European Journal of Scientific Research - EuroJournals

A Comparative Analysis of Gibberellic Acid Content with Respect to Tuber
Induction in Potato Plants Grown Under Differential Photoperiod and Temperature 408
Tuber induction is controlled by endogenous plant hormones (Melis and van Staden 1984;
Vreugdenhil and Struik 1989), which are governed by environmental factors, mainly photoperiod and
temperature. Potato plants integrate day-length measurement with temperature perception to ensure an
appropriate environmental regulation of tuberization. The length of photoperiod (the relative duration
of light and darkness during a day), the intensity of incident light, and the temperature requirements
vary with genotype (Snyder and Ewing 1989; Jackson 1999). Every cultivar requires a threshold
photoperiod and a critical temperature above which tuberization cannot occur (Ewing and Struik 1992).
Potato is best grown in temperate climate (Haverkort 1990). In general, short days, high-intensity light
and cool temperature (referred to as tuber climatic inducing conditions) promote tuberization whereas
long days, low-intensity light and high temperature (tuber non-inducing climatic conditions) delay or
inhibit the process (Jackson 1999; Ewing and Struik 1992).
The tuberization signal initiates cellular changes in the subapical region of the stolon (Xu et al.
1998b; Jackson 1999; Vreugdenhil et al. 1999; Hannapel 2004) which result in a sink organ for
carbohydrate and protein accumulation leading to the newly formed potato tuber (Fernie and
Willmitzer 2001; Hannapel et al 2004). The identity of the tuberization signal is still unknown
(Rodriguez-Falcon et al. 2006). Such a mobile signal is thought to include positive and negative
regulators of tuberization (Jackson 1999). The positive regulator is an inducing component formed
under inducing conditions whereas the negative regulator is an inhibitory component that forms under
non-inducing conditions. Tuber induction is believed to occur when the ratio of stimulus to inhibitor
exceeds a certain threshold.
Physiological and transgenic studies imply gibberellins in an inhibitory functional role, and
consequently, the inhibitory component of the tuber induction signal is believed to be a gibberellin
(Jackson et al. 1996; Jackson and Prat 1996; Amador et al. 2001; Martinez-Garcia et al. 2002)
Gibberellic acid (also known as gibberellin A3 or GA3) is a pentacyclic diterpenoid compound,
structure shown in Figure 1, and one of more than 126 members of a ubiquitous class of natural
products called gibberellins (Hedden and Phillips 2000; Mander 1992, 2003). GA3 is a plant hormone
that elicits profound effects on various phases of plant growth and development (Kende and Zeevart
1997).
Figure 1: Molecular structures of the gibberellic acid (left) isolated from S. tuberosum and 7-hydroxycoumarin-
4-acetic acid (right) used as internal standard in the present analysis.
HO
O
O
H
HO
H
O
OH
HO
COOH
O O
The most common gibberellin used in physiological studies of potato plant growth and
development is GA3. A central question in the understanding of tuber induction mechanism is the
extent to which GA3 biosynthesis is limited by environmental conditions while establishing a
regulatory role proposed for this compound in the tuberization process. Traditionally, investigation of
this question has been designed to give a qualitative answer by studying the effects of exogenously
applied GA3 on the morphological development of potato plants. Clearly, a more complete
understanding of the role of GA3 in tuber induction and development will require quantification of GA3
profiles in potato tissues grown in certain photothermal regimes. It is difficult to draw conclusions
from measurements of gibberellin-like activity or total content of GA3 from samples harvested at just a
single time point. In the present work, we have used a time-course quantitative approach to the
problem. A detailed time-course experiment was conducted to determine the effect of temperature and