Reproductive frost tolerance in field pea (Pisum sativum L..

Abstract

Radiant frost during spring is a significant problem for field pea (Pisum sativum L.) grown in Mediterranean environments as plants are at the vulnerable reproductive stage when frost occurs. In such environments, radiant frost events after the commencement of flowering of field pea may lead to severe frost injuries on plants, and can adversely affect the grain yield. Despite the importance of the impact of frost on grain yield, no dedicated study has been conducted on reproductive frost tolerance (RFT) in field pea. One aim of this research was to develop a simple and reliable screening method to evaluate frost tolerance of eight reproductive organs (from immature buds to mature pods) which are often present at the same time on a single plant. A controlled environment screening method that exposed plants to a defined temperature regime, including a minimum temperature of -4.8°C for 4 hr, was developed. A scoring key was devised to record frost symptoms on each reproductive organ, and five categories were defined to evaluate frost damage on seeds. Using this screening method, a diverse collection of germplasm was screened, including 83 accessions sourced from high altitude and frost prone areas in 39 countries. A locally adapted variety, Kaspa, the most widely cultivated field pea variety in southern Australia, was included in the screening. The flowering stage was found to be more susceptible to frost than the pod development stage. Buds and set pods were found to be the most frost-susceptible reproductive organs, and mature pods were the most frost-tolerant reproductive organs. Genetic variation was found among field pea genotypes for frost tolerance at the flowering stage. Eight accessions, ATC 104, ATC 377, ATC 947, ATC 968, ATC 1564, ATC 3489, ATC 3992 and ATC 4204, each from a different country, were identified with more than 20 % frost survival of flowering stage organs. Kaspa was highly susceptible to frost at reproductive stages, with no buds, flowers or pods surviving the frost treatment. A BC1F1 population was derived from frost- tolerant ATC 1564 and frost-sensitive Kaspa, and segregation of the frost survival trait and SSR markers was studied. Little marker polymorphism was observed between the two genotypes, with only 41 (12.3 %) of the 332 primer pairs assayed on DNA samples of the parental lines, exhibiting polymorphic products in polyacrylamide gel electrophoresis. Unfortunately, most of these markers were not linked with any other loci, and only two linkage groups were developed: one with three markers, and the other with only two. No strong marker-trait associations were observed for frost tolerance. Responses of reproductive-stage plants to low positive temperature (10/5°C day/night, and 150 – 250 μmole m⁻² s⁻¹ PPFD) for 7, 14 and 21 days were studiedas were the effects of these cold treatments on survival of vegetative and reproductive tissues after frost, for frost-tolerant (ATC 968 and ATC 1564) and frost-sensitive (ATC 1040 and Kaspa) genotypes. Under long exposures (21 days), all genotypes exhibited an ability to maintain the photosynthetic rate. All genotypes were found to be adversely affected by chilling at the reproductive stage, however frost-sensitive genotypes were more responsive to low positive temperatures (cold) than frost tolerant genotypes. Evidence of symptoms of chilling injuries was found in the frost-sensitive genotype: distortion in the ultrastructure of chloroplasts was observed in parenchyma cells of stipules in Kaspa. A decrease and/or non-accumulation of soluble sugar in vegetative and reproductive tissues found in all genotypes under cold conditions reflected the inability in reproductive stage plants to acclimate. In contrast to what has previously been observed for pea seedlings, cold treatment of reproductive-stage pea plants did not result in acclimation, did not improve reproductive frost tolerance, and in fact reduced frost tolerance. In conclusion, a drop in temperature under radiant frost conditions is lethal for reproductive stage pea plants. Reproductive organs are inherently sensitive to frost, and severe frost damage may lead to abortion of buds, flowers and set pods, and significantly reduce the seed weight.