Salt Tolerance in Crop Plants Monitored by Chlorophyll Fluorescence In Vivo

Abstract

The potential of measurements of chlorophyll [Chl] fluorescence in vivo to detect cellular responses to salinity and degrees of salt stress in leaves was investigated for 3 crop plants. Sugar beet (Beta vulgaris L.) (salt tolerant), sunflower (Helianthus annuus L.) (moderately salt tolerant) and bean (Phaseolus vulgaris L. cv. Canadian Wonder) (salt intolerant) were grown in pots and watered with mineral nutrient solution containing 100 mm NaCl. The fast rise in variable Chl fluorescence yield that is correlated with photoreduction of photosystem [PS] II acceptors increased in leaves of sugar beet plants treated with salt suggesting stimulation of PS II activity relative to PS I. In sunflower, this fast rise was depressed by .apprx. 25% and the subsequent slow rate of quenching of the Chl fluorescence was stimulated. These differences were more marked in the older mature leaves indicating an increasing gradient of salt response down the plant. The salt effect in vivo was reversible since chloroplasts isolated from mature leaves of salt-treated and control sunflower plants gave similar PS II activities. Unlike in sugar beet and sunflower, leaves of salt-treated bean progressively lost Chl. The rate of slow quenching of Chl fluorescence decreased indicating development of a partial block after PS II and possible initial stimulation of PS II activity. With further loss of Chl, PS II activity declined. Evidently, measurements of Chl fluorescence in vivo can provide a rapid means of detecting salt stress in leaves, including instances where photosynthesis is reduced in the absence of visible symptoms. The possible application to screening for salt tolerance is discussed.