Dry Matter Requirement for Growth and Respiration of Coconut

The present study determined the dry matter (DM) requirement for potential growth (PGR), growth respiration (R g ) and maintenance respiration (R m ) offruits, leaves, stem and root system of adult coconut palms (25-26 years) ofTall x Tall cultivar under resource-unlimiting conditions. The total OM requirement ofa palm was about 21 kg/palm/month and 63% ofthat total was required for growth and respiration of vegetative organs (leaves, stem and roots) whilst the rest (37%) was for fruits. Similarly, the OM required for respiration (62%) was nearly two third ofthe total requirement of a palm and the balance 38% was required for growth oforgans. The total dry matter requirement (TOMR) ofindividual fruits varied with developing stage and TOMR offruits / palm varied between months and years depending on the fruit load of different development stages. Irrespective of the year, TOMR of fruits was highest during November to February (about 10-11 kg/palm/month) and lowest in June-july (about 5-6 kg/palm/month). However, the OM requirement of vegetative organs (a total of approximately 13 kg/palm/month) did not vary between months.


INTRODUCTION
Crop yield in tree fruit crops like coconut depends on fruit set and the ability of the tree to supply photosynthates to sustain the growth and respiration requirements of developing fruits.Because of this relationship to crop yield, there has been some research on the factors controlling fruit set and photosynthesis in coconut (Nainanayake et af. 2008;Ranasinghe et af. 2012).But, even though fruits are widely recognized as major 'sinks' for photo-assimilates and the competition for assimilates among fruits is one of the major factors controlling fruit set (Navarro et ai, 2008; Ranasinghe et af.2012), there have been no attempts to quantify actual assimilate requirement for growth and respiration of fruits and other vegetative organs of coconut palms.Furthermore, quantitative information on assimilate requirement (sink strength) of different organs and its interaction with critical climatic factors is necessary for developing source/sink based crop forecasting models.
Coconut palm shows an indeterminate growth pattern, producing an inflorescence at each leaf axil at intervals varying from 25 to 30 days, depending on the environmental conditions and age ofthe palm (Liyanage, 1950).Consequently, at any time a healthy coconut palm carries 12-14 inflorescences (bunches) with varying numbers offruits at different developing stages.Thus, the total assimilate (dry matter) requirement ofa coconut palm is the dry matter required for potential growth and respiration of vegetative and reproductive organs.The supply of assimilates, originating from the photosynthesis, is divided over the organs in pmportion to their fractional contribution to the total sink strength (Bertin, 1995;Marcelis, 1996).In a perennial crop like coconut with simultaneous growth of vegetative and reproductive organs, the dry matter requirement of vegetative organs has the priority over fruits (Navarro et af. 2008) and therefore, when the total dry matter requirement of the palm is high, flowers and young fruits are not able to compete for assimilates with the fast-growing fruits and hence abort resulting low fruit set.
The dry matter requirement for 'potential growth' of fruits and vegetative organs can be estimated by determining the growth rate of these organs under resource 'nonlimiting' conditions, i.e. with no soil moisture stress, no nutrient deficiency, at favorable temperature and solar radiation intensity and with minimum competition (for fruits) (Henson, 2007;Ranasinghe et af. 2012).The dry matter requirement for respiration (growth and maintenance respiration) of each organ can be determined using potential growth rate, biomass (dry weight) of the respective organs and the respiration coefficients.Although this information is available for some tree fruit crops (De long and Walton, 1989) and glasshouse grown crops (Marcel is and Hofrnan-Eijer, 1995;Marcelis et af. 2004), detailed quantitative information on above factors is so far lacking in coconut or any other palm species.Therefore, the objective of this study was to estimate and quantify the monthly dry matter requirement (sink strength) for growth (dry matter accumulation) and respiration of coconut fruits and vegetative organs of a palm.Dry matter requirement for potential growth of fruits (PGR F ), leaves (PGR L ) and stem (pGR s ) Dry matter requirement for PGR from pollinated flower to mature fruit, was obtained by non-destructive measurements on 'potentially growing' fruits.Conditions for potential fruit growth were created by tagging fruits in each inflorescence and recording their growth rate under 'resource unlimited' conditions.To accomplish this condition, four fruits on newly pollinated youngest inflorescence (first), two fruits each on second and third inflorescences and one fruit each on 'other developing inflorescences were retained and all other developing flowers and fruits were removed (since some fruits ofthe first three inflorescences are prone to abort, more than one fruit was retained on the three youngest inftorescences).The developmental stages were approximately equally spaced in time.Data collection was done under highly favourable soil (S2 Land suitability class) (Somasiri et at. 1994) and environmental conditions (no soil moisture stress, favourable temperature and solar radiation intensity).The fruits were assumed to grow with minimum competition under optimum environmental conditions to attain the potential size of each stage.Vertical circumference ofthe tagged fruits was measured at time t, and after one month at time t 2 and fruit dry matter content at t 1 and t 2 was estimated non-destructively by a fitted empirical relationship developed for the Tall coconut cultivar (Equation I).The potential dry matter increase of fruit / month (potential growth rate, PGR) for each developing stage was calculated by the difference in estimated dry weight of fruits between months (t 2 -tJ For determining the dry matter requirement for PG~, vertical growth rate ofstem under nonstressed conditions was monitored by marking a line just below the leaf crown.The increased volume of the stem over the period (considering that there is no detectable increase in stem The carbon requirement for R ,R and g m total respiration of a single fruit of each stage was determined using above equations and the carbon values were converted to dry mass by multiplying it by 2 (assuming 0.5 g C gDM• 1 (Matthews, 1993;Navarro et at. 2008)).Then, the dry matter requirement for growth (PGR), R g and R m of each bunch was calculated separately by multiplying the single fruit's requirement by the actual number of fruits remaining in a bunch.This was determined for continuous five months period from April to August 2009 using 16 palms.The total dry matter requirement of fruits / palm / month was estimated by summing up the requirement of all bunches on a palm.The variation of total dry matter requirement of fruits was studied in detail using fruit load data of 16 palms in the same field (Urapola, wet agro-climatic zone, WL 3 ) for a period of two years (2009-20 I 0).

