PhD

The present investigation entitled “Evaluation of CO 2 enrichment on growth,
development and soft rot tolerance in ginger (Zingiber officinale Rosc.)” was undertaken
with the objective to assess the impact of elevated CO 2 on growth, yield, quality and
tolerance to soft rot in ginger through morphological, physiological, biochemical and
molecular analysis. Two sets of pot culture experiments were conducted during 2017-
2019. The technology utilized for CO 2 enrichment was Open Top Chamber (OTC) system
established under Department of Plant Physiology, College of Agriculture, Vellayani.
In the first experiment three varieties of ginger Aswathy, Athira and Maran were
utilized to assess the impact of elevated CO 2 on growth, yield and quality of ginger. One
set of three month old plants were shifted to OTC and another set was retained under
ambient CO 2. The plants were harvested at 4 th , 5 th , 7 th and 8 th months after planting
(MAP). Observations on growth, physiological and biochemical parameters were
recorded at each harvest. Yield, yield parameters, tissue nutrient status and quality
parameters were assessed at 8 MAP. Second experiment was conducted to assess the
impact of elevated CO 2 on tolerance to soft rot in three ginger varieties Aswathy, Athira,
Maran and wild ginger congener Zingiber zerumbet. One set of three and a half month
old plants were shifted to OTC and another set retained at ambient CO 2 and were
challenged with Pythium aphanidermatum at 4 MAP. The CO 2 concentration of 500 ppm
was maintained in the OTC.
The results of the first experiment indicated an improvement in growth parameters
like plant height, number of leaves, root weight, shoot weight, stem diameter and number
of tillers, physiological and biochemical parameters like total chlorophyll, photosynthetic
rate and total sugars in the rhizome by recording significantly higher values upon CO 2
enrichment when compared to ambient CO 2 condition. Significant reduction
in
transpiration rate, stomatal conductance and specific leaf area were recorded upon CO 2
enrichment.
The per cent increase in yield parameters like number of fingers (38.19%), rhizome
spread (11.32%), thickness (42.72%) and dry weight of rhizomes (31.43%) recordedhigher values upon CO 2 enrichment, based on average performance of all the selected
varieties, leading to greater yield. The per cent increase in the fresh weight of rhizome
recorded higher for variety Aswathy (38.34%), followed by Maran (20.00%) and Athira
(12.12%) with a total fresh rhizome yield of 165.35 g plant -1 under ambient and 228.75 g
plant -1 under elevated CO 2 condition in the case of Aswathy.
Upon CO 2 enrichment the quality of rhizome was modified with increase in fiber
(60.26%), gingerol (9.06%), phenol (19.25%), total oleoresin (11.52%), starch (44.84%)
and essential oils (31.10%). Significant reduction in protein and nutritional status in the
rhizomes were also recorded. Nitrogen, potassium, iron and zinc contents decreased by
15.78%, 3.22%, 11.78% and 18.57% respectively. Phosphorous and copper showed
decrease in trend though not significant. Among the three varieties of ginger, Aswathy
was found to be performing better compared to Athira and Maran not only in terms of
yield, also for majority of quality parameters assessed upon exposure to elevated CO 2
condition.
In the case of impact of elevated CO 2 on soft rot development, there was no
variation in the per cent disease incidence. However, lesser number of days was taken for
development of symptom. The per cent disease index was reduced by 11.10% upon CO 2
enrichment. The reduction in the fresh weight of rhizome upon pathogen inoculation was
also less under elevated CO 2 (14.34%) when compared to ambient CO 2 (17.60%)
showing that increasing CO 2 levels might bring down the disease severity.
Upon P. aphanidermatum inoculation, the retention of photosynthetic pigments like
chlorophyll a (0.90 mg g -1 FW) chlorophyll b (0.74 mg g -1 FW) total chlorophyll (1.64
mg g -1 FW) carotenoids (0.58 mg g -1 FW), xanthophyll (0.40 mg g -1 FW) and
photosynthetic rate (5.92 μmol CO 2 m -2 s -1 ) were higher under elevated CO 2 when
compared to ambient CO 2 . Higher membrane integrity values and lower malondialdehyde
(0.75 μg g -1 FW) content in all the selected varieties were evident upon pathogen
inoculation under CO 2 enrichment.
Enhanced activity of ascorbate peroxidase (4.86 activity Units mg -1 total protein),
PAL (Phenylalanine ammonia lyase) (20.91 Units mg -1 total protein), and glucanase (4.57Units mg -1 total protein) activity, as well as higher accumulation of phenols (32.40 mg g -1
DW) and ascorbic acid (7.14 mg 100 g -1 FW) were found in the inoculated plants under
CO 2 enrichment compared to the ambient CO 2 . The catalase activity was found to reduce
upon pathogen inoculation and the reduction was higher (0.11 Units mg -1 total protein)
under ambient CO 2 as compared to elevated CO 2 (0.09 Units mg -1 total protein).
Highest levels of hydrogen peroxide, superoxide and cell death were observed upon
pathogen inoculation under ambient CO 2 through staining techniques and it was highest
for variety Aswathy and least for Z. zerumbet. After pathogen inoculation maximum
values for variable to maximum fluorescence Fv/Fm (0.63), photosynthetic electron
transport rate (82.60 micro equiv m -2 s -1 ) and quantum yield of PS II photochemistry
(0.29) were recorded under elevated CO 2. Protein profiling performed in varieties Athira
and Aswathy revealed lower expressions of 20-25 kDa and 25-35 kDa protein under
pathogen inoculation in variety Aswathy. The expression of 55 kDa (Rubisco Large Sub-
Unit) was down regulated to a greater extent under pathogen inoculation in both the
varieties. Higher expression levels of the defense responsive genes Phenylalanine
ammonium-lyase (PAL), Cinnamyl alcohol dehydrogenase (CAD) and Chalcone synthase
(CSH) were also observed upon pathogen inoculation under CO 2 enrichment in both the
varieties. Between the varieties, Athira had higher relative fold changes of PAL, CAD and
CSH under both ambient and elevated CO 2 upon pathogen inoculation.
The present study revealed that CO 2 enrichment can improve the performance of
ginger in terms of growth, yield and quality. It also indicates changes in photosynthetic
allocation pattern and phytochemical profiles with enhanced contents of oleoresins and
essential oils. Ginger plants showed better tolerance against P. aphanidermatum upon
CO 2 enrichment. This was apparent in the form of reduced disease severity, oxidative
stress level and less reduction in yield upon pathogen infestation under elevated CO 2
condition. This was evidently achieved through the activation of multiple mechanisms,
including enhanced production of anti-oxidants and up regulation of pathogen resistance
proteins. This is one of the first studies to comprehend on possible effects of elevated
CO 2 on the susceptibility of ginger to soft rot. The study also demonstrated the varietal
variations existing in the response to CO 2 enrichment which will help in selection ofsuitable varieties for changing climatic scenario. The information generated indicates the
possibility of enhancing both qualitative and quantitative yield of ginger through low cost
CO 2 enrichment technologies. The insights obtained on the tolerance mechanisms
involved will help to restructure the soft rot disease management strategies in the coming
future.