Agriculture Reference
In-Depth Information
10.4
REACTIONS OF VEGETABLE CROPS TO HEAT STRESS
Heat stress due to increase in temperature is a major problem for vegetable
crops. Most of the vegetables show marked reduction in growth above a
temperature of 35 °C and above 50 °C thermal growth of the crop is said to
occur (Hazra and Som, 2006). A constantly high temperature causes an ar-
ray of morpho-anatomical changes in plant which affect the seed germina-
tion, plant growth, flower shedding, pollen viability, gametic fertilization,
fruit setting, fruit size, fruit weight, fruit quality, etc.
The optimum soil temperature for most of the vegetable crops ranges
from 20 to 30 °C. The maximum temperature for the warm season veg-
etables (tomato, brinjal, chili, cucurbits, beans, etc.) is 35 to 40 °Cwhile
that of cool season vegetables is 32-35 °C. However, above 30 °C the
germination of vegetable crops is impaired.
Vegetative and reproductive processes are strongly modified by tem-
perature alone or in conjunction with other environmental factors (Abdalla
and Verkerk, 1968). Heat stress above 35ºC has become a major limiting
factor for seed germination, seedling and vegetative growth, flowering and
fruit setting, and ripening in most vegetable crops. The reproductive devel-
opment is always more sensitive to high temperatures than vegetative de-
velopment. The optimum temperatures for tomato cultivation are between
25 °C and 30 °C during the photoperiod and 20 °C during the dark period.
However, only 2-4 °C increase in optimal temperature adversely affected
gamete development and inhibited the ability of pollinated flowers into
seeded fruits and thus, reduced crops yields (Firon et al., 2006; Peet et al.,
1997). High temperatures also interfere with floral bud development due to
flower abortion. High temperatures can cause significant losses in tomato
productivity due to reduced fruit set, and smaller and lower quality fruits
(Stevens and Rudich, 1978). In tomato bud drop, abnormal flower devel-
opment, poor pollen production, dehiscence and viability, ovule abortion
and poor viability, reduced carbohydrate availability, and other reproduc-
tive abnormalities have been reported due to high temperature by Hazra et
al. (2007). In leafy vegetables like palak, high temperatures lead to bolting
thus reducing the leaf yield. Likewise in all other vegetables, high temper-
atures leads to reduced fruit set thus lowering the yield as well as diminish
the quality of the produce. Significant inhibition of photosynthesis occurs
at temperatures above optimum, resulting in considerable loss of potential
productivity. Brief exposure of plants to high temperatures during seed