Forty healthy coconut palms
PGR ofstem and leaves was taken as the net primary productivity (NPP) of respective organs.Dry weight ofstem was estimated by multiplying the volume of the stem with the mean density (The shape of the coconut stem was assumed to be cylindrical and tapering of the stem towards the top was not taken into account.The same approach has been taken by many researchers working on coconut) (Friend and Corley, 1994;  Navarro et al. 2008)).Dry weight oftotal fronds per palm was estimated by using the actual dry weight of most mature frond and the crown leaf load (Navarro et al. 2008; Ranasinghe and  Thimothias, 2012).The R m of coconut root system was estimated using estimated root weight (17% of above-ground weight for tree species) and maintenance respiration coefficients published by Navarro et al. (2008).Since the potential growth rate (pG~) of roots were not estimated, the dry matter requirement for growth respiration (Rg) was not estimated for roots in the present study.

Analysis ofdata:
The data were analysed using General Linear Model (PROC GLM) of the SAS statistical package and (version 8.2).

Dry matter requirement offruits
Crown of coconut palms consists of about 14 fruit bunches of different developing stages.The time course of fruit weight increase was approximately sigmoidal with a substantial linear phase during which the greater part of the nut weight was attained (Fig 1).The period of rapid growth was from about sixth to tenth development stage after pollination.At harvest the weight of a coconut fruit was about 800 g (dry weight).
The DM requirement for potential growth (PGR) of a single fruit increased with age from development stage one to nine and the rate of increase was highest from stages five to six (39 g fruit-I month-I) (Fig 2a).
(R m ) increased with maturity up to 11 th stage and remained constant.Consequently, the DM requirement for cumulative respiration (Total R) of a fruit was highest at ninth to tenth development stage and it was about 80 g fruit-I month-I.The total dry matter requirement (TDI\;lR) for growth and respiration of a fruit was also highest at ninth to tenth stage and it was about 225 g fruit-I month-I (Fig 2a).Tn young fruits, DM requirement for R m was only a small fraction « 20%) of the total respiration, while from eighth to ninth stage, this fraction continuously increased with development stage (Fig 2b).DM requirement for R showed the g opposite trend.

