8342937079

Pertanika 14(1). 15-19(1991)

Exothermic Events in Cocoa (Theobroma Cacao L.) Seeds Associated

with Low Temperaturę Damage

Y.L. HOR¹ AND P.C.² STANWOOD

¹ Department of Agronomy and Horticullure, Faeulty of Agricutture,

i niversiti Pert a nian Malaysia,

43400 UPM Serdang, Selangor Darul Ehsan.

²National Seed Storage I.aboratory, Colorado State University, Fort Collins, Co 80523, USA.

Key words: Theobroma cacao, differential thermal analysis, cold injury.

ABSTRAK

Analisis terma pembeza (differential thermal analysis) unluk mengesankan fasa peralihan (pepejal kepada cecairdan cecairkepada pepejal) kandungan tisii embnodan kotiledon biji benih kokotelah dikaji. Fisu-tisu em Im o ber helem ha pa n antara 27% dan64 % menunjukkansatu eksoterm antara-14°Cdan-38°C. Eksoterm yangdisebabka n oleh airsejuk-beku i ni tidak didapati dańpada tisu embńo yang kering, iaitu yang mempunyai kelembapan kurang dańpada 27% - 15%. Fisu-tisu kotiledon yang berkelembapan antara 15% dan 27% jaga menunjukkan satu eksoterm yang sama antara suhu -19 °C dan -25 °C. Di samping itu. satu eksoterm tambahan yang tidak disebabkan oleh air sejuk-beku fuga didapati antara suhu +!5°Cdan + 1 l°Cdari setiap tisu kotiledon. tan pa rnengira kandungan kelembapannya. Pengesanan Jasa yang dilakukan pada suhu yang tinggi ini (+15ⁿC kepada +] I°C) menyebabkan kerosakan yang serius dan memusnahkan perrambahan biji benih. walaupun embńonya tidak dicederakan. Kepentingan kejadian eksoterm terhadap kehilangan perrambahan biji benih koko semasa penyimpanan korwensional dan knogenik dibinrangkan.

ABSTRACT

Embryonic axes and cotyledon tissues from indiyidual cocoa (Theobroma cacao /..) seeds were subjected to differential thermal analysis (DTA) to detect phase transitions (lifjuid to solid and solid to liąuid phases) oj tissue constituents. Embryonic axes at 27% to 64% moisture content (fresh weight basis) exhibited a single exotherm between -14° C to -38° C. Theexothenn was caused by thepresenceoffreezable water and was absent in tissues dńed helów 27% to 15% moisture. Cotyledon tissues at moisture contents between 15% to 27% exhibited a simiUir aąueous exotherm between -19°C to -25° C. In addition, a non~aqueous exotherrn was also detected between +/ 5° C to +//° C in all tissues, inespectwe of moisture content. The phase transition which occurred at lliis temperaturę (+15° C to +11° C) was suffidently damaging to cotyledon function so as to preuent seed germination, although the axis remained unaffected. The importance oj these exothermic euents to seed deaih duńngconuentional (alnwefreezing) and nyogenic (sufefreezing) storage of cocoa seeds is dtscussed.

INTRODUCTION

Cocoa ( Theobroma cacao La.) seeds are recalcitrant and unablc to wilhsiand even moderate dehydra-tion (Barton 1965;Swarbrick 1965).Germination is destroyed whcn seeds are dried below 17% moisture content (Mumford & Brett 1982). Like other recalcitrant seeds, cocoa also isverysensitive tocool temperatures (0°C to 15°C). Germination is ra-pidly reduced when seeds are stored at 15°C (1 lor et al. 1984). Sensitivity to cool temperaturę and desiccation makes cocoa seed difficult to presene

even for short periods of time.

The sensitivity of cocoa seeds to low temperaturę is a function of temperaturę and time of exposure. For example, exposure of cocoa seeds to 12 °C for 8 and 32 min resulted in 88% and 56% germination respectively; while at 8° C. the corresponding percentages were 36% and 0% (Boroughs 8c Hunter 1963). The inereased ratę of germination loss at lower temperatures could be related to morę rapid cooling of seed tissues to a critical temperaturę. Ibanez (1963) reported that