desiccation method. The method was developed by Blum et al. (1983a, b) as a fast
and relatively simple field assay for revealing the capacity for seed filling from stem
reserves. The method is based on the application of a chemical desiccant to plant
canopies after flowering as a means for inhibiting plant photosynthesis and thus
revealing the capacity for seed filling by stem reserves. The treatment does not
simulate drought stress. It simulates the effect of stress by inhibiting current assimilation. With this method a chemical desiccant (potassium iodide; 0.2% w/v) is
sprayed to complete wetting over the whole canopy, at seed filling stage (R5 plus
8–10 days) in soybean (Bhatia et al. 2014), which mainly destroys chlorophyll and
simulates natural senescence. Chemical desiccation can be incorporated into breeding programs in two ways: (1) it can be used to assess responses of individual
advanced lines or families, always compared with non-treated controls under
non-stress conditions and (2) the method can be used for early generation advancement through mass selection where F 2 bulks are chemically desiccated and selections
are made for seed size divergently by mechanical sieving. After two cycles of early
generation selection, vigorous lines were selected and tested for their response to
chemical desiccation stress. Mass selection for large grains under chemical desiccation significantly improved grain weight and grain yield of the population under
desiccation stress, as compared to control where selection for grain size was
performed without chemical desiccation (Blum et al. 1991; Haley and Quick 1993;
Annual Report 2019).
Physiological processes such as delayed leaf senescence, water status, and canopy
temperature are very crucial for drought stress tolerance capability in plants. Delayed
leaf senescence in a flowering plant induces extreme drought tolerance and
evidenced by suppression of drought-induced leaf senescence in transgenic tobacco
plants expressing isopentenyltransferase (IPT), an enzyme that catalyzes the ratelimiting step in cytokinin synthesis, resulted in outstanding drought tolerance as
shown by vigorous growth after a long drought period, among other responses of
high water contents, retained photosynthetic activity albeit at a reduced level, and
displayed minimal yield loss when watered with only 30% of the amount of water
used under control conditions during the drought (Rivero et al. 2007). A “slowwilting” line has been recognized in soybean (Fletcher et al. 2007). The visual
scoring of delayed leaf senescence of a given genotype must be based on an
integrated impression of the symptom in the whole plant or even the whole plot.
Scoring is performed on 1–5 arbitrary rating scale with 1 being sensitive with dried
leaves chaffy matured pods and 5 being delayed leaf senescence with well-filled
matured pods and green leaves. Very small variations in leaf senescence score, even
if they are statistically significant, are of no real consequence in breeding for
dehydration avoidance and large and prominent differences are sought (Blum 2011).
Leaf relative water content (RWC) is a simple, standard, and effective estimate
and a reliable indicator of water status in plants (Sinclair and Ludlow 1985) with
respect to dehydration avoidance. Usually the top-most fully expanded sun-lit leaf
must be sampled to determine leaf RWC as per Blum (2011). Boyer et al. (2008)
cautioned against excessive rehydration of samples which can result in excessive
absorption of water by the leaf sample, beyond its normal full turgor capacity. This
86
G. K. Satpute et al.
and relatively simple field assay for revealing the capacity for seed filling from stem
reserves. The method is based on the application of a chemical desiccant to plant
canopies after flowering as a means for inhibiting plant photosynthesis and thus
revealing the capacity for seed filling by stem reserves. The treatment does not
simulate drought stress. It simulates the effect of stress by inhibiting current assimilation. With this method a chemical desiccant (potassium iodide; 0.2% w/v) is
sprayed to complete wetting over the whole canopy, at seed filling stage (R5 plus
8–10 days) in soybean (Bhatia et al. 2014), which mainly destroys chlorophyll and
simulates natural senescence. Chemical desiccation can be incorporated into breeding programs in two ways: (1) it can be used to assess responses of individual
advanced lines or families, always compared with non-treated controls under
non-stress conditions and (2) the method can be used for early generation advancement through mass selection where F 2 bulks are chemically desiccated and selections
are made for seed size divergently by mechanical sieving. After two cycles of early
generation selection, vigorous lines were selected and tested for their response to
chemical desiccation stress. Mass selection for large grains under chemical desiccation significantly improved grain weight and grain yield of the population under
desiccation stress, as compared to control where selection for grain size was
performed without chemical desiccation (Blum et al. 1991; Haley and Quick 1993;
Annual Report 2019).
Physiological processes such as delayed leaf senescence, water status, and canopy
temperature are very crucial for drought stress tolerance capability in plants. Delayed
leaf senescence in a flowering plant induces extreme drought tolerance and
evidenced by suppression of drought-induced leaf senescence in transgenic tobacco
plants expressing isopentenyltransferase (IPT), an enzyme that catalyzes the ratelimiting step in cytokinin synthesis, resulted in outstanding drought tolerance as
shown by vigorous growth after a long drought period, among other responses of
high water contents, retained photosynthetic activity albeit at a reduced level, and
displayed minimal yield loss when watered with only 30% of the amount of water
used under control conditions during the drought (Rivero et al. 2007). A “slowwilting” line has been recognized in soybean (Fletcher et al. 2007). The visual
scoring of delayed leaf senescence of a given genotype must be based on an
integrated impression of the symptom in the whole plant or even the whole plot.
Scoring is performed on 1–5 arbitrary rating scale with 1 being sensitive with dried
leaves chaffy matured pods and 5 being delayed leaf senescence with well-filled
matured pods and green leaves. Very small variations in leaf senescence score, even
if they are statistically significant, are of no real consequence in breeding for
dehydration avoidance and large and prominent differences are sought (Blum 2011).
Leaf relative water content (RWC) is a simple, standard, and effective estimate
and a reliable indicator of water status in plants (Sinclair and Ludlow 1985) with
respect to dehydration avoidance. Usually the top-most fully expanded sun-lit leaf
must be sampled to determine leaf RWC as per Blum (2011). Boyer et al. (2008)
cautioned against excessive rehydration of samples which can result in excessive
absorption of water by the leaf sample, beyond its normal full turgor capacity. This
86
G. K. Satpute et al.