PGR
These variations in DM requirement with ontogeny of the fruit may be attributed to the physico-chemical changes that take place within fruit during the post-pollination phase (from fertilization to maturity).The fruit of coconut consists of four main components, namely, the husk, shell, liquid endosperm (nut water) and solid endosperm (kernel including lipids).
The growth during the post-pollination phase, that is, from fertilization to maturity (10-11 month period), is mainly by cell enlargement which is associated with the accumulation of food reserves.The rapid growth phase of husk, shell and kernel extends from 3-7 months, 5-9 months and 6-10 months, respectively.It was evident that the commencement and term ination of the growth of fruit components, husk, shell and endosperm, take place at different times.
However, the growth of all three components overlapped during the period from fifth to ninth stage.Accumulation of dry matter in the solid endosperm (kernel) ceases after eleventh stage and with that the increase in dry weight of the whole fruit also ceases indicating that no assim ilates are transported into the fruit after the full development of the endosperm (Jayasuriya and Perera, 1985).
Using the single fruit's requirements and fruit load of palms, total dry matter requirement of fruits/palm (TDMR) was estimated and it was approximately 8 kg /palm / month (Fig 3).
Fruit growth (PGR) and respiration accounted for about 60% and 40% of the total dry matter requirements, respectively.The two components of respiration required the same percentage of dry matter (20% each).There have been few attempts to quantify the actual dry matter requirement for growth and respiration of tree The total dry matter requirement for growth and respiration of leaves was almost similar to that of fruits, however the contribution of different components to the total requirement was appreciably different from those of fruits (Fig 4a).For instance, about 60% of the total DM was required for maintenance respiration (R m ) of the leaf crown whilst only 34% was required for growth of leaves (PGR).The percentage DM required for growth respiration (R ) ofleaves was comparatively low (6%).The g total DM requirement of stem was significantly lower (about 3 kg /palm/month) compared to other organs and almost 75 % of that amount was required for maintenance respiration (Fig 4b).Since the stem showed the lowest growth rate, the DM requirement for Rg was lowest for the stem.The DM requirement for R m of root system was 2.15 kg / palm / month and the DM requirement for growth (PGR) and R of roots was not estimated in the present study.

Contribution oftotal dry matter requirement of a palm by different organs and processes
The total DM requirement offruits and vegetative organs was about 21 kg/palm/month and nearly two-third of that was required for growth and respiration of vegetative organs (leaves, stem and roots, 63%) and only one-third (37%) of the total was required by fruits (Fig 5a).Similarly, the DM required for respiration (62%) was nearly two third of the total requirement and only 38% was required for growth of organs (Fig 5b).

Bin
Ri; conditions in a recent study (Ranasinghe et al. 2015a).Under such conditions wher~the total dry matter supply is lower than the requirement, there is a high competition for assimilates between vegetative organs and developing circumference over time) and stem density were used to determine the PGRs per month.The number of new leaves emerged per month was nearly one for each palm.Therefore, the dry weight of leaves was used to determine the dry matter requirement for PGR L • The potential growth rate of roots (PGR R ) was not estimated in the present study.DM log = 0.1486+(0.1472(LverticaJ)) -(0.000741 (Lvertical) Equation I DM; dry matter content of the fruit (g), LverticaJ; circumference of the fruit along the vertical axis (cm) (Ranasinghe et ai., 2008) C S Ranasinghe and R D tV Premasiri Dry matter requirement for growth respiration (R ), maintenance respiration (R ) and total g m (heterotrophic respiration R) of fruits, leaves, stem and roots Dry matter requirement for respiration of a single fruit of all 14 developmental stages, leaves (approximately 28-30 fronds), stem and roots (only maintenance respiration) ofa palm was estimated by a general model proposed by Penning de Vries et al. (1983) and Navarro et al. (2008).R = R + R = a NPP + 0.4 f3 B a g m R -Respiration of organ (g C m• 2 day•I), subscripts a, g and m denote total, growth and maintenance respiration, respectively a -Growth respiration coefficient (g C gDM-I ), (estimated from 0.67 g CO 2 g DM• I for fruits, 0.32 g CO 2 gDM-1 for leaf and 0.25 g CO 2 gDM• 1 for stem (Navarro et al. 2008)) NPP -Net Primary Production of each organ (g DM m• 2 day-I), 0.4 -C: CH20 molecular mass ratio f3 -Maintenance respiration coefficient (g CHp g DM• 1 day•I), (0.00184 for fruits, 0.00245 for leaf, 0.0005 for stem and 0.0019 for roots (Navarro et al. 2008)) B -Biomass of the organ (g DM m-2 )
Fig 4: Dry matter requirement for growth (PGRl, respiration (maintenance (Rml, growth (R,) and total of growth and respiration (TDMRl of leaves (a) and stem (b).

Fig 5 :Fig 6 :
Fig 5: Total dry matter requirement of an adult coconut palm.(a) it's contribution by different organs and (b) its contribution by different processes (potential growth (PGR), maintenance respiration (R.,l, growth respiration (R,l and total of growth and respiration (TDMR) (Cocos nucifera L., variety typica, Tall x Tall) selected from