Changes in Metabolites and Allelopathic Effects of Non-pigmented and Black-pigmented Lowland indica Rice Varieties in Phosphorus Deficiency

Phosphorus (P) levels alters the allelopathic activity of rice seedlings against lettuce seeds. Here, we investigated the effect of P deficiency on allelopathic potential in non-pigmented and pigmented rice varieties. Rice seedlings of the white variety Khao Dawk Mali (KDML105, non-pigmented) and the black varieties Jao Hom Nin (JHN, pigmented) and Riceberry (RB, pigmented) were grown in high P (HP) and low P (LP) conditions. Morphological and metabolic responses to P deficiency were investigated. P deficiency inhibited shoot growth but promoted root growth of rice seedlings in all three varieties. Moreover, P deficiency led to decreased cytosolic phosphate (Pi) and total P in shoot and root tissues. The subsequent reduction of internal P enhanced the accumulation of phenolic contents in both shoot and root tissues of the seedlings. Subsequently, allelopathy-based inter- and intraspecific interactions were validated by using water extracts from seedlings of the three varieties grown under HP and LP conditions. The extracts were tested on seeds of lettuce, the weed Dactyloctenium aegyptium, and the same rice variety. Shoot and root extracts of P-deficient seedlings reduced the germination of all the receiver plants. The shoot extract of P-deficient KDML105 seedlings reduced the germination index of lettuce seeds to 1%, while the shoot extracts of P-deficient RB and JHN seedlings produced GIs of 32% and 42%, respectively. However, when rice seeds were exposed to their own LP shoot and root extracts, their germination indices were enhanced up to 4-fold, compared with the HP extracts. Shoot extracts of P-deficient plants also promoted the germination of D. aegyptium by about 2–3-fold whereas root extracts of P-deficient plants did not. Therefore, P starvation led to the accumulation and exudation of phenolics in the shoots and roots of rice seedlings, which then changed their allelopathic activities. To thrive under P deficiency, rice seedlings potentially release signaling chemicals to suppress different species nearby and simultaneously promote their own germination and growth.