which was kept higher or slightly lower than 0.10 mol m-2 s-1, a limit indicative of severe drought in C3 species. Positive relationships were found between A, leaf N concentration and SLA. SLA is an indicator of resource-use strategy and can provide useful information about abiotic factor effects on sugar beet physiology and yield. Δ was negatively related with A/gs showing that it can be an indirect assessment of WUE at leaf level.