Transient daily heat stress during the early reproductive phase disrupts pod and seed development in Brassica napus L.

Abstract

Transient daily heat stress during flowering of canola (Brassica napus L.) as a result of global warming is an increasing threat to grain production in this important oilseed crop. We investigated the intensity and duration of transient daily heat stress treatment at different stages of reproductive development in three B. napus genotypes under controlled environment conditions. Heat stress treatments during the week before first open flower on the main stem (S0) or during the first week (S1) or second week (S2) following first open flower greatly reduced pod number and seed yield on the main stem. Heat stress treatment during the third week (S3), fourth week (S4), or fifth week (S5) reduced seed yield more on the branches and less on the main stem as time progressed. Pod number and seed yield were reduced by moderate heat stress (TC2; 32°C/22°C) and high heat stress (TC3; 35°C/25°C), compared with the control (TC1, 25°C/15°C). Each duration of heat stress treatment (3, 5 or 7 days) caused the same reduction in pod number and seed per pod on the main stem. Leaf stomatal conductance, leaf chlorophyll index, plant height, and dry weight of above‐ground biomass increased from TC1 to TC3, which indicates that heat stress in the absence of drought stress does not inhibit vegetative growth. Cool night temperatures (15°C) resulted in recovery of pod number and seed yield after moderate (32°C) but not high (35°C) daily transient heat stress. The range of genotype responses to heat stress was greater under TC2 than under TC3. This research brings forward the critical period for heat stress sensitivity in B. napus to one week before first open flower and defines the conditions for controlled environment screening for heat tolerance in B. napus.

Effects of long-term high-temperature stress on reproductive growth and seed development in development inBrassica napus

“…We did not find any biologically significant changes in pollen viability, which could cause a decrease in fertilization rate and seed number per pod. These findings do not correspond with published data of reduced pollen viability after 7-day heat treatment in B. napus and B. rapa (Chen et al , 2020a; Young et al , 2004; Annisa et al , 2013). Our growth setup includes lower night temperatures (18 °C in all conditions) and ramping up and down to stress temperatures during the day.…”

“…A significant decrease in fertilization rate (occurrence of unfertilized ovules) may be caused by the infertility of the ovules, lower pollen germination (Young et al , 2004), or lower transmission of pollen tubes through the transmitting tract. The percentage of aborted seeds increased with increasing temperatures, which was also noticed without further investigations (Chen et al ., 2020a). In our study, only DH12075 had a very low abortion rate for all tested temperatures.…”

“…As previous studies applied heat stress for limited periods, it hardly showed the changes on the whole plant level, although partial hints in accordance with our results were published. Indeed, HT increased plant growth and their above-ground biomass regardless of the genotype (Chen et al , 2020a). And significantly higher production of lateral inflorescences led to higher production after stress release and return to control conditions (Young et al , 2004).…”

“…Despite many studies reporting damage of pollen grains due to HS in various species, rapeseed pollen manifests a limited HS sensitivity. The average viability of pollen grains decreases from 96.4 % at 25/15 °C to 84 % at 35/25 °C (Chen et al , 2020a). A higher difference in pollen viability (58 % in 35/18 °C compared to 86 % in 23/18 °C) was shown together with reduced pollen germination rates (17.5 % in HS versus 59.2 % in control) (Young et al , 2004).…”

“…Seed weight and seed number per pod were reduced in cultivation above 29.5 °C of canola cultivars (Morrison and Steward, 2002). Reduced pod number and seed yield were caused by 32/22 °C and 35/25 °C day/night temperatures in a 7-day treatment during the week before and after flowering initiation of 12 different cultivars (Chen et al , 2020a). High variability among cultivars was observed for pollen viability and germination in the temperature range 5-35 °C with a significant decrease above 30 °C (Singh et al , 2008).…”